SR15_Chapter_1_HR.txt

Chapters

491
Coordinating Lead Authors:
Myles R. Allen (UK), Opha Pauline Dube (Botswana), William Solecki (USA)
Lead Authors:
Fernando Aragón-Durand (Mexico), Wolfgang Cramer (France/Germany), Stephen Humphreys (UK/
Ireland), Mikiko Kainuma (Japan), Jatin Kala (Australia), Natalie Mahowald (USA), Yacob Mulugetta 
(UK/Ethiopia), Rosa Perez (Philippines), Morgan Wairiu (Solomon Islands), Kirsten Zickfeld (Canada/
Germany) 
Contributing Authors:
Purnamita Dasgupta (India), Haile Eakin (USA), Bronwyn Hayward (New Zealand), Diana Liverman 
(USA), Richard Millar (UK), Graciela Raga (Mexico/Argentina), Aurélien Ribes (France), Mark Richardson 
(USA/UK), Maisa Rojas (Chile), Roland Séférian (France), Sonia I. Seneviratne (Switzerland), Christopher 
Smith (UK), Will Steffen (Australia), Peter Thorne (Ireland/UK)
Chapter Scientist: 
Richard Millar (UK)
Review Editors:
Ismail Elgizouli Idris (Sudan), Andreas Fischlin (Switzerland), Xuejie Gao (China) 
This chapter should be cited as:
Allen, M.R., O.P . Dube, W. Solecki, F . Aragón-Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, 
Y . Mulugetta, R. Perez, M.  Wairiu, and K. Zickfeld, 2018: Framing and Context. In: Global Warming of 1.5°C. An IPCC 
Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse 
gas emission pathways, in the context of strengthening the global response to the threat of climate change, 
sustainable development, and efforts to eradicate poverty  [Masson-Delmotte, V., P .  Zhai, H.-O. Pörtner, D. Roberts, J. 
Skea, P .R. Shukla, A.  Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y . Chen, X.  Zhou, 
M.I. Gomis, E. Lonnoy, T. Maycock, M.  Tignor, and T. Waterfield (eds.)]. Cambridge University Press, Cambridge, UK 
and New York, NY , USA, pp. 49-92. https://doi.org/ 10.1017/9781009157940.003.Framing and Context

50
Chapter 1 Framing and Context
1Executive Summary  ..................................................................... 51
1.1 Assessing the Knowledge Base  
  for a 1.5°C Warmer World  ............................................ 53
 Box 1.1: The Anthropocene: Strengthening the  
 Global Response to 1.5°C Global Warming  ............................ 54
 1.1.1 Equity and a 1.5°C Warmer World  ................................ 54
 1.1.2 Eradication of Poverty  .................................................. 55
 1.1.3 Sustainable Development and a 1.5°C  
   Warmer World  .............................................................. 55
1.2 Understanding 1.5°C: Reference Levels,  
  Probability, Transience, Overshoot,  
  and Stabilization  ............................................................. 56
 1.2.1 Working Definitions of 1.5°C and 2°C  
   Warming Relative to Pre-Industrial Levels  .................... 56
 1.2.2 Global versus Regional and Seasonal Warming  ............ 59
 1.2.3 Definition of 1.5°C Pathways: Probability,  
   Transience, Stabilization and Overshoot  ....................... 59
 Cross-Chapter Box 1: Scenarios and Pathways  ...................... 62
 1.2.4 Geophysical Warming Commitment  ............................. 64
 Cross-Chapter Box 2: Measuring Progress to Net Zero  
 Emissions Combining Long-Lived and Short-Lived  
 Climate Forcers  .......................................................................... 66
1.3 Impacts at 1.5°C and Beyond  ..................................... 68
 1.3.1 Definitions  .................................................................... 68
 1.3.2 Drivers of Impacts  ........................................................ 69
 1.3.3 Uncertainty and Non-Linearity of Impacts  .................... 69
1.4 Strengthening the Global Response  ......................... 70
 1.4.1 Classifying Response Options  ....................................... 70
 1.4.2 Governance, Implementation and Policies  .................... 71
 Cross-Chapter Box 3: Framing Feasibility:  
 Key Concepts and Conditions for Limiting  
 Global Temperature Increases to 1.5°C  .................................. 71
 1.4.3 Transformation, Transformation Pathways,  
   and Transition: Evaluating Trade-Offs and  
   Synergies Between Mitigation, Adaptation  
   and Sustainable Development Goals  ............................ 73
 Cross-Chapter Box 4: Sustainable Development  
 and the Sustainable Development Goals  ............................... 731.5 Assessment Frameworks and Emerging  
  Methodologies that Integrate Climate  
  Change Mitigation and Adaptation  
  with Sustainable Development  .................................. 75
 1.5.1 Knowledge Sources and Evidence  
   Used in the Report  ....................................................... 75
 1.5.2 Assessment Frameworks and Methodologies  ............... 76
1.6 Confidence, Uncertainty and Risk  .............................. 77
1.7 Storyline of the Report  ................................................. 77
Frequently Asked Questions
 FAQ 1.1:  Why are we Talking about 1.5°C?  ........................ 79
 FAQ 1.2:  How Close are we to 1.5°C?  ................................. 81
References  ..................................................................................... 83Table of Contents

51
1Framing and Context Chapter 1Executive Summary
This chapter frames the context, knowledge-base and assessment 
approaches used to understand the impacts of 1.5°C global warming 
above pre-industrial levels and related global greenhouse gas 
emission pathways, building on the IPCC Fifth Assessment Report 
(AR5), in the context of strengthening the global response to the 
threat of climate change, sustainable development and efforts to 
eradicate poverty. 
Human-induced warming reached approximately 1°C ( likely  
between 0.8°C and 1.2°C) above pre-industrial levels in 2017, 
increasing at 0.2°C ( likely  between 0.1°C and 0.3°C) per 
decade ( high confidence ). Global warming is defined in this report 
as an increase in combined surface air and sea surface temperatures 
averaged over the globe and over a 30-year period. Unless otherwise 
specified, warming is expressed relative to the period 1850–1900, 
used as an approximation of pre-industrial temperatures in AR5. 
For periods shorter than 30 years, warming refers to the estimated 
average temperature over the 30 years centred on that shorter 
period, accounting for the impact of any temperature fluctuations 
or trend within those 30 years. Accordingly, warming from pre-
industrial levels to the decade 2006–2015 is assessed to be 0.87°C 
(likely  between 0.75°C and 0.99°C). Since 2000, the estimated level 
of human-induced warming has been equal to the level of observed 
warming with a likely range of ±20% accounting for uncertainty due 
to contributions from solar and volcanic activity over the historical 
period ( high confidence ). {1.2.1}
Warming greater than the global average has already been 
experienced in many regions and seasons, with higher average 
warming over land than over the ocean ( high confidence ). Most 
land regions are experiencing greater warming than the global average, 
while most ocean regions are warming at a slower rate. Depending 
on the temperature dataset considered, 20–40% of the global human 
population live in regions that, by the decade 2006–2015, had already 
experienced warming of more than 1.5°C above pre-industrial in at 
least one season ( medium confidence ). {1.2.1, 1.2.2}
Past emissions alone are unlikely  to raise global-mean 
temperature to 1.5°C above pre-industrial levels ( medium 
confidence ), but past emissions do commit to other changes, 
such as further sea level rise ( high confidence ). If all 
anthropogenic emissions (including aerosol-related) were reduced 
to zero immediately, any further warming beyond the 1°C already 
experienced would likely  be less than 0.5°C over the next two to 
three decades ( high confidence ), and likely  less than 0.5°C on a 
century time scale ( medium confidence ), due to the opposing effects 
of different climate processes and drivers. A warming greater than 
1.5°C is therefore not geophysically unavoidable: whether it will 
occur depends on future rates of emission reductions. {1.2.3, 1.2.4}
1.5°C emission pathways are defined as those that, given 
current knowledge of the climate response, provide a one-
in-two to two-in-three chance of warming either remaining 
below 1.5°C or returning to 1.5°C by around 2100 following an overshoot. Overshoot pathways are characterized by the peak 
magnitude of the overshoot, which may have implications for 
impacts. All 1.5°C pathways involve limiting cumulative emissions 
of long-lived greenhouse gases, including carbon dioxide and nitrous 
oxide, and substantial reductions in other climate forcers ( high 
confidence ). Limiting cumulative emissions requires either reducing 
net global emissions of long-lived greenhouse gases to zero before 
the cumulative limit is reached, or net negative global emissions 
(anthropogenic removals) after the limit is exceeded. {1.2.3, 1.2.4, 
Cross-Chapter Boxes 1 and 2}
This report assesses projected impacts at a global average 
warming of 1.5°C and higher levels of warming. Global warming 
of 1.5°C is associated with global average surface temperatures 
fluctuating naturally on either side of 1.5°C, together with warming 
substantially greater than 1.5°C in many regions and seasons ( high 
confidence ), all of which must be considered in the assessment of 
impacts. Impacts at 1.5°C of warming also depend on the emission 
pathway to 1.5°C. Very different impacts result from pathways 
that remain below 1.5°C versus pathways that return to 1.5°C 
after a substantial overshoot, and when temperatures stabilize at 
1.5°C versus a transient warming past 1.5°C ( medium confidence ). 
{1.2.3, 1.3} 
Ethical considerations, and the principle of equity in particular, 
are central to this report, recognizing that many of the impacts 
of warming up to and beyond 1.5°C, and some potential 
impacts of mitigation actions required to limit warming to 
1.5°C, fall disproportionately on the poor and vulnerable ( high 
confidence ). Equity has procedural and distributive dimensions and 
requires fairness in burden sharing both between generations and 
between and within nations. In framing the objective of holding the 
increase in the global average temperature rise to well below 2°C 
above pre-industrial levels, and to pursue efforts to limit warming to 
1.5°C, the Paris Agreement associates the principle of equity with the 
broader goals of poverty eradication and sustainable development, 
recognising that effective responses to climate change require a 
global collective effort that may be guided by the 2015 United 
Nations Sustainable Development Goals. {1.1.1}
Climate adaptation refers to the actions taken to manage 
impacts of climate change by reducing vulnerability and 
exposure to its harmful effects and exploiting any potential 
benefits. Adaptation takes place at international, national and 
local levels. Subnational jurisdictions and entities, including urban 
and rural municipalities, are key to developing and reinforcing 
measures for reducing weather- and climate-related risks. Adaptation 
implementation faces several barriers including lack of up-to-date and 
locally relevant information, lack of finance and technology, social 
values and attitudes, and institutional constraints ( high confidence ). 
Adaptation is more likely  to contribute to sustainable development 
when policies align with mitigation and poverty eradication goals 
(medium confidence ). {1.1, 1.4} 
Ambitious mitigation actions are indispensable to limit 
warming to 1.5°C while achieving sustainable development 
and poverty eradication ( high confidence ). Ill-designed responses, 

52
Chapter 1 Framing and Context
1however, could pose challenges especially – but not exclusively – for 
countries and regions contending with poverty and those requiring 
significant transformation of their energy systems. This report focuses 
on ‘climate-resilient development pathways’, which aim to meet the 
goals of sustainable development, including climate adaptation and 
mitigation, poverty eradication and reducing inequalities. But any 
feasible pathway that remains within 1.5°C involves synergies and 
trade-offs ( high confidence ). Significant uncertainty remains as to 
which pathways are more consistent with the principle of equity. 
{1.1.1, 1.4}
Multiple forms of knowledge, including scientific evidence, 
narrative scenarios and prospective pathways, inform the 
understanding of 1.5°C. This report is informed by traditional 
evidence of the physical climate system and associated impacts and 
vulnerabilities of climate change, together with knowledge drawn 
from the perceptions of risk and the experiences of climate impacts 
and governance systems. Scenarios and pathways are used to 
explore conditions enabling goal-oriented futures while recognizing 
the significance of ethical considerations, the principle of equity, and 
the societal transformation needed. {1.2.3, 1.5.2} 
There is no single answer to the question of whether it 
is feasible to limit warming to 1.5°C and adapt to the 
consequences. Feasibility is considered in this report as the 
capacity of a system as a whole to achieve a specific outcome. The 
global transformation that would be needed to limit warming to 
1.5°C requires enabling conditions that reflect the links, synergies 
and trade-offs between mitigation, adaptation and sustainable 
development. These enabling conditions are assessed across many 
dimensions of feasibility – geophysical, environmental-ecological, 
technological, economic, socio-cultural and institutional – that 
may be considered through the unifying lens of the Anthropocene, 
acknowledging profound, differential but increasingly geologically 
significant human influences on the Earth system as a whole. This 
framing also emphasises the global interconnectivity of past, present 
and future human–environment relations, highlighting the need and 
opportunities for integrated responses to achieve the goals of the 
Paris Agreement. {1.1, Cross-Chapter Box 1}

53
1Framing and Context Chapter 11.1 Assessing the Knowledge Base 
for a 1.5°C Warmer World 
Human influence on climate has been the dominant cause of observed 
warming since the mid-20th century, while global average surface 
temperature warmed by 0.85°C between 1880 and 2012, as reported 
in the IPCC Fifth Assessment Report, or AR5 (IPCC, 2013b). Many 
regions of the world have already greater regional-scale warming, 
with 20–40% of the global population (depending on the temperature 
dataset used) having experienced over 1.5°C of warming in at least 
one season (Figure 1.1; Chapter 3 Section 3.3.2.1). Temperature rise 
to date has already resulted in profound alterations to human and 
natural systems, including increases in droughts, floods, and some 
other types of extreme weather; sea level rise; and biodiversity loss – 
these changes are causing unprecedented risks to vulnerable persons 
and populations (IPCC, 2012a, 2014a; Mysiak et al., 2016; Chapter 
3 Sections 3.4.5–3.4.13). The most affected people live in low and 
middle income countries, some of which have experienced a decline 
in food security, which in turn is partly linked to rising migration and 
poverty (IPCC, 2012a). Small islands, megacities, coastal regions, and 
high mountain ranges are likewise among the most affected (Albert 
et al., 2017). Worldwide, numerous ecosystems are at risk of severe 
impacts, particularly warm-water tropical reefs and Arctic ecosystems 
(IPCC, 2014a).
This report assesses current knowledge of the environmental, technical, 
economic, financial, socio-cultural, and institutional dimensions of a 
1.5°C warmer world (meaning, unless otherwise specified, a world 
in which warming has been limited to 1.5°C relative to pre-industrial 
levels). Differences in vulnerability and exposure arise from numerous non-climatic factors (IPCC, 2014a). Global economic growth has been 
accompanied by increased life expectancy and income in much of 
the world; however, in addition to environmental degradation and 
pollution, many regions remain characterised by significant poverty 
and severe inequality in income distribution and access to resources, 
amplifying vulnerability to climate change (Dryzek, 2016; Pattberg 
and Zelli, 2016; Bäckstrand et al., 2017; Lövbrand et al., 2017). World 
population continues to rise, notably in hazard-prone small and 
medium-sized cities in low- and moderate-income countries (Birkmann 
et al., 2016). The spread of fossil-fuel-based material consumption and 
changing lifestyles is a major driver of global resource use, and the 
main contributor to rising greenhouse gas (GHG) emissions (Fleurbaey 
et al., 2014). 
The overarching context of this report is this: human influence has 
become a principal agent of change on the planet, shifting the world 
out of the relatively stable Holocene period into a new geological 
era, often termed the Anthropocene (Box 1.1). Responding to climate 
change in the Anthropocene will require approaches that integrate 
multiple levels of interconnectivity across the global community. 
This chapter is composed of seven sections linked to the remaining 
four chapters of the report. This introductory Section 1.1 situates the 
basic elements of the assessment within the context of sustainable 
development; considerations of ethics, equity and human rights; and the 
problem of poverty. Section 1.2 focuses on understanding 1.5°C, global 
versus regional warming, 1.5°C pathways, and associated emissions. 
Section 1.3 frames the impacts at 1.5°C and beyond on natural and 
human systems. The section on strengthening the global response (1.4) 
frames responses, governance and implementation, and trade-offs 
and synergies between mitigation, adaptation, and the Sustainable 
Figure 1.1 |  Human experience of present-day warming. Different shades of pink to purple indicated by the inset histogram show estimated warming for the season 
that has warmed the most at a given location between the periods 1850–1900 and 2006–2015, during which global average temperatures rose by 0.91°C in this dataset 
(Cowtan and Way, 2014) and 0.87°C in the multi-dataset average (Table 1.1 and Figure 1.3). The density of dots indicates the population (in 2010) in any 1° × 1° grid box. 
The underlay shows national Sustainable Development Goal (SDG) Global Index Scores indicating performance across the 17 SDGs. Hatching indicates missing SDG index data 
(e.g., Greenland). The histogram shows the population (in 2010) living in regions experiencing different levels of warming (at 0.25°C increments). See Supplementary Material 
1.SM for further details.

54
Chapter 1 Framing and Context
1Development Goals (SDGs) under transformation, transformation 
pathways, and transition. Section 1.5 provides assessment frameworks 
and emerging methodologies that integrate climate change mitigation and adaptation with sustainable development. Section 1.6 defines 
approaches used to communicate confidence, uncertainty and risk, 
while 1.7 presents the storyline of the whole report.
1.1.1 Equity and a 1.5°C Warmer World
The AR5 suggested that equity, sustainable development, and 
poverty eradication are best understood as mutually supportive 
and co-achievable within the context of climate action and are 
underpinned by various other international hard and soft law 
instruments (Denton et al., 2014; Fleurbaey et al., 2014; Klein et al., 2014; Olsson et al., 2014; Porter et al., 2014; Stavins et al., 2014). 
The aim of the Paris Agreement under the UNFCCC to ‘pursue 
efforts to limit’ the rise in global temperatures to 1.5°C above pre-
industrial levels raises ethical concerns that have long been central 
to climate debates (Fleurbaey et al., 2014; Kolstad et al., 2014). 
The Paris Agreement makes particular reference to the principle 
of equity, within the context of broader international goals of Box 1.1 |  The Anthropocene: Strengthening the Global Response to 1.5°C Global Warming
Introduction  
The concept of the Anthropocene can be linked to the aspiration of the Paris Agreement. The abundant empirical evidence of the 
unprecedented rate and global scale of impact of human influence on the Earth System (Steffen et al., 2016; Waters et al., 2016) has 
led many scientists to call for an acknowledgement that the Earth has entered a new geological epoch: the Anthropocene (Crutzen 
and Stoermer, 2000; Crutzen, 2002; Gradstein et al., 2012). Although rates of change in the Anthropocene are necessarily assessed 
over much shorter periods than those used to calculate long-term baseline rates of change, and therefore present challenges for direct 
comparison, they are nevertheless striking. The rise in global CO2 concentration since 2000 is about 20 ppm per decade, which is up to 
10 times faster than any sustained rise in CO2 during the past 800,000 years (Lüthi et al., 2008; Bereiter et al., 2015). AR5 found that 
the last geological epoch with similar atmospheric CO2 concentration was the Pliocene, 3.3 to 3.0 Ma (Masson-Delmotte et al., 2013). 
Since 1970 the global average temperature has been rising at a rate of 1.7°C per century, compared to a long-term decline over the 
past 7,000 years at a baseline rate of 0.01°C per century (NOAA, 2016; Marcott et al., 2013). These global-level rates of human-driven 
change far exceed the rates of change driven by geophysical or biosphere forces that have altered the Earth System trajectory in the past 
(e.g., Summerhayes, 2015; Foster et al., 2017); even abrupt geophysical events do not approach current rates of human-driven change. 
The Geological Dimension of the Anthropocene and 1.5°C Global Warming 
The process of formalising the Anthropocene is on-going (Zalasiewicz et al., 2017), but a strong majority of the Anthropocene Working 
Group (AWG) established by the Subcommission on Quaternary Stratigraphy of the International Commission on Stratigraphy have 
agreed that: (i) the Anthropocene has a geological merit; (ii) it should follow the Holocene as a formal epoch in the Geological Time 
Scale; and, (iii) its onset should be defined as the mid-20th century. Potential markers in the stratigraphic record include an array of 
novel manufactured materials of human origin, and “these combined signals render the Anthropocene stratigraphically distinct from 
the Holocene and earlier epochs” (Waters et al., 2016). The Holocene period, which itself was formally adopted in 1885 by geological 
science community, began 11,700 years ago with a more stable warm climate providing for emergence of human civilisation and 
growing human-nature interactions that have expanded to give rise to the Anthropocene (Waters et al., 2016).
The Anthropocene and the Challenge of a 1.5° C Warmer World 
The Anthropocene can be employed as a “boundary concept” (Brondizio et al., 2016) that frames critical insights into understanding the 
drivers, dynamics and specific challenges in responding to the ambition of keeping global temperature well below 2°C while pursuing 
efforts towards and adapting to a 1.5°C warmer world. The United Nations Framework Convention on Climate Change (UNFCCC) and 
its Paris Agreement recognize the ability of humans to influence geophysical planetary processes (Chapter 2, Cross-Chapter Box 1 in this 
chapter). The Anthropocene offers a structured understanding of the culmination of past and present human–environmental relations 
and provides an opportunity to better visualize the future to minimize pitfalls (Pattberg and Zelli, 2016; Delanty and Mota, 2017),  while 
acknowledging the differentiated responsibility and opportunity to limit global warming and invest in prospects for climate-resilient 
sustainable development (Harrington, 2016) (Chapter 5). The Anthropocene also provides an opportunity to raise questions regarding 
the regional differences, social inequities, and uneven capacities and drivers of global social–environmental changes, which in turn 
inform the search for solutions as explored in Chapter 4 of this report (Biermann et al., 2016). It links uneven influences of human 
actions on planetary functions to an uneven distribution of impacts (assessed in Chapter 3) as well as the responsibility and response 
capacity to, for example, limit global warming to no more than a 1.5°C rise above pre-industrial levels. Efforts to curtail greenhouse gas 
emissions without incorporating the intrinsic interconnectivity and disparities associated with the Anthropocene world may themselves 
negatively affect the development ambitions of some regions more than others and negate sustainable development efforts (see 
Chapter 2 and Chapter 5). 

55
1Framing and Context Chapter 1sustainable development and poverty eradication. Equity is a long-
standing principle within international law and climate change law 
in particular (Shelton, 2008; Bodansky et al., 2017).
The AR5 describes equity as having three dimensions: intergenerational 
(fairness between generations), international (fairness between 
states), and national (fairness between individuals) (Fleurbaey et al., 
2014). The principle is generally agreed to involve both procedural 
justice (i.e., participation in decision making) and distributive justice 
(i.e., how the costs and benefits of climate actions are distributed) 
(Kolstad et al., 2014; Savaresi, 2016; Reckien et al., 2017). Concerns 
regarding equity have frequently been central to debates around 
mitigation, adaptation and climate governance (Caney, 2005; 
Schroeder et al., 2012; Ajibade, 2016; Reckien et al., 2017; Shue, 
2018). Hence, equity provides a framework for understanding the 
asymmetries between the distributions of benefits and costs relevant 
to climate action (Schleussner et al., 2016; Aaheim et al., 2017). 
Four key framing asymmetries associated with the conditions of a 
1.5°C warmer world have been noted (Okereke, 2010; Harlan et al., 
2015; Ajibade, 2016; Savaresi, 2016; Reckien et al., 2017) and are 
reflected in the report’s assessment. The first concerns differential 
contributions to the problem: the observation that the benefits from 
industrialization have been unevenly distributed and those who 
benefited most historically also have contributed most to the current 
climate problem and so bear greater responsibility (Shue, 2013; 
McKinnon, 2015; Otto et al., 2017; Skeie et al., 2017). The second 
asymmetry concerns differential impact: the worst impacts tend to 
fall on those least responsible for the problem, within states, between 
states, and between generations (Fleurbaey et al., 2014; Shue, 2014; 
Ionesco et al., 2016). The third is the asymmetry in capacity to shape 
solutions and response strategies, such that the worst-affected states, 
groups, and individuals are not always well represented (Robinson 
and Shine, 2018). Fourth, there is an asymmetry in future response 
capacity: some states, groups, and places are at risk of being left 
behind as the world progresses to a low-carbon economy (Fleurbaey 
et al., 2014; Shue, 2014; Humphreys, 2017). 
A sizeable and growing literature exists on how best to 
operationalize climate equity considerations, drawing on other 
concepts mentioned in the Paris Agreement, notably its explicit 
reference to human rights (OHCHR, 2009; Caney, 2010; Adger et 
al., 2014; Fleurbaey et al., 2014; IBA, 2014; Knox, 2015; Duyck 
et al., 2018; Robinson and Shine, 2018). Human rights comprise 
internationally agreed norms that align with the Paris ambitions of 
poverty eradication, sustainable development, and the reduction of 
vulnerability (Caney, 2010; Fleurbaey et al., 2014; OHCHR, 2015). 
In addition to defining substantive rights (such as to life, health, 
and shelter) and procedural rights (such as to information and 
participation), human rights instruments prioritise the rights of 
marginalized groups, children, vulnerable and indigenous persons, 
and those discriminated against on grounds such as gender, race, 
age or disability (OHCHR, 2017). Several international human 
rights obligations are relevant to the implementation of climate 
actions and consonant with UNFCCC undertakings in the areas 
of mitigation, adaptation, finance, and technology transfer (Knox, 
2015; OHCHR, 2015; Humphreys, 2017). Much of this literature is still new and evolving (Holz et al., 2017; 
Dooley et al., 2018; Klinsky and Winkler, 2018), permitting the 
present report to examine some broader equity concerns raised 
both by possible failure to limit warming to 1.5°C and by the range 
of ambitious mitigation efforts that may be undertaken to achieve 
that limit. Any comparison between 1.5°C and higher levels of 
warming implies risk assessments and value judgements and cannot 
straightforwardly be reduced to a cost-benefit analysis (Kolstad et 
al., 2014). However, different levels of warming can nevertheless be 
understood in terms of their different implications for equity – that 
is, in the comparative distribution of benefits and burdens for specific 
states, persons, or generations, and in terms of their likely impacts 
on sustainable development and poverty (see especially Sections   
2.3.4.2, 2.5, 3.4.5–3.4.13, 3.6, 5.4.1, 5.4.2, 5.6 and Cross-Chapter 
boxes 6 in Chapter 3 and 12 in Chapter 5).
1.1.2 Eradication of Poverty
This report assesses the role of poverty and its eradication in the 
context of strengthening the global response to the threat of 
climate change and sustainable development. A wide range of 
definitions for poverty  exist. The AR5 discussed ‘poverty’ in terms 
of its multidimensionality, referring to ‘material circumstances’ 
(e.g., needs, patterns of deprivation, or limited resources), as well 
as to economic conditions (e.g., standard of living, inequality, or 
economic position), and/or social relationships (e.g., social class, 
dependency, lack of basic security, exclusion, or lack of entitlement; 
Olsson et al., 2014). The UNDP now uses a Multidimensional Poverty 
Index and estimates that about 1.5 billion people globally live in 
multidimensional poverty, especially in rural areas of South Asia and 
Sub-Saharan Africa, with an additional billion at risk of falling into 
poverty (UNDP , 2016). 
A large and rapidly growing body of knowledge explores the 
connections between climate change and poverty. Climatic 
variability and climate change are widely recognized as factors that 
may exacerbate poverty, particularly in countries and regions where 
poverty levels are high (Leichenko and Silva, 2014). The AR5 noted 
that climate change-driven impacts often act as a threat multiplier 
in that the impacts of climate change compound other drivers of 
poverty (Olsson et al., 2014). Many vulnerable and poor people are 
dependent on activities such as agriculture that are highly susceptible 
to temperature increases and variability in precipitation patterns 
(Shiferaw et al., 2014; Miyan, 2015). Even modest changes in rainfall 
and temperature patterns can push marginalized people into poverty 
as they lack the means to recover from associated impacts. Extreme 
events, such as floods, droughts, and heat waves, especially when 
they occur in series, can significantly erode poor people’s assets and 
further undermine their livelihoods in terms of labour productivity, 
housing, infrastructure and social networks (Olsson et al., 2014).
1.1.3 Sustainable Development and a 1.5°C  
Warmer World
AR5 (IPCC, 2014c) noted with high confidence  that ‘equity is an 
integral dimension of sustainable development’ and that ‘mitigation 
and adaptation measures can strongly affect broader sustainable 

56
Chapter 1 Framing and Context
1development and equity objectives’ (Fleurbaey et al., 2014). Limiting 
global warming to 1.5°C would require substantial societal and 
technological transformations, dependent in turn on global and 
regional sustainable development pathways. A range of pathways, 
both sustainable and not, are explored in this report, including 
implementation strategies to understand the enabling conditions and 
challenges required for such a transformation. These pathways and 
connected strategies are framed within the context of sustainable 
development, and in particular the United Nations 2030 Agenda for 
Sustainable Development (UN, 2015b) and Cross-Chapter Box 4 on 
SDGs (in this chapter). The feasibility of staying within 1.5°C depends 
upon a range of enabling conditions with geophysical, environmental–
ecological, technological, economic, socio-cultural, and institutional 
dimensions. Limiting warming to 1.5°C also involves identifying 
technology and policy levers to accelerate the pace of transformation 
(see Chapter 4). Some pathways are more consistent than others with 
the requirements for sustainable development (see Chapter 5). Overall, 
the three-pronged emphasis on sustainable development, resilience, 
and transformation provides Chapter 5 an opportunity to assess 
the conditions of simultaneously reducing societal vulnerabilities, 
addressing entrenched inequalities, and breaking the circle of poverty.
The feasibility of any global commitment to a 1.5°C pathway depends, 
in part, on the cumulative influence of the nationally determined 
contributions (NDCs), committing nation states to specific GHG 
emission reductions. The current NDCs, extending only to 2030, do 
not limit warming to 1.5°C. Depending on mitigation decisions after 
2030, they cumulatively track toward a warming of 3°-4°C above 
pre-industrial temperatures by 2100, with the potential for further 
warming thereafter (Rogelj et al., 2016a; UNFCCC, 2016). The analysis 
of pathways in this report reveals opportunities for greater decoupling 
of economic growth from GHG emissions. Progress towards limiting 
warming to 1.5°C requires a significant acceleration of this trend. AR5 
concluded that climate change constrains possible development paths, 
that synergies and trade-offs exist between climate responses and 
socio-economic contexts, and that opportunities for effective climate 
responses overlap with opportunities for sustainable development, 
noting that many existing societal patterns of consumption are 
intrinsically unsustainable (Fleurbaey et al., 2014). 
1.2 Understanding 1.5°C: Reference 
Levels, Probability, Transience, 
Overshoot, and Stabilization
1.2.1 Working Definitions of 1.5°C and 2°C 
Warming Relative to Pre-Industrial Levels
What is meant by ‘the increase in global average temperature… above 
pre-industrial levels’ referred to in the Paris Agreement depends on 
the choice of pre-industrial reference period, whether 1.5°C refers to 
total warming or the human-induced component of that warming, 
and which variables and geographical coverage are used to define 
global average temperature change. The cumulative impact of these 
definitional ambiguities (e.g., Hawkins et al., 2017; Pfleiderer et al., 
2018) is comparable to natural multi-decadal temperature variability on continental scales (Deser et al., 2012) and primarily affects the 
historical period, particularly that prior to the early 20th century when 
data is sparse and of less certain quality. Most practical mitigation 
and adaptation decisions do not depend on quantifying historical 
warming to this level of precision, but a consistent working definition 
is necessary to ensure consistency across chapters and figures. We 
adopt definitions that are as consistent as possible with key findings 
of AR5 with respect to historical warming. 
This report defines ‘warming’, unless otherwise qualified, as an 
increase in multi-decade global mean surface temperature (GMST) 
above pre-industrial levels. Specifically, warming at a given point 
in time is defined as the global average of combined land surface 
air and sea surface temperatures for a 30-year period centred on 
that time, expressed relative to the reference period 1850–1900 
(adopted for consistency with Box SPM.1 Figure 1 of IPCC (2014a)) 
‘as an approximation of pre-industrial levels’, excluding the impact of 
natural climate fluctuations within that 30-year period and assuming 
any secular trend continues throughout that period, extrapolating 
into the future if necessary. There are multiple ways of accounting 
for natural fluctuations and trends (e.g., Foster and Rahmstorf, 2011; 
Haustein et al., 2017; Medhaug et al., 2017; Folland et al., 2018; 
Visser et al., 2018), but all give similar results. A major volcanic 
eruption might temporarily reduce observed global temperatures, 
but would not reduce warming as defined here (Bethke et al., 2017). 
Likewise, given that the level of warming is currently increasing at 
0.3°C–0.7°C per 30 years ( likely  range quoted in Kirtman et al., 2013 
and supported by Folland et al., 2018), the level of warming in 2017 
was 0.15°C–0.35°C higher than average warming over the 30-year 
period 1988–2017. 
In summary, this report adopts a working definition of ‘1.5°C relative 
to pre-industrial levels’ that corresponds to global average combined 
land surface air and sea surface temperatures either 1.5°C warmer 
than the average of the 51-year period 1850–1900, 0.87°C warmer 
than the 20-year period 1986–2005, or 0.63°C warmer than the 
decade 2006–2015. These offsets are based on all available published 
global datasets, combined and updated, which show that 1986–
2005 was 0.63°C warmer than 1850–1900 (with a 5–95% range 
of 0.57°C–0.69°C based on observational uncertainties alone), and 
2006–2015 was 0.87°C warmer than 1850–1900 (with a likely range 
of 0.75°C–0.99°C, also accounting for the possible impact of natural 
fluctuations). Where possible, estimates of impacts and mitigation 
pathways are evaluated relative to these more recent periods. Note 
that the 5–95% intervals often quoted in square brackets in AR5 
correspond to very likely  ranges, while likely  ranges correspond to 
17–83%, or the central two-thirds, of the distribution of uncertainty.  
1.2.1.1 Definition of global average temperature
The IPCC has traditionally defined changes in observed GMST as a 
weighted average of near-surface air temperature (SAT) changes 
over land and sea surface temperature (SST) changes over the oceans 
(Morice et al., 2012; Hartmann et al., 2013), while modelling studies 
have typically used a simple global average SAT. For ambitious 
mitigation goals, and under conditions of rapid warming or declining 
sea ice (Berger et al., 2017), the difference can be significant. Cowtan 

57
1Framing and Context Chapter 1et al. (2015) and Richardson et al. (2016) show that the use of 
blended SAT/SST data and incomplete coverage together can give 
approximately 0.2°C less warming from the 19th century to the 
present relative to the use of complete global-average SAT (Stocker 
et al., 2013, Figure TFE8.1 and Figure 1.2). However, Richardson et al. 
(2018) show that this is primarily an issue for the interpretation of 
the historical record to date, with less absolute impact on projections 
of future changes, or estimated emissions budgets, under ambitious 
mitigation scenarios. 
The three GMST reconstructions used in AR5 differ in their treatment 
of missing data. GISTEMP (Hansen et al., 2010) uses interpolation 
to infer trends in poorly observed regions like the Arctic (although 
even this product is spatially incomplete in the early record), while 
NOAAGlobalTemp (Vose et al., 2012) and HadCRUT (Morice et al., 
2012) are progressively closer to a simple average of available 
observations. Since the AR5, considerable effort has been devoted 
to more sophisticated statistical modelling to account for the impact of incomplete observation coverage (Rohde et al., 2013; Cowtan and 
Way, 2014; Jones, 2016). The main impact of statistical infilling is to 
increase estimated warming to date by about 0.1°C (Richardson et 
al., 2018 and Table 1.1). 
We adopt a working definition of warming over the historical period 
based on an average of the four available global datasets that are 
supported by peer-reviewed publications: the three datasets used in the 
AR5, updated (Karl et al., 2015), together with the Cowtan-Way infilled 
dataset (Cowtan and Way, 2014). A further two datasets, Berkeley 
Earth (Rohde et al., 2013) and that of the Japan Meteorological Agency 
(JMA), are provided in Table 1.1. This working definition provides an 
updated estimate of 0.86°C for the warming over the period 1880–
2012 based on a linear trend. This quantity was quoted as 0.85°C in 
the AR5. Hence the inclusion of the Cowtan-Way dataset does not 
introduce any inconsistency with the AR5, whereas redefining GMST 
to represent global SAT could increase this figure by up to 20% (Table 
1.1, blue lines in Figure 1.2 and Richardson et al., 2016). 
Figure 1.2 |  Evolution of global mean surface temperature (GMST) over the period of instrumental observations. Grey shaded line shows monthly mean GMST 
in the HadCRUT4, NOAAGlobalTemp, GISTEMP and Cowtan-Way datasets, expressed as departures from 1850–1900, with varying grey line thickness indicating inter-dataset 
range. All observational datasets shown represent GMST as a weighted average of near surface air temperature over land and sea surface temperature over oceans. Human-
induced (yellow) and total (human- and naturally-forced, orange) contributions to these GMST changes are shown calculated following Otto et al. (2015) and Haustein et al. 
(2017). Fractional uncertainty in the level of human-induced warming in 2017 is set equal to ±20% based on multiple lines of evidence. Thin blue lines show the modelled 
global mean surface air temperature (dashed) and blended surface air and sea surface temperature accounting for observational coverage (solid) from the CMIP5 historical 
ensemble average extended with RCP8.5 forcing (Cowtan et al., 2015; Richardson et al., 2018). The pink shading indicates a range for temperature fluctuations over the 
Holocene (Marcott et al., 2013). Light green plume shows the AR5 prediction for average GMST over 2016–2035 (Kirtman et al., 2013). See Supplementary Material 1.SM for 
further details. 
1.2.1.2 Choice of reference period
Any choice of reference period used to approximate ‘pre-
industrial’ conditions is a compromise between data coverage 
and representativeness of typical pre-industrial solar and volcanic 
forcing conditions. This report adopts the 51-year reference period, 
1850–1900 inclusive, assessed as an approximation of pre-industrial 
levels in AR5 (Box TS.5, Figure 1 of Field et al., 2014). The years 
1880–1900 are subject to strong but uncertain volcanic forcing, but in the HadCRUT4 dataset, average temperatures over 1850–1879, 
prior to the largest eruptions, are less than 0.01°C from the average 
for 1850–1900. Temperatures rose by 0.0°C–0.2°C from 1720–
1800 to 1850–1900 (Hawkins et al., 2017), but the anthropogenic 
contribution to this warming is uncertain (Abram et al., 2016; Schurer 
et al., 2017). The 18th century represents a relatively cool period in 
the context of temperatures since the mid-Holocene (Marcott et al., 
2013; Lüning and Vahrenholt, 2017; Marsicek et al., 2018), which is 
indicated by the pink shaded region in Figure 1.2.

58
Chapter 1 Framing and Context
1Projections of responses to emission scenarios, and associated 
impacts, may use a more recent reference period, offset by historical 
observations, to avoid conflating uncertainty in past and future 
changes (e.g., Hawkins et al., 2017; Millar et al., 2017b; Simmons 
et al., 2017). Two recent reference periods are used in this report: 
1986–2005 and 2006–2015. In the latter case, when using a single 
decade to represent a 30-year average centred on that decade, it 
is important to consider the potential impact of internal climate 
variability. The years 2008–2013 were characterised by persistent 
cool conditions in the Eastern Pacific (Kosaka and Xie, 2013; Medhaug 
et al., 2017), related to both the El Niño-Southern Oscillation (ENSO) 
and, potentially, multi-decadal Pacific variability (e.g., England et al., 
2014), but these were partially compensated for by El Niño conditions 
in 2006 and 2015. Likewise, volcanic activity depressed temperatures 
in 1986–2005, partly offset by the very strong El Niño event in 1998. 
Figure 1.2 indicates that natural variability (internally generated and 
externally driven) had little net impact on average temperatures 
over 2006–2015, in that the average temperature of the decade is similar to the estimated externally driven warming. When solar, 
volcanic and ENSO-related variability is taken into account following 
the procedure of Foster and Rahmstorf (2011), there is no indication 
of average temperatures in either 1986–2005 or 2006–2015 being 
substantially biased by short-term variability (see Supplementary 
Material 1.SM.2). The temperature difference between these two 
reference periods (0.21°C–0.27°C over 15 years across available 
datasets) is also consistent with the AR5 assessment of the current 
warming rate of 0.3°C–0.7°C over 30 years (Kirtman et al., 2013). 
On the definition of warming used here, warming to the decade 
2006–2015 comprises an estimate of the 30-year average centred 
on this decade, or 1996–2025, assuming the current trend continues 
and that any volcanic eruptions that might occur over the final seven 
years are corrected for. Given this element of extrapolation, we use 
the AR5 near-term projection to provide a conservative uncertainty 
range. Combining the uncertainty in observed warming to 1986–
2005 (±0.06°C) with the likely  range in the current warming trend as 
Diagnostic 
/ dataset1850–1900 
to (1)
2006–20151850–1900 
to (2)
1986–20051986–2005 
to (3)
2006–20151850–1900 
to (4)
1981–20101850–1900 
to (5)
1998–2017Trend (6)
1880–2012Trend (6)
1880–2015
HadCRUT4.60.84 
[0.79–0.89]0.60 
[0.57–0.66]0.22 
[0.21–0.23]0.62 
[0.58–0.67]0.83 
[0.78–0.88]0.83 
[0.77–0.90]0.88 
[0.83–0.95]
NOAAGlobalTemp 
(7)0.86 0.62 0.22 0.63 0.85 0.85 0.91
GISTEMP (7) 0.89 0.65 0.23 0.66 0.88 0.89 0.94
Cowtan-Way0.91 
[0.85–0.99]0.65
[0.60–0.72]0.26 
[0.25–0.27]0.65 
[0.60–0.72]0.88 
[0.82–0.96]0.88 
[0.79–0.98]0.93 
[0.85–1.03]
Average (8) 0.87 0.63 0.23 0.64 0.86 0.86 0.92
Berkeley (9) 0.98 0.73 0.25 0.73 0.97 0.97 1.02
JMA (9) 0.82 0.59 0.17 0.60 0.81 0.82 0.87
ERA-Interim N/A N/A 0.26 N/A N/A N/A N/A
JRA-55 N/A N/A 0.23 N/A N/A N/A N/A
CMIP5 global 
SAT (10)0.99 
[0.65–1.37]0.62 
[0.38–0.94]0.38
[0.24–0.62]0.62 
[0.34–0.93]0.89 
[0.62–1.29]0.81 
[0.58–1.31]0.86 
[0.63–1.39]
CMIP5 SAT/SST 
blend -masked0.86 
[0.54–1.18]0.50 
[0.31–0.79]0.34 
[0.19–0.54]0.48 
[0.26–0.79]0.75 
[0.52–1.11]0.68 
[0.45–1.08]0.74 
[0.51–1.14]
Notes:  
 1) Most recent reference period used in this report.
 2) Most recent reference period used in AR5.
 3) Difference between recent reference periods.
 4) Current WMO standard reference periods.
 5) Most recent 20-year period. 
 6) Linear trends estimated by a straight-line fit, expressed in degrees yr−1 multiplied by 133 or 135 years respectively, with uncertainty ranges incorporating observational uncertainty only.
 7) To estimate changes in the NOAAGlobalTemp and GISTEMP datasets relative to the 1850–1900 reference period, warming is computed relative to 1850–1900 using the HadCRUT4.6  
  dataset and scaled by the ratio of the linear trend 1880–2015 in the NOAAGlobalTemp or GISTEMP dataset with the corresponding linear trend computed from HadCRUT4. 
 8) Average of diagnostics derived – see (7) – from four peer-reviewed global datasets, HadCRUT4.6, NOAA, GISTEMP & Cowtan-Way. Note that differences between averages may not  
  coincide with average differences because of rounding.
 9) No peer-reviewed publication available for these global combined land–sea datasets.
 10) CMIP5 changes estimated relative to 1861–80 plus 0.02°C for the offset in HadCRUT4.6 from 1850–1900. CMIP5 values are the mean of the RCP8.5 ensemble, with 5–95% ensemble  
  range. They are included to illustrate the difference between a complete global surface air temperature record (SAT) and a blended surface air and sea surface temperature (SST) record  
  accounting for incomplete coverage (masked), following Richardson et al. (2016). Note that 1986–2005 temperatures in CMIP5 appear to have been depressed more than observed temperatures 
by the eruption of Mount Pinatubo. Table 1.1 | Observed increase in global average surface temperature in various datasets. 
 Numbers in square brackets correspond to 5–95% uncertainty ranges from individual datasets, encompassing known sources of observational uncertainty only.

59
1Framing and Context Chapter 1for observational and forcing uncertainty and internal variability. 
Applying their method to the average of the four datasets shown in 
Figure 1.2 gives an average level of human-induced warming in 2017 
of 1.04°C. They also estimate a human-induced warming trend over 
the past 20 years of 0.17°C (0.13°C–0.33°C) per decade, consistent 
with estimates of the total observed trend of Foster and Rahmstorf 
(2011) (0.17° ± 0.03°C per decade, uncertainty in linear trend only), 
Folland et al. (2018) and Kirtman et al. (2013) (0.3°C–0.7°C over 30 
years, or 0.1°C–0.23°C per decade, likely  range), and a best-estimate 
warming rate over the past five years of 0.215°C/decade (Leach et al., 
2018). Drawing on these multiple lines of evidence, human-induced 
warming is assessed to have reached 1.0°C in 2017, having increased 
by 0.13°C from the mid-point of 2006–2015, with a likely  range 
of 0.8°C to 1.2°C (reduced from 5–95% to account for additional 
forcing and model uncertainty), increasing at 0.2°C per decade (with 
a likely  range of 0.1°C to 0.3°C per decade: estimates of human-
induced warming given to 0.1°C precision only). 
Since warming is here defined in terms of a 30-year average, corrected 
for short-term natural fluctuations, when warming is considered to be 
at 1.5°C, global temperatures would fluctuate equally on either side 
of 1.5°C in the absence of a large cooling volcanic eruption (Bethke et 
al., 2017). Figure 1.2 indicates there is a substantial chance of GMST in 
a single month fluctuating over 1.5°C between now and 2020 (or, by 
2030, for a longer period: Henley and King, 2017), but this would not 
constitute temperatures ‘reaching 1.5°C’ on our working definition. 
Rogelj et al. (2017) show limiting the probability of annual GMST 
exceeding 1.5°C to less than one-year-in-20 would require limiting 
warming, on the definition used here, to 1.31°C or lower. 
1.2.2 Global versus Regional and Seasonal Warming
Warming is not observed or expected to be spatially or seasonally 
uniform (Collins et al., 2013). A 1.5°C increase in GMST will be 
associated with warming substantially greater than 1.5°C in many 
land regions, and less than 1.5°C in most ocean regions. This is 
illustrated by Figure 1.3, which shows an estimate of the observed 
change in annual and seasonal average temperatures between 
the 1850–1900 pre-industrial reference period and the decade 
2006–2015 in the Cowtan-Way dataset. These regional changes are 
associated with an observed GMST increase of 0.91°C in the dataset 
shown here, or 0.87°C in the four-dataset average (Table 1.1). This 
observed pattern reflects an on-going transient warming: features 
such as enhanced warming over land may be less pronounced, but still 
present, in equilibrium (Collins et al., 2013). This figure illustrates the 
magnitude of spatial and seasonal differences, with many locations, 
particularly in Northern Hemisphere mid-latitude winter (December–
February), already experiencing regional warming more than double 
the global average. Individual seasons may be substantially warmer, 
or cooler, than these expected changes in the long-term average.
1.2.3 Definition of 1.5°C Pathways: Probability, 
Transience, Stabilization and Overshoot
Pathways considered in this report, consistent with available literature 
on 1.5°C, primarily focus on the time scale up to 2100, recognising 
that the evolution of GMST after 2100 is also important. Two broad assessed by AR5 (±0.2°C/30 years), assuming these are uncorrelated, 
and using observed warming relative to 1850–1900 to provide the 
central estimate (no evidence of bias from short-term variability), 
gives an assessed warming to the decade 2006–2015 of 0.87°C with 
a ±0.12°C likely range. This estimate has the advantage of traceability 
to the AR5, but more formal methods of quantifying externally driven 
warming (e.g., Bindoff et al., 2013; Jones et al., 2016; Haustein et 
al., 2017; Ribes et al., 2017), which typically give smaller ranges of 
uncertainty, may be adopted in the future.
1.2.1.3 Total versus human-induced warming and  
warming rates 
Total warming refers to the actual temperature change, irrespective 
of cause, while human-induced warming refers to the component 
of that warming that is attributable to human activities. Mitigation 
studies focus on human-induced warming (that is not subject to 
internal climate variability), while studies of climate change impacts 
typically refer to total warming (often with the impact of internal 
variability minimised through the use of multi-decade averages). 
In the absence of strong natural forcing due to changes in solar or 
volcanic activity, the difference between total and human-induced 
warming is small: assessing empirical studies quantifying solar and 
volcanic contributions to GMST from 1890 to 2010, AR5 (Figure 10.6 
of Bindoff et al., 2013) found their net impact on warming over the 
full period to be less than plus or minus 0.1°C. Figure 1.2 shows that 
the level of human-induced warming has been indistinguishable from 
total observed warming since 2000, including over the decade 2006–
2015. Bindoff et al. (2013) assessed the magnitude of human-induced 
warming over the period 1951–2010 to be 0.7°C ( likely  between 
0.6°C and 0.8°C), which is slightly greater than the 0.65°C observed 
warming over this period (Figures 10.4 and 10.5) with a likely  range 
of ±14%. The key surface temperature attribution studies underlying 
this finding (Gillett et al., 2013; Jones et al., 2013; Ribes and Terray, 
2013) used temperatures since the 19th century to constrain human-
induced warming, and so their results are equally applicable to the 
attribution of causes of warming over longer periods. Jones et al. 
(2016) show (Figure 10) human-induced warming trends over the 
period 1905–2005 to be indistinguishable from the corresponding 
total observed warming trend accounting for natural variability using 
spatio-temporal detection patterns from 12 out of 15 CMIP5 models 
and from the multi-model average. Figures from Ribes and Terray 
(2013), show the anthropogenic contribution to the observed linear 
warming trend 1880–2012 in the HadCRUT4 dataset (0.83°C in Table 
1.1) to be 0.86°C using a multi-model average global diagnostic, with 
a 5–95% confidence interval of 0.72°C–1.00°C (see figure 1.SM.6). 
In all cases, since 2000 the estimated combined contribution of solar 
and volcanic activity to warming relative to 1850–1900 is found to be 
less than ±0.1°C (Gillett et al., 2013), while anthropogenic warming 
is indistinguishable from, and if anything slightly greater than, the 
total observed warming, with 5–95% confidence intervals typically 
around ±20%.
Haustein et al. (2017) give a 5–95% confidence interval for 
human-induced warming in 2017 of 0.87°C–1.22°C, with a best 
estimate of 1.02°C, based on the HadCRUT4 dataset accounting 

60
Chapter 1 Framing and Context
1categories of 1.5°C pathways can be used to characterise mitigation 
options and impacts: pathways in which warming (defined as 30-year 
averaged GMST relative to pre-industrial levels, see Section 1.2.1) 
remains below 1.5°C throughout the 21st century, and pathways 
in which warming temporarily exceeds (‘overshoots’) 1.5°C and 
returns to 1.5°C either before or soon after 2100. Pathways in which 
warming exceeds 1.5°C before 2100, but might return to that level in 
some future century, are not considered 1.5°C pathways.
Because of uncertainty in the climate response, a ‘prospective’ 
mitigation pathway (see Cross-Chapter Box 1 in this chapter), in which 
emissions are prescribed, can only provide a level of probability of 
warming remaining below a temperature threshold. This probability 
cannot be quantified precisely since estimates depend on the method 
used (Rogelj et al., 2016b; Millar et al., 2017b; Goodwin et al., 2018; 
Tokarska and Gillett, 2018). This report defines a ‘1.5°C pathway’ 
as a pathway of emissions and associated possible temperature 
responses in which the majority of approaches using presently 
available information assign a probability of approximately one-in-
two to two-in-three to warming remaining below 1.5°C or, in the case 
of an overshoot pathway, to warming returning to 1.5°C by around 
2100 or earlier. Recognizing the very different potential impacts and 
risks associated with high-overshoot pathways, this report singles 
Figure 1.3 |  Spatial and seasonal pattern of present-day warming: Regional warming for the 2006–2015 decade relative to 1850–1900 for the annual mean (top), 
the average of December, January, and February (bottom left) and for June, July, and August (bottom right). Warming is evaluated by regressing regional changes in the Cowtan 
and Way (2014) dataset onto the total (combined human and natural) externally forced warming (yellow line in Figure 1.2). See Supplementary Material 1.SM for further details 
and versions using alternative datasets. The definition of regions (green boxes and labels in top panel) is adopted from the AR5 (Christensen et al., 2013).
out 1.5°C pathways with no or limited (<0.1°C) overshoot in many 
instances and pursues efforts to ensure that when the term ‘1.5°C 
pathway’ is used, the associated overshoot is made explicit where 
relevant. In Chapter 2, the classification of pathways is based on one 
modelling approach to avoid ambiguity, but probabilities of exceeding 
1.5°C are checked against other approaches to verify that they lie 
within this approximate range. All these absolute probabilities are 
imprecise, depend on the information used to constrain them, and 
hence are expected to evolve in the future. Imprecise probabilities 
can nevertheless be useful for decision-making, provided the 
imprecision is acknowledged (Hall et al., 2007; Kriegler et al., 2009; 
Simpson et al., 2016). Relative and rank probabilities can be assessed 
much more consistently: approaches may differ on the absolute 
probability assigned to individual outcomes, but typically agree on 
which outcomes are more probable. 
Importantly, 1.5°C pathways allow a substantial (up to one-in-two) 
chance of warming still exceeding 1.5°C. An ‘adaptive’ mitigation 
pathway in which emissions are continuously adjusted to achieve 
a specific temperature outcome (e.g., Millar et al., 2017b) reduces 
uncertainty in the temperature outcome while increasing uncertainty 
in the emissions required to achieve it. It has been argued (Otto et 
al., 2015; Xu and Ramanathan, 2017) that achieving very ambitious 

61
1Framing and Context Chapter 1temperature goals will require such an adaptive approach to 
mitigation, but very few studies have been performed taking this 
approach (e.g., Jarvis et al., 2012).
Figure 1.4 illustrates categories of (a) 1.5°C pathways and associated 
(b) annual and (c) cumulative emissions of CO2. It also shows (d) 
an example of a ‘time-integrated impact’ that continues to increase 
even after GMST has stabilised, such as sea level rise. This schematic 
assumes for the purposes of illustration that the fractional contribution 
of non-CO2 climate forcers to total anthropogenic forcing (which is 
currently increasing, Myhre et al., 2017) is approximately constant 
from now on. Consequently, total human-induced warming is 
proportional to cumulative CO2 emissions (solid line in c), and GMST 
stabilises when emissions reach zero. This is only the case in the most 
ambitious scenarios for non-CO2 mitigation (Leach et al., 2018). A 
simple way of accounting for varying non-CO2 forcing in Figure 1.4 
would be to note that every 1 W m−2 increase in non-CO2 forcing 
between now and the decade or two immediately prior to the time 
of peak warming reduces cumulative CO2 emissions consistent with 
the same peak warming by approximately 1100 GtCO2, with a range 
of 900-1500 GtCO2 (using values from AR5: Myhre et al., 2013; Allen 
et al., 2018; Jenkins et al., 2018; Cross-Chapter Box 2 in this chapter).
1.2.3.1 Pathways remaining below 1.5°C
In this category of 1.5°C pathways, human-induced warming either 
rises monotonically to stabilise at 1.5°C (Figure 1.4, brown lines) 
or peaks at or below 1.5°C and then declines (yellow lines). Figure 
1.4b demonstrates that pathways remaining below 1.5°C require net 
annual CO2 emissions to peak and decline to near zero or below, 
depending on the long-term adjustment of the carbon cycle and 
non-CO2 emissions (Bowerman et al., 2013; Wigley, 2018). Reducing 
emissions to zero corresponds to stabilizing cumulative CO2 emissions 
(Figure 1.4c, solid lines) and falling concentrations of CO2 in the 
atmosphere (panel c dashed lines) (Matthews and Caldeira, 2008; 
Solomon et al., 2009), which is required to stabilize GMST if non-CO2 
climate forcings are constant and positive. Stabilizing atmospheric 
greenhouse gas concentrations would result in continued warming 
(see Section 1.2.4). 
If emission reductions do not begin until temperatures are close to 
the proposed limit, pathways remaining below 1.5°C necessarily 
involve much faster rates of net CO2 emission reductions (Figure 1.4, 
green lines), combined with rapid reductions in non-CO2 forcing and 
these pathways also reach 1.5°C earlier. Note that the emissions 
associated with these schematic temperature pathways may not 
correspond to feasible emission scenarios, but they do illustrate the 
fact that the timing of net zero emissions does not in itself determine 
peak warming: what matters is total cumulative emissions up to that time. Hence every year’s delay before initiating emission reductions 
decreases by approximately two years the remaining time available 
to reach zero emissions on a pathway still remaining below 1.5°C 
(Allen and Stocker, 2013; Leach et al., 2018). 
1.2.3.2 Pathways temporarily exceeding 1.5°C
With the pathways in this category, also referred to as overshoot 
pathways, GMST rises above 1.5°C relative to pre-industrial before 
peaking and returning to 1.5°C around or before 2100 (Figure 1.4, 
blue lines), subsequently either stabilising or continuing to fall. This 
allows initially slower or delayed emission reductions, but lowering 
GMST requires net negative global CO2 emissions (net anthropogenic 
removal of CO2; Figure 1.4b). Cooling, or reduced warming, through 
sustained reductions of net non-CO2 climate forcing (Cross-Chapter 
Box 2 in this chapter) is also required, but their role is limited because 
emissions of most non-CO2 forcers cannot be reduced to below zero. 
Hence the feasibility and availability of large-scale CO2 removal 
limits the possible rate and magnitude of temperature decline. In 
this report, overshoot pathways are referred to as 1.5°C pathways, 
but qualified by the amount of the temperature overshoot, which 
can have a substantial impact on irreversible climate change impacts 
(Mathesius et al., 2015; Tokarska and Zickfeld, 2015).
1.2.3.3 Impacts at 1.5°C warming associated with different 
pathways: transience versus stabilisation
Figure 1.4 also illustrates time scales associated with different 
impacts. While many impacts scale with the change in GMST itself, 
some (such as those associated with ocean acidification) scale with 
the change in atmospheric CO2 concentration, indicated by the 
fraction of cumulative CO2 emissions remaining in the atmosphere 
(dotted lines in Figure 1.4c). Others may depend on the rate of 
change of GMST, while ‘time-integrated impacts’, such as sea level 
rise, shown in Figure 1.4d continue to increase even after GMST has 
stabilised.
Hence impacts that occur when GMST reaches 1.5°C could be very 
different depending on the pathway to 1.5°C. CO2 concentrations will 
be higher as GMST rises past 1.5°C (transient warming) than when 
GMST has stabilized at 1.5°C, while sea level and, potentially, global 
mean precipitation (Pendergrass et al., 2015) would both be lower 
(see Figure 1.4). These differences could lead to very different impacts 
on agriculture, on some forms of extreme weather (e.g., Baker et al., 
2018), and on marine and terrestrial ecosystems (e.g., Mitchell et al., 
2017 and Boxes 3.1 and 3.2). Sea level would be higher still if GMST 
returns to 1.5°C after an overshoot (Figure 1.4 d), with potentially 
significantly different impacts in vulnerable regions. Temperature 
overshoot could also cause irreversible impacts (see Chapter 3). 

62
Chapter 1 Framing and Context
1Figure 1.4 |  Different 1.5°C pathways1: Schematic illustration of the relationship between (a) global mean surface temperature (GMST) change; (b) annual rates of CO2 
emissions, assuming constant fractional contribution of non-CO2 forcing to total human-induced warming; (c) total cumulative CO2 emissions (solid lines) and the fraction 
thereof remaining in the atmosphere (dashed lines; these also indicates changes in atmospheric CO2 concentrations); and (d) a time-integrated impact, such as sea level rise, 
that continues to increase even after GMST has stabilized. Colours indicate different 1.5°C pathways. Brown: GMST remaining below and stabilizing at 1.5°C in 2100; Green: a 
delayed start but faster emission reductions pathway with GMST remaining below and reaching 1.5°C earlier; Blue: a pathway temporarily exceeding 1.5°C, with temperatures 
reduced to 1.5°C by net negative CO2 emissions after temperatures peak; and Yellow: a pathway peaking at 1.5°C and subsequently declining. Temperatures are anchored 
to 1°C above pre-industrial in 2017; emissions–temperature relationships are computed using a simple climate model (Myhre et al., 2013; Millar et al., 2017a; Jenkins et al., 
2018) with a lower value of the Transient Climate Response (TCR) than used in the quantitative pathway assessments in Chapter 2 to illustrate qualitative differences between 
pathways: this figure is not intended to provide quantitative information. The time-integrated impact is illustrated by the semi-empirical sea level rise model of Kopp et al. (2016).
1 An animated version of Figure 1.4 will be embedded in the web-based version of this Special ReportCross-Chapter Box 1 |  Scenarios and Pathways
Contributing Authors:   
Mikiko Kainuma (Japan), Kristie L. Ebi (USA), Sabine Fuss (Germany), Elmar Kriegler (Germany), Keywan Riahi (Austria), Joeri Rogelj 
(Austria/Belgium), Petra Tschakert (Australia/Austria), Rachel Warren (UK)
Climate change scenarios have been used in IPCC assessments since the First Assessment Report (Leggett et al., 1992). The SRES 
scenarios  (named after the IPCC Special Report on Emissions Scenarios published in 2000; IPCC, 2000), consist of four scenarios that 
do not take into account any future measures to limit greenhouse gas (GHG) emissions. Subsequently, many policy scenarios have been 
developed based upon them (Morita et al., 2001). The SRES scenarios are superseded by a set of scenarios based on the Representative 
Concentration Pathways (RCPs) and Shared Socio-Economic Pathways (SSPs) (Riahi et al., 2017). The RCPs comprise a set of four GHG 
concentration trajectories that jointly span a large range of plausible human-caused climate forcing ranging from 2.6 W m−2 (RCP2.6) 
to 8.5 W m−2 (RCP8.5) by the end of the 21st century (van Vuuren et al., 2011). They were used to develop climate projections in the 
Coupled Model Intercomparison Project Phase 5 (CMIP5; Taylor et al., 2012) and were assessed in the IPCC Fifth Assessment Report 
(AR5). Based on the CMIP5 ensemble, RCP2.6, provides a better than two-in-three chance of staying below 2°C and a median warming 
of 1.6°C relative to 1850–1900 in 2100 (Collins et al., 2013). 
The SSPs were developed to complement the RCPs with varying socio-economic challenges to adaptation and mitigation. SSP-based 
scenarios were developed for a range of climate forcing levels, including the end-of-century forcing levels of the RCPs (Riahi et al., 2017) 
and a level below RCP2.6 to explore pathways limiting warming to 1.5°C above pre-industrial levels (Rogelj et al., 2018). The SSP-based 
1.5°C pathways are assessed in Chapter 2 of this report. These scenarios offer an integrated perspective on socio-economic, energy-
system (Bauer et al., 2017), land use (Popp et al., 2017), air pollution (Rao et al., 2017) and, GHG emissions developments (Riahi et al.,  

63
1Framing and Context Chapter 1 
2017). Because of their harmonised assumptions, scenarios developed with the SSPs facilitate the integrated analysis of future climate 
impacts, vulnerabilities, adaptation and mitigation.
Scenarios and Pathways in this Report
This report focuses on pathways that could limit the increase of global mean surface temperature (GMST) to 1.5°C above pre-industrial 
levels and pathways that align with the goals of sustainable development and poverty eradication. The pace and scale of mitigation 
and adaptation are assessed in the context of historical evidence to determine where unprecedented change is required (see Chapter 
4). Other scenarios are also assessed, primarily as benchmarks for comparison of mitigation, impacts, and/or adaptation requirements. 
These include baseline scenarios that assume no climate policy; scenarios that assume some kind of continuation of current climate 
policy trends and plans, many of which are used to assess the implications of the nationally determined contributions (NDCs); and 
scenarios holding warming below 2°C above pre-industrial levels. This report assesses the spectrum from global mitigation scenarios 
to local adaptation choices – complemented by a bottom-up assessment of individual mitigation and adaptation options, and their 
implementation (policies, finance, institutions, and governance, see Chapter 4). Regional, national, and local scenarios, as well as 
decision-making processes involving values and difficult trade-offs are important for understanding the challenges of limiting GMST 
increase to 1.5°C and are thus indispensable when assessing implementation.
Different climate policies result in different temperature pathways, which result in different levels of climate risks and actual climate 
impacts with associated long-term implications. Temperature pathways are classified into continued warming pathways (in the cases of 
baseline and reference scenarios), pathways that keep the temperature increase below a specific limit (like 1.5°C or 2°C), and pathways 
that temporarily exceed and later fall to a specific limit (overshoot pathways). In the case of a temperature overshoot, net negative CO2 
emissions are required to remove excess CO2 from the atmosphere (Section 1.2.3). 
In a ‘prospective’ mitigation pathway, emissions (or sometimes concentrations) are prescribed, giving a range of GMST outcomes 
because of uncertainty in the climate response. Prospective pathways are considered ‘1.5°C pathways’ in this report if, based on current 
knowledge, the majority of available approaches assign an approximate probability of one-in-two to two-in-three to temperatures 
either remaining below 1.5°C or returning to 1.5°C either before or around 2100. Most pathways assessed in Chapter 2 are prospective 
pathways, and therefore even ‘1.5°C pathways’ are also associated with risks of warming higher than 1.5°C, noting that many risks 
increase non-linearly with increasing GMST. In contrast, the ‘risks of warming of 1.5°C’ assessed in Chapter 3 refer to risks in a 
world in which GMST is either passing through (transient) or stabilized at 1.5°C, without considering probabilities of different GMST 
levels (unless otherwise qualified). To stay below any desired temperature limit, mitigation measures and strategies would need to 
be adjusted as knowledge of the climate response is updated (Millar et al., 2017b; Emori et al., 2018). Such pathways can be called 
‘adaptive’ mitigation pathways. Given there is always a possibility of a greater-than-expected climate response (Xu and Ramanathan, 
2017), adaptive mitigation pathways are important to minimise climate risks, but need also to consider the risks and feasibility (see 
Cross-Chapter Box 3 in this chapter) of faster-than-expected emission reductions. Chapter 5 includes assessments of two related topics: 
aligning mitigation and adaptation pathways with sustainable development pathways, and transformative visions for the future that 
would support avoiding negative impacts on the poorest and most disadvantaged populations and vulnerable sectors.
Definitions of Scenarios and Pathways
Climate scenarios and pathways are terms that are sometimes used interchangeably, with a wide range of overlapping definitions 
(Rosenbloom, 2017).
A ‘scenario ’ is an internally consistent, plausible, and integrated description of a possible future of the human–environment system, 
including a narrative with qualitative trends and quantitative projections (IPCC, 2000). Climate change scenarios provide a framework 
for developing and integrating projections of emissions, climate change, and climate impacts, including an assessment of their inherent 
uncertainties. The long-term and multi-faceted nature of climate change requires climate scenarios to describe how socio-economic 
trends in the 21st century could influence future energy and land use, resulting emissions and the evolution of human vulnerability and 
exposure. Such driving forces include population, GDP , technological innovation, governance and lifestyles. Climate change scenarios 
are used for analysing and contrasting climate policy choices.
The notion of a ‘ pathway ’ can have multiple meanings in the climate literature. It is often used to describe the temporal evolution 
of a set of scenario features, such as GHG emissions and socio-economic development. As such, it can describe individual scenario 
components or sometimes be used interchangeably with the word ‘scenario’. For example, the RCPs describe GHG concentration 
trajectories (van Vuuren et al., 2011) and the SSPs are a set of narratives of societal futures augmented by quantitative projections 
of socio-economic determinants such as population, GDP and urbanization (Kriegler et al., 2012; O’Neill et al., 2014). Socio-economic  Cross-Chapter Box 1 (continued)

64
Chapter 1 Framing and Context
1 
driving forces consistent with any of the SSPs can be combined with a set of climate policy assumptions (Kriegler et al., 2014) that 
together would lead to emissions and concentration outcomes consistent with the RCPs (Riahi et al., 2017). This is at the core of the 
scenario framework for climate change research that aims to facilitate creating scenarios integrating emissions and development 
pathways dimensions (Ebi et al., 2014; van Vuuren et al., 2014).
In other parts of the literature, ‘pathway’ implies a solution-oriented trajectory describing a pathway from today’s world to achieving a 
set of future goals. Sustainable Development Pathways  describe national and global pathways where climate policy becomes part of 
a larger sustainability transformation (Shukla and Chaturvedi, 2013; Fleurbaey et al., 2014; van Vuuren et al., 2015). The AR5 presented 
climate-resilient pathways  as sustainable development pathways that combine the goals of adaptation and mitigation (Denton et 
al., 2014), more broadly defined as iterative processes for managing change within complex systems in order to reduce disruptions 
and enhance opportunities associated with climate change (IPCC, 2014a). The AR5 also introduced the notion of climate-resilient 
development pathways , with a more explicit focus on dynamic livelihoods, multi-dimensional poverty, structural inequalities, and 
equity among poor and non-poor people (Olsson et al., 2014). Adaptation pathways  are understood as a series of adaptation choices 
involving trade-offs between short-term and long-term goals and values (Reisinger et al., 2014). They are decision-making processes 
sequenced over time with the purpose of deliberating and identifying socially salient solutions in specific places (Barnett et al., 2014; 
Wise et al., 2014; Fazey et al., 2016). There is a range of possible pathways for transformational change, often negotiated through 
iterative and inclusive processes (Harris et al., 2017; Fazey et al., 2018; Tàbara et al., 2018).Cross-Chapter Box 1 (continued)
1.2.4 Geophysical Warming Commitment
It is frequently asked whether limiting warming to 1.5°C is ‘feasible’ 
(Cross-Chapter Box 3 in this chapter). There are many dimensions to 
this question, including the warming ‘commitment’ from past emissions 
of greenhouse gases and aerosol precursors. Quantifying commitment 
from past emissions is complicated by the very different behaviour of 
different climate forcers affected by human activity: emissions of long-
lived greenhouse gases such as CO2 and nitrous oxide (N2O) have a 
very persistent impact on radiative forcing (Myhre et al., 2013), lasting 
from over a century (in the case of N2O) to hundreds of thousands 
of years (for CO2). The radiative forcing impact of short-lived climate 
forcers (SLCFs) such as methane (CH4) and aerosols, in contrast, 
persists for at most about a decade (in the case of methane) down to 
only a few days. These different behaviours must be taken into account 
in assessing the implications of any approach to calculating aggregate 
emissions (Cross-Chapter Box 2 in this chapter).
Geophysical warming commitment is defined as the unavoidable 
future warming resulting from physical Earth system inertia. Different 
variants are discussed in the literature, including (i) the ‘constant 
composition commitment’ (CCC), defined by Meehl et al. (2007) as 
the further warming that would result if atmospheric concentrations 
of GHGs and other climate forcers were stabilised at the current level; 
and (ii) and the ‘zero emissions commitment’ (ZEC), defined as the 
further warming that would still occur if all future anthropogenic 
emissions of greenhouse gases and aerosol precursors were 
eliminated instantaneously (Meehl et al., 2007; Collins et al., 2013). 
The CCC is primarily associated with thermal inertia of the ocean 
(Hansen et al., 2005), and has led to the misconception that 
substantial future warming is inevitable (Matthews and Solomon, 
2013). The CCC takes into account the warming from past emissions, 
but also includes warming from future emissions (declining but still 
non-zero) that are required to maintain a constant atmospheric composition. It is therefore not relevant to the warming commitment 
from past emissions alone.
The ZEC, although based on equally idealised assumptions, allows 
for a clear separation of the response to past emissions from the 
effects of future emissions. The magnitude and sign of the ZEC 
depend on the mix of GHGs and aerosols considered. For CO2, which 
takes hundreds of thousands of years to be fully removed from the 
atmosphere by natural processes following its emission (Eby et al., 
2009; Ciais et al., 2013), the multi-century warming commitment 
from emissions to date in addition to warming already observed 
is estimated to range from slightly negative (i.e., a slight cooling 
relative to present-day) to slightly positive (Matthews and Caldeira, 
2008; Lowe et al., 2009; Gillett et al., 2011; Collins et al., 2013). 
Some studies estimate a larger ZEC from CO2, but for cumulative 
emissions much higher than those up to present day (Frölicher et al., 
2014; Ehlert and Zickfeld, 2017). The ZEC from past CO2 emissions 
is small because the continued warming effect from ocean thermal 
inertia is approximately balanced by declining radiative forcing due 
to CO2 uptake by the ocean (Solomon et al., 2009; Goodwin et al., 
2015; Williams et al., 2017). Thus, although present-day CO2-induced 
warming is irreversible on millennial time scales (without human 
intervention such as active carbon dioxide removal or solar radiation 
modification; Section 1.4.1), past CO2 emissions do not commit to 
substantial further warming (Matthews and Solomon, 2013). 
Sustained net zero anthropogenic emissions of CO2 and declining net 
anthropogenic non-CO2 radiative forcing over a multi-decade period 
would halt anthropogenic global warming over that period, although 
it would not halt sea level rise or many other aspects of climate system 
adjustment. The rate of decline of non-CO2 radiative forcing must be 
sufficient to compensate for the ongoing adjustment of the climate 
system to this forcing (assuming it remains positive) due to ocean 
thermal inertia. It therefore depends on deep ocean response time 
scales, which are uncertain but of order centuries, corresponding to 

65
1Framing and Context Chapter 1decline rates of non-CO2 radiative forcing of less than 1% per year. In 
the longer term, Earth system feedbacks such as the release of carbon 
from melting permafrost may require net negative CO2 emissions to 
maintain stable temperatures (Lowe and Bernie, 2018).
For warming SLCFs, meaning those associated with positive radiative 
forcing such as methane, the ZEC is negative. Eliminating emissions 
of these substances results in an immediate cooling relative to the 
present (Figure 1.5, magenta lines) (Frölicher and Joos, 2010; Matthews 
and Zickfeld, 2012; Mauritsen and Pincus, 2017). Cooling SLCFs (those 
associated with negative radiative forcing) such as sulphate aerosols 
create a positive ZEC, as elimination of these forcers results in rapid 
increase in radiative forcing and warming (Figure 1.5, green lines) 
(Matthews and Zickfeld, 2012; Mauritsen and Pincus, 2017; Samset 
et al., 2018). Estimates of the warming commitment from eliminating 
aerosol emissions are affected by large uncertainties in net aerosol 
radiative forcing (Myhre et al., 2013, 2017) and the impact of other measures affecting aerosol loading (e.g., Fernández et al., 2017). 
If present-day emissions of all GHGs (short- and long-lived) and 
aerosols (including sulphate, nitrate and carbonaceous aerosols) are 
eliminated (Figure 1.5, yellow lines) GMST rises over the following 
decade, driven by the removal of negative aerosol radiative forcing. 
This initial warming is followed by a gradual cooling driven by the 
decline in radiative forcing of short-lived greenhouse gases (Matthews 
and Zickfeld, 2012; Collins et al., 2013). Peak warming following 
elimination of all emissions was assessed at a few tenths of a degree in 
AR5, and century-scale warming was assessed to change only slightly 
relative to the time emissions are reduced to zero (Collins et al., 2013). 
New evidence since AR5 suggests a larger methane forcing (Etminan 
et al., 2016) but no revision in the range of aerosol forcing (although 
this remains an active field of research, e.g., Myhre et al., 2017). This 
revised methane forcing estimate results in a smaller peak warming 
and a faster temperature decline than assessed in AR5 (Figure 1.5, 
yellow line).
Figure 1.5 |  Warming commitment from past emissions of greenhouse gases and aerosols:  Radiative forcing (top) and global mean surface temperature change 
(bottom) for scenarios with different combinations of greenhouse gas and aerosol precursor emissions reduced to zero in 2020. Variables were calculated using a simple 
climate–carbon cycle model (Millar et al., 2017a) with a simple representation of atmospheric chemistry (Smith et al., 2018). The bars on the right-hand side indicate the median 
warming in 2100 and 5–95% uncertainty ranges (also indicated by the plume around the yellow line) taking into account one estimate of uncertainty in climate response, 
effective radiative forcing and carbon cycle sensitivity, and constraining simple model parameters with response ranges from AR5 combined with historical climate observations 
(Smith et al., 2018). Temperatures continue to increase slightly after elimination of CO2 emissions (blue line) in response to constant non-CO2 forcing. The dashed blue line 
extrapolates one estimate of the current rate of warming, while dotted blue lines show a case where CO2 emissions are reduced linearly to zero assuming constant non-CO2 
forcing after 2020. Under these highly idealized assumptions, the time to stabilize temperatures at 1.5°C is approximately double the time remaining to reach 1.5°C at the 
current warming rate.


66
Chapter 1 Framing and Context
1Expert judgement based on the available evidence (including model 
simulations, radiative forcing and climate sensitivity) suggests that if 
all anthropogenic emissions were reduced to zero immediately, any 
further warming beyond the 1°C already experienced would likely  be 
less than 0.5°C over the next two to three decades, and also likely 
less than 0.5°C on a century time scale. 
Since most sources of emissions cannot, in reality, be brought to 
zero instantaneously due to techno-economic inertia, the current 
rate of emissions also constitutes a conditional commitment to 
future emissions and consequent warming depending on achievable 
rates of emission reductions. The current level and rate of human-
induced warming determines both the time left before a temperature 
threshold is exceeded if warming continues (dashed blue line 
in Figure 1.5) and the time over which the warming rate must be 
reduced to avoid exceeding that threshold (approximately indicated 
by the dotted blue line in Figure 1.5). Leach et al. (2018) use a central 
estimate of human-induced warming of 1.02°C in 2017, increasing 
at 0.215°C per decade (Haustein et al., 2017), to argue that it will 
take 13–32 years (one-standard-error range) to reach 1.5°C if the 
current warming rate continues, allowing 25–64 years to stabilise 
temperatures at 1.5°C if the warming rate is reduced at a constant rate of deceleration starting immediately.  Applying a similar approach 
to the multi-dataset average GMST used in this report gives an 
assessed likely range for the date at which warming reaches 1.5°C 
of 2030 to 2052. The lower bound on this range, 2030, is supported 
by multiple lines of evidence, including the AR5 assessment for the 
likely  range of warming (0.3°C–0.7°C) for the period 2016–2035 
relative to 1986–2005. The upper bound, 2052, is supported by fewer 
lines of evidence, so we have used the upper bound of the 5–95% 
confidence interval given by the Leach et al. (2018) method applied to 
the multi-dataset average GMST, expressed as the upper limit of the 
likely  range, to reflect the reliance on a single approach. Results are 
sensitive both to the confidence level chosen and the number of years 
used to estimate the current rate of anthropogenic warming (5 years 
used here, to capture the recent acceleration due to rising non-CO2 
forcing). Since the rate of human-induced warming is proportional 
to the rate of CO2 emissions (Matthews et al., 2009; Zickfeld et al., 
2009) plus a term approximately proportional to the rate of increase 
in non-CO2 radiative forcing (Gregory and Forster, 2008; Allen et al., 
2018; Cross-Chapter Box 2 in this chapter), these time scales also 
provide an indication of minimum emission reduction rates required 
if a warming greater than 1.5°C is to be avoided (see Figure 1.5, 
Supplementary Material 1.SM.6 and FAQ 1.2).
Cross-Chapter Box 2 | Measuring Progress to Net Zero Emissions Combining Long-Lived and Short-
Lived Climate Forcers 
Contributing Authors:   
Piers Forster (UK), Myles R. Allen (UK), Elmar Kriegler (Germany), Joeri Rogelj (Austria/Belgium), Seth Schultz (USA), Drew Shindell 
(USA), Kirsten Zickfeld (Canada/Germany)
Emissions of many different climate forcers will affect the rate and magnitude of climate change over the next few decades (Myhre et al., 
2013). Since these decades will determine when 1.5°C is reached or whether a warming greater than 1.5°C is avoided, understanding 
the aggregate impact of different forcing agents is particularly important in the context of 1.5°C pathways. Paragraph 17 of Decision 1 
of the 21st Conference of the Parties on the adoption of the Paris Agreement specifically states that this report is to identify aggregate 
greenhouse gas emission levels compatible with holding the increase in global average temperatures to 1.5°C above pre-industrial 
levels (see Chapter 2). This request highlights the need to consider the implications of different methods of aggregating emissions of 
different gases, both for future temperatures and for other aspects of the climate system (Levasseur et al., 2016; Ocko et al., 2017). 
To date, reporting of GHG emissions under the UNFCCC has used Global Warming Potentials (GWPs) evaluated over a 100-year time 
horizon (GWP100) to combine multiple climate forcers. IPCC Working Group 3 reports have also used GWP100 to represent multi-gas 
pathways (Clarke et al., 2014). For reasons of comparability and consistency with current practice, Chapter 2 in this Special Report 
continues to use this aggregation method. Numerous other methods of combining different climate forcers have been proposed, such 
as the Global Temperature-change Potential (GTP; Shine et al., 2005) and the Global Damage Potential (Tol et al., 2012; Deuber et al., 
2013).
Climate forcers fall into two broad categories in terms of their impact on global temperature (Smith et al., 2012): long-lived GHGs, such 
as CO2 and nitrous oxide (N2O), whose warming impact depends primarily on the total cumulative amount emitted over the past century 
or the entire industrial epoch; and short-lived climate forcers (SLCFs), such as methane and black carbon, whose warming impact 
depends primarily on current and recent annual emission rates (Reisinger et al., 2012; Myhre et al., 2013; Smith et al., 2013; Strefler et 
al., 2014). These different dependencies affect the emissions reductions required of individual forcers to limit warming to 1.5°C or any 
other level.
Natural processes that remove CO2 permanently from the climate system are so slow that reducing the rate of CO2-induced warming 
to zero requires net zero global anthropogenic CO2 emissions (Archer and Brovkin, 2008; Matthews and Caldeira, 2008; Solomon et al.,  

67
1Framing and Context Chapter 12009), meaning almost all remaining anthropogenic CO2 emissions must be compensated for by an equal rate of anthropogenic carbon 
dioxide removal (CDR). Cumulative CO2 emissions are therefore an accurate indicator of CO2-induced warming, except in periods of 
high negative CO2 emissions (Zickfeld et al., 2016), and potentially in century-long periods of near-stable temperatures (Bowerman et 
al., 2011; Wigley, 2018). In contrast, sustained constant emissions of a SLCF such as methane, would (after a few decades) be consistent 
with constant methane concentrations and hence very little additional methane-induced warming (Allen et al., 2018; Fuglestvedt et al., 
2018). Both GWP and GTP would equate sustained SLCF emissions with sustained constant CO2 emissions, which would continue to 
accumulate in the climate system, warming global temperatures indefinitely. Hence nominally ‘equivalent’ emissions of CO2 and SLCFs, 
if equated conventionally using GWP or GTP , have very different temperature impacts, and these differences are particularly evident 
under ambitious mitigation characterizing 1.5°C pathways.
Since the AR5, a revised usage of GWP has been proposed (Lauder et al., 2013; Allen et al., 2016), denoted GWP* (Allen et al., 
2018), that addresses this issue by equating a permanently sustained change in the emission rate of an SLCF or SLCF-precursor (in 
tonnes-per-year), or other non-CO2 forcing (in watts per square metre), with a one-off pulse  emission (in tonnes) of a fixed amount 
of CO2. Specifically, GWP* equates a 1 tonne-per-year increase in emission rate of an SLCF with a pulse emission of GWPH x H tonnes 
of CO2, where GWPH is the conventional GWP of that SLCF evaluated over time GWPH for SLCFs decreases with increasing time H, 
GWPH x H for SLCFs is less dependent on the choice of time horizon. Similarly, a permanent 1 W m−2 increase in radiative forcing has 
a similar temperature impact as the cumulative emission of H/AGWPH tonnes of CO2, where AGWPH is the Absolute Global Warming 
Potential of CO2 (Shine et al., 2005; Myhre et al., 2013; Allen et al., 2018). This indicates approximately how future changes in non-
CO2 radiative forcing affect cumulative CO2 emissions consistent with any given level of peak warming.
When combined using GWP*, cumulative aggregate GHG emissions are closely proportional to total GHG-induced warming, while 
the annual rate of GHG-induced warming is proportional to the annual rate of aggregate GHG emissions (see Cross-Chapter Box 2, 
Figure 1). This is not the case when emissions are aggregated using GWP or GTP , with discrepancies particularly pronounced when 
SLCF emissions are falling. Persistent net zero CO2-equivalent emissions containing a residual positive forcing contribution from 
SLCFs and aggregated using GWP100 or GTP would result in a steady decline of GMST. Net zero global emissions aggregated using 
GWP* (which corresponds to zero net emissions of CO2 and other long-lived GHGs like nitrous oxide, combined with near-constant 
SLCF forcing – see Figure 1.5) results in approximately stable GMST (Allen et al., 2018; Fuglestvedt et al., 2018 and Cross-Chapter 
Box 2, Figure 1, below).
Whatever method is used to relate emissions of different greenhouse gases, scenarios achieving stable GMST well below 2°C 
require both near-zero net emissions of long-lived greenhouse gases and deep reductions in warming SLCFs (Chapter 2), in part to 
compensate for the reductions in cooling SLCFs that are expected to accompany reductions in CO2 emissions (Rogelj et al., 2016b; 
Hienola et al., 2018). Understanding the implications of different methods of combining emissions of different climate forcers is, 
however, helpful in tracking progress towards temperature stabilisation and ‘balance between anthropogenic emissions by sources 
and removals by sinks of greenhouse gases’ as stated in Article 4 of the Paris Agreement. Fuglestvedt et al. (2018) and Tanaka and 
O’Neill (2018) show that when, and even whether, aggregate GHG emissions need to reach net zero before 2100 to limit warming 
to 1.5°C depends on the scenario, aggregation method and mix of long-lived and short-lived climate forcers.
The comparison of the impacts of different climate forcers can also consider more than their effects on GMST (Johansson, 2012; Tol 
et al., 2012; Deuber et al., 2013; Myhre et al., 2013; Cherubini and Tanaka, 2016). Climate impacts arise from both magnitude and 
rate of climate change, and from other variables such as precipitation (Shine et al., 2015). Even if GMST is stabilised, sea level rise 
and associated impacts will continue to increase (Sterner et al., 2014), while impacts that depend on CO2 concentrations such as 
ocean acidification may begin to reverse. From an economic perspective, comparison of different climate forcers ideally reflects the 
ratio of marginal economic damages if used to determine the exchange ratio of different GHGs under multi-gas regulation (Tol et 
al., 2012; Deuber et al., 2013; Kolstad et al., 2014). 
Emission reductions can interact with other dimensions of sustainable development (see Chapter 5). In particular, early action on 
some SLCFs (including actions that may warm the climate, such as reducing sulphur dioxide emissions) may have considerable 
societal co-benefits, such as reduced air pollution and improved public health with associated economic benefits (OECD, 2016; 
Shindell et al., 2016). Valuation of broadly defined social costs attempts to account for many of these additional non-climate factors 
along with climate-related impacts (Shindell, 2015; Sarofim et al., 2017; Shindell et al., 2017). See Chapter 4, Section 4.3.6, for a 
discussions of mitigation options, noting that mitigation priorities for different climate forcers depend on multiple economic and 
social criteria that vary between sectors, regions and countries.Cross-Chapter Box 2 (continued)

68
Chapter 1 Framing and Context
1Cross-Chapter Box 2, Figure 1 |  Implications of different approaches to calculating aggregate greenhouse gas emissions on a pathway to net 
zero. (a) Aggregate emissions of well-mixed greenhouse gases (WMGHGs) under the RCP2.6 mitigation scenario expressed as CO2-equivalent using GWP100 (blue); 
GTP100 (green) and GWP* (yellow). Aggregate WMGHG emissions appear to fall more rapidly if calculated using GWP* than using either GWP or GTP , primarily 
because GWP* equates a falling methane emission rate with negative CO2 emissions, as only active CO2 removal would have the same impact on radiative forcing 
and GMST as a reduction in methane emission rate. (b) Cumulative emissions of WMGHGs combined as in panel (a) (blue, green and yellow lines & left hand axis) 
and warming response to combined emissions (black dotted line and right hand axis, Millar et al. (2017a). The temperature response under ambitious mitigation is 
closely correlated with cumulative WMGHG emissions aggregated using GWP*, but with neither emission rate nor cumulative emissions if aggregated using GWP 
or GTP .
Cross-Chapter Box 2 (continued)
1.3 Impacts at 1.5°C and Beyond
1.3.1 Definitions
Consistent with the AR5 (IPCC, 2014a), ‘impact’ in this report refers 
to the effects of climate change on human and natural systems. 
Impacts may include the effects of changing hazards, such as the 
frequency and intensity of heat waves. ‘Risk’ refers to potential 
negative impacts of climate change where something of value is at 
stake, recognizing the diversity of values. Risks depend on hazards, 
exposure, vulnerability (including sensitivity and capacity to respond) 
and likelihood. Climate change risks can be managed through efforts 
to mitigate climate change forcers, adaptation of impacted systems, 
and remedial measures (Section 1.4.1).
In the context of this report, regional  impacts of global  warming at 
1.5°C and 2°C are assessed in Chapter 3. The ‘ warming experience at 
1.5°C ’ is that of regional climate change (temperature, rainfall, and 
other changes) at the time when global average temperatures, as 
defined in Section 1.2.1, reach 1.5°C above pre-industrial (the same 
principle applies to impacts at any other global mean temperature). 
Over the decade 2006–2015, many regions have experienced higher 
than average levels of warming and some are already now 1.5°C or 
more warmer with respect to the pre-industrial period (Figure 1.3). At a global warming of 1.5°C, some seasons will be substantially 
warmer than 1.5°C above pre-industrial (Seneviratne et al., 2016). 
Therefore, most regional impacts of a global mean warming of 1.5°C 
will be different from those of a regional warming by 1.5°C. 
The impacts of 1.5°C global warming will vary in both space and 
time (Ebi et al., 2016). For many regions, an increase in global 
mean temperature by 1.5°C or 2°C implies substantial increases 
in the occurrence and/or intensity of some extreme events (Fischer 
and Knutti, 2015; Karmalkar and Bradley, 2017; King et al., 2017; 
Chevuturi et al., 2018), resulting in different impacts (see Chapter 
3). By comparing impacts at 1.5°C versus those at 2°C, this report 
discusses the ‘avoided impacts’ by maintaining global temperature 
increase at or below 1.5°C as compared to 2°C, noting that these 
also depend on the pathway taken to 1.5°C (see Section 1.2.3 and 
Cross-Chapter Box 8 in Chapter 3 on 1.5°C warmer worlds). Many 
impacts take time to observe, and because of the warming trend, 
impacts over the past 20 years were associated with a level of human-
induced warming that was, on average, 0.1°C–0.23°C colder than 
its present level, based on the AR5 estimate of the warming trend 
over this period (Section 1.2.1 and Kirtman et al., 2013). Attribution 
studies (e.g., van Oldenborgh et al., 2017) can address this bias, but 
informal estimates of ‘recent impact experience’ in a rapidly warming 
world necessarily understate the temperature-related impacts of the 
current level of warming.

69
1Framing and Context Chapter 11.3.2 Drivers of Impacts
Impacts of climate change are due to multiple environmental drivers 
besides rising temperatures, such as rising atmospheric CO2, shifting 
rainfall patterns (Lee et al., 2018), rising sea levels, increasing ocean 
acidification, and extreme events, such as floods, droughts, and heat 
waves (IPCC, 2014a). Changes in rainfall affect the hydrological cycle 
and water availability (Schewe et al., 2014; Döll et al., 2018; Saeed 
et al., 2018). Several impacts depend on atmospheric composition,  
increasing atmospheric carbon dioxide levels leading to changes in 
plant productivity (Forkel et al., 2016), but also to ocean acidification 
(Hoegh-Guldberg et al., 2007). Other impacts are driven by changes 
in ocean heat content such as the destabilization of coastal ice sheets 
and sea level rise (Bindoff et al., 2007; Chen et al., 2017), whereas 
impacts due to heat waves depend directly on ambient air or ocean 
temperature (Matthews et al., 2017). Impacts can be direct, such as 
coral bleaching due to ocean warming, and indirect, such as reduced 
tourism due to coral bleaching. Indirect impacts can also arise from 
mitigation efforts such as changed agricultural management (Section 
3.6.2) or remedial measures such as solar radiation modification 
(Section 4.3.8, Cross-Chapter Box 10 in Chapter 4). 
Impacts may also be triggered by combinations of factors, including 
‘impact cascades’ (Cramer et al., 2014) through secondary 
consequences of changed systems. Changes in agricultural water 
availability caused by upstream changes in glacier volume are a 
typical example. Recent studies also identify compound events 
(e.g., droughts and heat waves), that is, when impacts are induced 
by the combination of several climate events (AghaKouchak et al., 
2014; Leonard et al., 2014; Martius et al., 2016; Zscheischler and 
Seneviratne, 2017).
There are now techniques to attribute impacts formally to 
anthropogenic global warming and associated rainfall changes 
(Rosenzweig et al., 2008; Cramer et al., 2014; Hansen et al., 2016), 
taking into account other drivers such as land-use change (Oliver and 
Morecroft, 2014) and pollution (e.g., tropospheric ozone; Sitch et al., 
2007). There are multiple lines of evidence that climate change has 
observable and often severely negative effects on people, especially 
where climate-sensitive biophysical conditions and socio-economic 
and political constraints on adaptive capacities combine to create 
high vulnerabilities (IPCC, 2012a, 2014a; World Bank, 2013). The 
character and severity of impacts depend not only on the hazards 
(e.g., changed climate averages and extremes) but also on the 
vulnerability (including sensitivities and adaptive capacities) of 
different communities and their exposure to climate threats. These 
impacts also affect a range of natural and human systems, such 
as terrestrial, coastal and marine ecosystems and their services; 
agricultural production; infrastructure; the built environment; human 
health; and other socio-economic systems (Rosenzweig et al., 2017).
Sensitivity to changing drivers varies markedly across systems 
and regions. Impacts of climate change on natural and managed 
ecosystems can imply loss or increase in growth, biomass or diversity 
at the level of species populations, interspecific relationships such as 
pollination, landscapes or entire biomes. Impacts occur in addition 
to the natural variation in growth, ecosystem dynamics, disturbance, succession and other processes, rendering attribution of impacts 
at lower levels of warming difficult in certain situations. The same 
magnitude of warming can be lethal during one phase of the life 
of an organism and irrelevant during another. Many ecosystems 
(notably forests, coral reefs and others) undergo long-term 
successional processes characterised by varying levels of resilience 
to environmental change over time. Organisms and ecosystems may 
adapt to environmental change to a certain degree, through changes 
in physiology, ecosystem structure, species composition or evolution. 
Large-scale shifts in ecosystems may cause important feedbacks, 
in terms of changing water and carbon fluxes through impacted 
ecosystems – these can amplify or dampen atmospheric change at 
regional to continental scale. Of particular concern is the response of 
most of the world’s forests and seagrass ecosystems, which play key 
roles as carbon sinks (Settele et al., 2014; Marbà et al., 2015).
Some ambitious efforts to constrain atmospheric greenhouse gas 
concentrations may themselves impact ecosystems. In particular, 
changes in land use, potentially required for massively enhanced 
production of biofuels (either as simple replacement of fossil fuels, or 
as part of bioenergy with carbon capture and storage, BECCS) impact 
all other land ecosystems through competition for land (e.g., Creutzig, 
2016) (see Cross-Chapter Box 7 in Chapter 3, Section 3.6.2.1).
Human adaptive capacity to a 1.5°C warmer world varies markedly 
for individual sectors and across sectors such as water supply, public 
health, infrastructure, ecosystems and food supply. For example, den -
sity and risk exposure, infrastructure vulnerability and resilience, gov -
ernance, and institutional capacity all drive different impacts across 
a range of human settlement types (Dasgupta et al., 2014; Revi et al., 
2014; Rosenzweig et al., 2018). Additionally, the adaptive capacity of 
communities and human settlements in both rural and urban areas, 
especially in highly populated regions, raises equity, social justice and 
sustainable development issues. Vulnerabilities due to gender, age, 
level of education and culture act as compounding factors (Arora-
Jonsson, 2011; Cardona et al., 2012; Resurrección, 2013; Olsson et 
al., 2014; Vincent et al., 2014).
1.3.3 Uncertainty and Non-Linearity of Impacts
Uncertainties in projections of future climate change and impacts 
come from a variety of different sources, including the assumptions 
made regarding future emission pathways (Moss et al., 2010), the 
inherent limitations and assumptions of the climate models used for 
the projections, including limitations in simulating regional climate 
variability (James et al., 2017), downscaling and bias-correction 
methods (Ekström et al., 2015), the assumption of a linear scaling 
of impacts with GMST used in many studies (Lewis et al., 2017; King 
et al., 2018b), and in impact models (e.g., Asseng et al., 2013). The 
evolution of climate change also affects uncertainty with respect 
to impacts. For example, the impacts of overshooting 1.5°C and 
stabilization at a later stage compared to stabilization at 1.5°C 
without overshoot may differ in magnitude (Schleussner et al., 2016). 
AR5 (IPCC, 2013b) and World Bank (2013) underscored the non-
linearity of risks and impacts as temperature rises from 2°C to 4°C of 
warming, particularly in relation to water availability, heat extremes, 

70
Chapter 1 Framing and Context
1bleaching of coral reefs, and more. Recent studies (Schleussner et al., 
2016; James et al., 2017; Barcikowska et al., 2018; King et al., 2018a) 
assess the impacts of 1.5°C versus 2°C warming, with the same 
message of non-linearity. The resilience of ecosystems, meaning 
their ability either to resist change or to recover after a disturbance, 
may change, and often decline, in a non-linear way. An example 
are reef ecosystems, with some studies suggesting that reefs will 
change, rather than disappear entirely, and with particular species 
showing greater tolerance to coral bleaching than others (Pörtner 
et al., 2014). A key issue is therefore whether ecosystems such as 
coral reefs survive an overshoot scenario, and to what extent they 
would be able to recover after stabilization at 1.5°C or higher levels 
of warming (see Box 3.4).
1.4 Strengthening the Global Response 
This section frames the implementation options, enabling conditions 
(discussed further in Cross-Chapter Box 3 on feasibility in this 
chapter), capacities and types of knowledge and their availability 
(Blicharska et al., 2017) that can allow institutions, communities 
and societies to respond to the 1.5°C challenge in the context of 
sustainable development and the Sustainable Development Goals 
(SDGs). It also addresses other relevant international agreements 
such as the Sendai Framework for Disaster Risk Reduction. Equity and 
ethics are recognised as issues of importance in reducing vulnerability 
and eradicating poverty. 
The connection between the enabling conditions for limiting global 
warming to 1.5°C and the ambitions of the SDGs are complex across 
scale and multi-faceted (Chapter 5). Climate mitigation–adaptation 
linkages, including synergies and trade-offs, are important when 
considering opportunities and threats for sustainable development. 
The IPCC AR5 acknowledged that ‘adaptation and mitigation 
have the potential to both contribute to and impede sustainable 
development, and sustainable development strategies and choices 
have the potential to both contribute to and impede climate change 
responses’ (Denton et al., 2014). Climate mitigation and adaptation 
measures and actions can reflect and enforce specific patterns 
of development and governance that differ amongst the world’s 
regions (Gouldson et al., 2015; Termeer et al., 2017). The role of 
limited adaptation and mitigation capacity, limits to adaptation and 
mitigation, and conditions of mal-adaptation and mal-mitigation are 
assessed in this report (Chapters 4 and 5).
1.4.1 Classifying Response Options
Key broad categories of responses to the climate change problem are 
framed here. Mitigation  refers to efforts to reduce or prevent the 
emission of greenhouse gases, or to enhance the absorption of gases 
already emitted, thus limiting the magnitude of future warming 
(IPCC, 2014b). Mitigation requires the use of new technologies, 
clean energy sources, reduced deforestation, improved sustainable 
agricultural methods, and changes in individual and collective 
behaviour. Many of these may provide substantial co-benefits for air 
quality, biodiversity and sustainable development. Mal-mitigation includes changes that could reduce emissions in the short-term but 
could lock in technology choices or practices that include significant 
trade-offs for effectiveness of future adaptation and other forms of 
mitigation (Chapters 2 and 4).
Carbon dioxide removal  (CDR) or ‘negative emissions’ activities 
are considered in this report as distinct from the above mitigation 
activities. While most mitigation activities focus on reducing the 
amount of carbon dioxide or other greenhouse gases emitted, 
CDR aims to reduce concentrations already in the atmosphere. 
Technologies for CDR are mostly in their infancy despite their 
importance to ambitious climate change mitigation pathways (Minx 
et al., 2017). Although some CDR activities such as reforestation 
and ecosystem restoration are well understood, the feasibility of 
massive-scale deployment of many CDR technologies remains an 
open question (IPCC, 2014b; Leung et al., 2014) (Chapters 2 and 4). 
Technologies for the active removal of other greenhouse gases, such 
as methane, are even less developed, and are briefly discussed in 
Chapter 4.
Climate change adaptation  refers to the actions taken to manage 
the impacts of climate change (IPCC, 2014a). The aim is to reduce 
vulnerability and exposure to the harmful effects of climate change 
(e.g., sea level rise, more intense extreme weather events or food 
insecurity). It also includes exploring the potential beneficial 
opportunities associated with climate change (for example, longer 
growing seasons or increased yields in some regions). Different 
adaptation pathways can be undertaken. Adaptation can be 
incremental, or transformational, meaning fundamental attributes 
of the system are changed (Chapter 3 and 4). There can be limits 
to ecosystem-based adaptation or the ability of humans to adapt 
(Chapter 4). If there is no possibility for adaptive actions that can 
be applied to avoid an intolerable risk, these are referred to as 
hard adaptation limits, while soft adaptation limits are identified 
when there are currently no options to avoid intolerable risks, but 
they are theoretically possible (Chapter 3 and 4). While climate 
change is a global issue, impacts are experienced locally. Cities and 
municipalities are at the frontline of adaptation (Rosenzweig et al., 
2018), focusing on reducing and managing disaster risks due to 
extreme and slow-onset weather and climate events, installing flood 
and drought early warning systems, and improving water storage 
and use (Chapters 3 and 4 and Cross-Chapter Box 12 in Chapter 5). 
Agricultural and rural areas, including often highly vulnerable remote 
and indigenous communities, also need to address climate-related 
risks by strengthening and making more resilient agricultural and 
other natural resource extraction systems. 
Remedial measures  are distinct from mitigation or adaptation, as 
the aim is to temporarily reduce or offset warming (IPCC, 2012b). 
One such measure is solar radiation modification (SRM), also referred 
to as solar radiation management in the literature, which involves 
deliberate changes to the albedo of the Earth system, with the net 
effect of increasing the amount of solar radiation reflected from the 
Earth to reduce the peak temperature from climate change (The Royal 
Society, 2009; Smith and Rasch, 2013; Schäfer et al., 2015). It should 
be noted that while some radiation modification measures, such as 
cirrus cloud thinning (Kristjánsson et al., 2016), aim at enhancing 

71
1Framing and Context Chapter 1outgoing long-wave radiation, SRM is used in this report to refer to 
all direct interventions on the planetary radiation budget. This report 
does not use the term ‘geo-engineering’ because of inconsistencies 
in the literature, which uses this term to cover SRM, CDR or both, 
whereas this report explicitly differentiates between CDR and SRM. 
Large-scale SRM could potentially be used to supplement mitigation 
in overshoot scenarios to keep the global mean temperature below 
1.5°C and temporarily reduce the severity of near-term impacts (e.g., 
MacMartin et al., 2018). The impacts of SRM (both biophysical and 
societal), costs, technical feasibility, governance and ethical issues 
associated need to be carefully considered (Schäfer et al., 2015; 
Section 4.3.8 and Cross-Chapter Box 10 in Chapter 4). 
1.4.2 Governance, Implementation and Policies
A challenge in creating the enabling conditions of a 1.5°C warmer 
world is the governance capacity of institutions to develop, implement 
and evaluate the changes needed within diverse and highly 
interlinked global social-ecological systems (Busby, 2016) (Chapter 
4). Policy arenas, governance structures and robust institutions are 
key enabling conditions for transformative climate action (Chapter 
4). It is through governance that justice, ethics and equity within 
the adaptation–mitigation–sustainable development nexus can be 
addressed (von Stechow et al., 2016) (Chapter 5).
Governance capacity includes a wide range of activities and efforts 
needed by different actors to develop coordinated climate mitigation 
and adaptation strategies in the context of sustainable development, 
taking into account equity, justice and poverty eradication. Significant 
governance challenges include the ability to incorporate multiple 
stakeholder perspectives in the decision-making process to reach 
meaningful and equitable decisions, interactions and coordination between different levels of government, and the capacity to raise 
financing and support for both technological and human resource 
development. For example, Lövbrand et al. (2017), argue that the 
voluntary pledges submitted by states and non-state actors to meet 
the conditions of the Paris Agreement will need to be more firmly 
coordinated, evaluated and upscaled.
Barriers for transitioning from climate change mitigation and 
adaptation planning to practical policy implementation include 
finance, information, technology, public attitudes, social values 
and practices (Whitmarsh et al., 2011; Corner and Clarke, 2017), 
and human resource constraints. Institutional capacity to deploy 
available knowledge and resources is also needed (Mimura et al., 
2014). Incorporating strong linkages across sectors, devolution of 
power and resources to sub-national and local governments with 
the support of national government, and facilitating partnerships 
among public, civic, private sectors and higher education institutions 
(Leal Filho et al., 2018) can help in the implementation of identified 
response options (Chapter 4). Implementation challenges of 1.5°C 
pathways are larger than for those that are consistent with limiting 
warming to well below 2°C, particularly concerning scale and speed 
of the transition and the distributional impacts on ecosystems and 
socio-economic actors. Uncertainties in climate change at different 
scales and capacities to respond combined with the complexities of 
coupled social and ecological systems point to a need for diverse and 
adaptive implementation options within and among different regions 
involving different actors. The large regional diversity between highly 
carbon-invested economies and emerging economies are important 
considerations for sustainable development and equity in pursuing 
efforts to limit warming to 1.5°C. Key sectors, including energy, food 
systems, health, and water supply, also are critical to understanding 
these connections. 
Cross-Chapter Box 3 |  Framing Feasibility: Key Concepts and Conditions for Limiting Global Temperature 
Increases to 1.5°C
Contributing Authors:   
William Solecki (USA), Anton Cartwright (South Africa), Wolfgang Cramer (France/Germany), James Ford (UK/Canada), Kejun Jiang 
(China), Joana Portugal Pereira (UK/Portugal), Joeri Rogelj (Austria/Belgium), Linda Steg (Netherlands), Henri Waisman (France)
This Cross-Chapter Box describes the concept of feasibility in relation to efforts to limit global warming to 1.5°C in the context of 
sustainable development and efforts to eradicate poverty and draws from the understanding of feasibility emerging within the IPCC 
(IPCC, 2017). Feasibility can be assessed in different ways, and no single answer exists as to the question of whether it is feasible to limit 
warming to 1.5°C. This implies that an assessment of feasibility would go beyond a ‘yes’ or a ‘no’. Rather, feasibility provides a frame 
to understand the different conditions and potential responses for implementing adaptation and mitigation pathways, and options 
compatible with a 1.5°C warmer world. This report assesses the overall feasibility of limiting warming to 1.5°C, and the feasibility of 
adaptation and mitigation options compatible with a 1.5°C warmer world, in six dimensions: 
Geophysical:  What global emission pathways could be consistent with conditions of a 1.5°C warmer world? What are the physical 
potentials for adaptation?
Environmental-ecological:  What are the ecosystem services and resources, including geological storage capacity and related rate 
of needed land-use change, available to promote transformations, and to what extent are they compatible with enhanced resilience?
Technological:  What technologies are available to support transformation? 
Economic: What economic conditions could support transformation? 

72
Chapter 1 Framing and Context
1Socio-cultural:  What conditions could support transformations in behaviour and lifestyles? To what extent are the transformations 
socially acceptable and consistent with equity?
Institutional: What institutional conditions are in place to support transformations, including multi-level governance, institutional 
capacity, and political support?
Assessment of feasibility in this report starts by evaluating the unavoidable warming from past emissions (Section 1.2.4) and identifying 
mitigation pathways that would lead to a 1.5°C world, which indicates that rapid and deep deviations from current emission pathways 
are necessary (Chapter 2). In the case of adaptation, an assessment of feasibility starts from an evaluation of the risks and impacts of 
climate change (Chapter 3). To mitigate and adapt to climate risks, system-wide technical, institutional and socio-economic transitions 
would be required, as well as the implementation of a range of specific mitigation and adaptation options. Chapter 4 applies various 
indicators categorised in these six dimensions to assess the feasibility of illustrative examples of relevant mitigation and adaptation 
options (Section 4.5.1). Such options and pathways have different effects on sustainable development, poverty eradication and 
adaptation capacity (Chapter 5). 
The six feasibility dimensions interact in complex and place-specific ways. Synergies and trade-offs may occur between the feasibility 
dimensions, and between specific mitigation and adaptation options (Section 4.5.4). The presence or absence of enabling conditions 
would affect the options that comprise feasibility pathways (Section 4.4), and can reduce trade-offs and amplify synergies between 
options. 
Sustainable development, eradicating poverty and reducing inequalities are not only preconditions for feasible transformations, but 
the interplay between climate action (both mitigation and adaptation options) and the development patterns to which they apply may 
actually enhance the feasibility of particular options (see Chapter 5).
The connections between the feasibility dimensions can be specified across three types of effects (discussed below). Each of these 
dimensions presents challenges and opportunities in realizing conditions consistent with a 1.5°C warmer world. 
Systemic effects:  Conditions that have embedded within them system-level functions that could include linear and non-linear 
connections and feedbacks. For example, the deployment of technology and large installations (e.g., renewable or low carbon energy 
mega-projects) depends upon economic conditions (costs, capacity to mobilize investments for R&D), social or cultural conditions 
(acceptability), and institutional conditions (political support; e.g., Sovacool et al., 2015). Case studies can demonstrate system-level 
interactions and positive or negative feedback effects between the different conditions (Jacobson et al., 2015; Loftus et al., 2015). This 
suggests that each set of conditions and their interactions need to be considered to understand synergies, inequities and unintended 
consequences.
Dynamic effects:  Conditions that are highly dynamic and vary over time, especially under potential conditions of overshoot or no 
overshoot. Some dimensions might be more time sensitive or sequential than others (i.e., if conditions are such that it is no longer 
geophysically feasible to avoid overshooting 1.5°C, the social and institutional feasibility of avoiding overshoot will be no longer 
relevant). Path dependencies, risks of legacy lock-ins related to existing infrastructures, and possibilities of acceleration permitted by 
cumulative effects (e.g., dramatic cost decreases driven by learning-by-doing) are all key features to be captured. The effects can play 
out over various time scales and thus require understanding the connections between near-term (meaning within the next several years 
to two decades) and long-term implications (meaning over the next several decades) when assessing feasibility conditions.
Spatial effects: Conditions that are spatially variable and scale dependent, according to context-specific factors such as regional-
scale environmental resource limits and endowment; economic wealth of local populations; social organisation, cultural beliefs, values 
and worldviews; spatial organisation, including conditions of urbanisation; and financial and institutional and governance capacity. 
This means that the conditions for achieving the global transformation required for a 1.5°C world will be heterogeneous and vary 
according to the specific context. On the other hand, the satisfaction of these conditions may depend upon global-scale drivers, such as 
international flows of finance, technologies or capacities. This points to the need for understanding feasibility to capture the interplay 
between the conditions at different scales.
With each effect, the interplay between different conditions influences the feasibility of both pathways (Chapter 2) and options (Chapter 
4), which in turn affect the likelihood of limiting warming to 1.5°C. The complexity of these interplays triggers unavoidable uncertainties, 
requiring transformations that remain robust under a range of possible futures that limit warming to 1.5°C. Cross-Chapter Box 3 (continued)

73
1Framing and Context Chapter 11.4.3 Transformation, Transformation Pathways, 
and Transition: Evaluating Trade-Offs and 
Synergies Between Mitigation, Adaptation 
and Sustainable Development Goals
Embedded in the goal of limiting warming to 1.5°C is the 
opportunity for intentional societal transformation (see Box 1.1 
on the Anthropocene). The form and process of transformation are 
varied and multifaceted (Pelling, 2011; O’Brien et al., 2012; O’Brien 
and Selboe, 2015; Pelling et al., 2015). Fundamental elements of 
1.5°C-related transformation include a decoupling of economic 
growth from energy demand and CO2 emissions; leap-frogging 
development to new and emerging low-carbon, zero-carbon and 
carbon-negative technologies; and synergistically linking climate 
mitigation and adaptation to global scale trends (e.g., global trade 
and urbanization) that will enhance the prospects for effective 
climate action, as well as enhanced poverty reduction and greater 
equity (Tschakert et al., 2013; Rogelj et al., 2015; Patterson et al., 
2017) (Chapters 4 and 5). The connection between transformative 
climate action and sustainable development illustrates a complex 
coupling of systems that have important spatial and time scale lag 
effects and implications for process and procedural equity, including 
intergenerational equity and for non-human species (Cross-Chapter 
Box 4 in this chapter, Chapter 5). Adaptation and mitigation transition 
pathways highlight the importance of cultural norms and values, 
sector-specific context, and proximate (i.e., occurrence of an extreme 
event) drivers that when acting together enhance the conditions for 
societal transformation (Solecki et al., 2017; Rosenzweig et al., 2018) 
(Chapters 4 and 5). 
Diversity and flexibility in implementation choices exist for adaptation, 
mitigation (including carbon dioxide removal, CDR) and remedial 
measures (such as solar radiation modification, SRM), and a potential 
for trade-offs and synergies between these choices and sustainable 
development (IPCC, 2014d; Olsson et al., 2014). The responses chosen could act to synergistically enhance mitigation, adaptation 
and sustainable development, or they may result in trade-offs 
which positively impact some aspects and negatively impact others. 
Climate change is expected to decrease the likelihood of achieving 
the Sustainable Development Goals (SDGs). While some strategies 
limiting warming towards 1.5°C are expected to significantly increase 
the likelihood of meeting those goals while also providing synergies 
for climate adaptation and mitigation (Chapter 5).
Dramatic transformations required to achieve the enabling conditions 
for a 1.5°C warmer world could impose trade-offs on dimensions 
of development (IPCC, 2014d; Olsson et al., 2014). Some choices 
of adaptation methods also could adversely impact development 
(Olsson et al., 2014). This report recognizes the potential for adverse 
impacts and focuses on finding the synergies between limiting 
warming, sustainable development, and eradicating poverty, thus 
highlighting pathways that do not constrain other goals, such as 
sustainable development and eradicating poverty.
The report is framed to address these multiple goals simultaneously 
and assesses the conditions to achieve a cost-effective and socially 
acceptable solution, rather than addressing these goals piecemeal 
(von Stechow et al., 2016) (Section 4.5.4 and Chapter 5), although 
there may be different synergies and trade-offs between a 2°C (von 
Stechow et al., 2016) and 1.5°C warmer world (Kainuma et al., 
2017). Climate-resilient development pathways (see Cross-Chapter 
Box 12 in Chapter 5 and Glossary) are trajectories that strengthen 
sustainable development, including mitigating and adapting to 
climate change and efforts to eradicate poverty while promoting 
fair and cross-scalar resilience in a changing climate. They take into 
account dynamic livelihoods; the multiple dimensions of poverty, 
structural inequalities; and equity between and among poor and 
non-poor people (Olsson et al., 2014). Climate-resilient development 
pathways can be considered at different scales, including cities, rural 
areas, regions or at global level (Denton et al., 2014; Chapter 5).
Cross-Chapter Box 4 | Sustainable Development and the Sustainable Development Goals
Contributing Authors:   
Diana Liverman (USA), Mustafa Babiker (Sudan), Purnamita Dasgupta (India), Riyanti Djanlante (Japan/Indonesia), Stephen Humphreys 
(UK/Ireland), Natalie Mahowald (USA), Yacob Mulugetta (UK/Ethiopia), Virginia Villariño (Argentina), Henri Waisman (France)
Sustainable development is most often defined as ‘development that meets the needs of the present without compromising the ability 
of future generations to meet their own needs’ (WCED, 1987) and includes balancing social well-being, economic prosperity and 
environmental protection. The AR5 used this definition and linked it to climate change (Denton et al., 2014). The most significant step 
since AR5 is the adoption of the UN Sustainable Development Goals, and the emergence of literature that links them to climate (von 
Stechow et al., 2015; Wright et al., 2015; Epstein and Theuer, 2017; Hammill and Price-Kelly, 2017; Kelman, 2017; Lofts et al., 2017; 
Maupin, 2017; Gomez-Echeverri, 2018).
In September 2015, the UN endorsed a universal agenda – ‘Transforming our World: the 2030 Agenda for Sustainable Development’ 
– which aims ‘to take the bold and transformative steps which are urgently needed to shift the world onto a sustainable and resilient 
path’. Based on a participatory process, the resolution in support of the 2030 agenda adopted 17 non-legally-binding Sustainable 
Development Goals (SDGs) and 169 targets to support people, prosperity, peace, partnerships and the planet (Kanie and Biermann, 
2017). 

74
Chapter 1 Framing and Context
1The SDGs expanded efforts to reduce poverty and other deprivations under the UN Millennium Development Goals (MDGs). There were 
improvements under the MDGs between 1990 and 2015, including reducing overall poverty and hunger, reducing infant mortality, and 
improving access to drinking water (UN, 2015a). However, greenhouse gas emissions increased by more than 50% from 1990 to 2015, 
and 1.6 billion people were still living in multidimensional poverty with persistent inequalities in 2015 (Alkire et al., 2015).
The SDGs raise the ambition for eliminating poverty, hunger, inequality and other societal problems while protecting the environment. 
They have been criticised: as too many and too complex, needing more realistic targets, overly focused on 2030 at the expense of 
longer-term objectives, not embracing all aspects of sustainable development, and even contradicting each other (Horton, 2014; Death 
and Gabay, 2015; Biermann et al., 2017; Weber, 2017; Winkler and Satterthwaite, 2017). 
Climate change is an integral influence on sustainable development, closely related to the economic, social and environmental 
dimensions of the SDGs. The IPCC has woven the concept of sustainable development into recent assessments, showing how climate 
change might undermine sustainable development, and the synergies between sustainable development and responses to climate 
change (Denton et al., 2014). Climate change is also explicit in the SDGs. SDG13 specifically requires ‘urgent action to address climate 
change and its impacts’. The targets include strengthening resilience and adaptive capacity to climate-related hazards and natural 
disasters; integrating climate change measures into national policies, strategies and planning; and improving education, awareness-
raising and human and institutional capacity. 
Targets also include implementing the commitment undertaken by developed-country parties to the UNFCCC to the goal of mobilizing 
jointly 100 billion USD annually by 2020 and operationalizing the Green Climate Fund, as well as promoting mechanisms for raising 
capacity for effective climate change-related planning and management in least developed countries and Small Island Developing 
States, including focusing on women, youth and local and marginalised communities. SDG13 also acknowledges that the UNFCCC is 
the primary international, intergovernmental forum for negotiating the global response to climate change.
Climate change is also mentioned in SDGs beyond SDG13, for example in goal targets 1.5, 2.4, 11.B, 12.8.1 related to poverty, hunger, 
cities and education respectively. The UNFCCC addresses other SDGs in commitments to ‘control, reduce or prevent anthropogenic 
emissions of greenhouse gases […] in all relevant sectors, including the energy, transport, industry, agriculture, forestry and waste 
management sectors’ (Art4, 1(c)) and to work towards ‘the conservation and enhancement, as appropriate, of […] biomass, forests and 
oceans as well as other terrestrial, coastal and marine ecosystems’ (Art4, 1(d)). This corresponds to SDGs that seek clean energy for all 
(Goal 7), sustainable industry (Goal 9) and cities (Goal 11) and the protection of life on land and below water (14 and 15). 
The SDGs and UNFCCC also differ in their time horizons. The SDGs focus primarily on 2030 whereas the Paris Agreement sets out that 
‘Parties aim […] to achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the 
second half of this century’. 
The IPCC decision to prepare this report on the impacts of 1.5°C and associated emission pathways explicitly asked for the assessment 
to be in the context of sustainable development and efforts to eradicate poverty. Chapter 1 frames the interaction between sustainable 
development, poverty eradication and ethics and equity. Chapter 2 assesses how risks and synergies of individual mitigation measures 
interact with 1.5°C pathways within the context of the SDGs and how these vary according to the mix of measures in alternative 
mitigation portfolios (Section 2.5). Chapter 3 examines the impacts of 1.5°C global warming on natural and human systems with 
comparison to 2°C and provides the basis for considering the interactions of climate change with sustainable development in Chapter 5. 
Chapter 4 analyses strategies for strengthening the response to climate change, many of which interact with sustainable development. 
Chapter 5 takes sustainable development, eradicating poverty and reducing inequalities as its focal point for the analysis of pathways 
to 1.5°C and discusses explicitly the linkages between achieving SDGs while eradicating poverty and reducing inequality. Cross-Chapter Box 4 (continued)

75
1Framing and Context Chapter 1Cross-Chapter Box 4, Figure 1 |  Climate action is number 13 of the UN Sustainable Development Goals.
Cross-Chapter Box 4 (continued)
1.5 Assessment Frameworks and Emerging 
Methodologies that Integrate Climate 
Change Mitigation and Adaptation 
with Sustainable Development 
This report employs information and data that are global in scope 
and include region-scale analysis. It also includes syntheses of 
municipal, sub-national, and national case studies. Global level 
statistics including physical and social science data are used, as 
well as detailed and illustrative case study material of particular 
conditions and contexts. The assessment provides the state of 
knowledge, including an assessment of confidence and uncertainty. 
The main time scale of the assessment is the 21st century and the 
time is separated into the near-, medium-, and long-term. Near-term 
refers to the coming decade, medium-term to the period 2030–2050, 
while long-term refers to 2050–2100. Spatial and temporal contexts 
are illustrated throughout, including: assessment tools that include 
dynamic projections of emission trajectories and the underlying 
energy and land transformation (Chapter 2); methods for assessing 
observed impacts and projected risks in natural and managed 
ecosystems and at 1.5°C and higher levels of warming in natural and 
managed ecosystems and human systems (Chapter 3); assessments 
of the feasibility of mitigation and adaptation options (Chapter 4); 
and linkages of the Shared Socioeconomic Pathways (SSPs) and 
Sustainable Development Goals (SDGs) (Cross-Chapter Boxes 1 and 
4 in this chapter, Chapter 2 and Chapter 5). 1.5.1 Knowledge Sources and Evidence 
Used in the Report
This report is based on a comprehensive assessment of documented 
evidence of the enabling conditions to pursuing efforts to limit the 
global average temperature rise to 1.5°C and adapting to this level 
of warming in the overarching context of the Anthropocene (Delanty 
and Mota, 2017). Two sources of evidence are used: peer-reviewed 
scientific literature and ‘grey’ literature in accordance with procedure 
on the use of literature in IPCC reports (IPCC, 2013a, Annex 2 to 
Appendix A), with the former being the dominant source. Grey 
literature is largely used on key issues not covered in peer-reviewed 
literature. 
The peer-reviewed literature includes the following sources: 1) 
knowledge regarding the physical climate system and human-induced 
changes, associated impacts, vulnerabilities, and adaptation options, 
established from work based on empirical evidence, simulations, 
modelling, and scenarios, with emphasis on new information since 
the publication of the IPCC AR5 to the cut-off date for this report 
(15th of May 2018); 2) humanities and social science theory and 
knowledge from actual human experiences of climate change 
risks and vulnerability in the context of social-ecological systems, 
development, equity, justice, and governance, and from indigenous 
knowledge systems; and 3) mitigation pathways based on climate 
projections into the future. 

76
Chapter 1 Framing and Context
1The grey literature category extends to empirical observations, 
interviews, and reports from government, industry, research institutes, 
conference proceedings and international or other organisations. 
Incorporating knowledge from different sources, settings and 
information channels while building awareness at various levels will 
advance decision-making and motivate implementation of context-
specific responses to 1.5°C warming (Somanathan et al., 2014). 
The assessment does not assess non-written evidence and does not 
use oral evidence, media reports or newspaper publications. With 
important exceptions, such as China, published knowledge from 
the most vulnerable parts of the world to climate change is limited 
(Czerniewicz et al., 2017).
1.5.2 Assessment Frameworks and Methodologies 
Climate models and associated simulations 
The multiple sources of climate model information used in this 
assessment are provided in Chapter 2 (Section 2.2) and Chapter 
3 (Section 3.2). Results from global simulations, which have also 
been assessed in previous IPCC reports and that are conducted as 
part of the World Climate Research Programme (WCRP) Coupled 
Models Intercomparison Project (CMIP) are used. The IPCC AR4 and 
Managing the Risks of Extreme Events and Disasters to Advance 
Climate Change Adaptation (SREX) reports were mostly based on 
simulations from the CMIP3 experiment, while the AR5 was mostly 
based on simulations from the CMIP5 experiment. The simulations 
of the CMIP3 and CMIP5 experiments were found to be very 
similar (e.g., Knutti and Sedláček, 2012; Mueller and Seneviratne, 
2014). In addition to the CMIP3 and CMIP5 experiments, results 
from coordinated regional climate model experiments (e.g., the 
Coordinated Regional Climate Downscaling Experiment, CORDEX) 
have been assessed and are available for different regions (Giorgi and 
Gutowski, 2015). For instance, assessments based on publications 
from an extension of the IMPACT2C project (Vautard et al., 2014; 
Jacob and Solman, 2017) are newly available for 1.5°C projections. 
Recently, simulations from the ‘Half a degree Additional warming, 
Prognosis and Projected Impacts’ (HAPPI) multimodel experiment 
have been performed to specifically assess climate changes at 1.5°C 
vs 2°C global warming (Mitchell et al., 2016). The HAPPI protocol 
consists of coupled land–atmosphere initial condition ensemble 
simulations with prescribed sea surface temperatures (SSTs); sea ice, 
GHG and aerosol concentrations; and solar and volcanic activity that 
coincide with three forced climate states: present-day (2006–2015) 
(see Section 1.2.1) and future (2091–2100) either with 1.5°C or 2°C 
global warming (prescribed by modified SSTs).
Detection and attribution of change in climate and impacted systems
Formalized scientific methods are available to detect and attribute 
impacts of greenhouse gas forcing on observed changes in climate 
(e.g., Hegerl et al., 2007; Seneviratne et al., 2012; Bindoff et al., 2013) 
and impacts of climate change on natural and human systems (e.g., 
Stone et al., 2013; Hansen and Cramer, 2015; Hansen et al., 2016). 
The reader is referred to these sources, as well as to the AR5 for more 
background on these methods.Global climate warming has already reached approximately 1°C 
(see Section 1.2.1) relative to pre-industrial conditions, and thus 
‘climate at 1.5°C global warming’ corresponds to approximately 
the addition of only half a degree of warming compared to the 
present day, comparable to the warming that has occurred since 
the 1970s (Bindoff et al., 2013). Methods used in the attribution of 
observed changes associate with this recent warming are therefore 
also applicable to assessments of future changes in climate at 1.5°C 
warming, especially in cases where no climate model simulations or 
analyses are available. 
Impacts of 1.5°C global warming can be assessed in part from 
regional and global climate changes that have already been detected 
and attributed to human influence (e.g., Schleussner et al., 2017) and 
are components of the climate system that are most responsive to 
current and projected future forcing. For this reason, when specific 
projections are missing for 1.5°C global warming, some of the 
assessments of climate change provided in Chapter 3 (Section 3.3) 
build upon joint assessments of (i) changes that were observed and 
attributed to human influence up to the present, that is, for 1°C 
global warming and (ii) projections for higher levels of warming (e.g., 
2°C, 3°C or 4°C) to assess the changes at 1.5°C. Such assessments 
are for transient changes only (see Chapter 3, Section 3.3).
Besides quantitative detection and attribution methods, assessments 
can also be based on indigenous and local knowledge (see Chapter 4, 
Box 4.3). While climate observations may not be available to assess 
impacts from a scientific perspective, local community knowledge 
can also indicate actual impacts (Brinkman et al., 2016; Kabir et al., 
2016). The challenge is that a community’s perception of loss due 
to the impacts of climate change is an area that requires further 
research (Tschakert et al., 2017).
Costs and benefits analysis
Cost–benefit analyses are common tools used for decision-making, 
whereby the costs of impacts are compared to the benefits from 
different response actions (IPCC,  2014a,  b). However, for the 
case of climate change, recognising the complex inter-linkages 
of the Anthropocene, cost–benefit analysis tools can be difficult 
to use because of disparate impacts versus costs and complex 
interconnectivity within the global social-ecological system (see 
Box 1.1 and Cross-Chapter Box 5 in Chapter 2). Some costs are 
relatively easily quantifiable in monetary terms but not all. Climate 
change impacts human lives and livelihoods, culture and values, and 
whole ecosystems. It has unpredictable feedback loops and impacts 
on other regions (IPCC, 2014a), giving rise to indirect, secondary, 
tertiary and opportunity costs that are typically extremely difficult to 
quantify. Monetary quantification is further complicated by the fact 
that costs and benefits can occur in different regions at very different 
times, possibly spanning centuries, while it is extremely difficult if not 
impossible to meaningfully estimate discount rates for future costs 
and benefits. Thus standard cost–benefit analyses become difficult 
to justify (IPCC, 2014a; Dietz et al., 2016) and are not used as an 
assessment tool in this report.

77
1Framing and Context Chapter 11.6 Confidence, Uncertainty and Risk
This report relies on the IPCC’s uncertainty guidance provided in 
Mastrandrea et al. (2011) and sources given therein. Two metrics for 
qualifying key findings are used: 
Confidence:  Five qualifiers are used to express levels of confidence 
in key findings, ranging from very low , through low, medium, 
high, to very high . The assessment of confidence involves at least 
two dimensions, one being the type, quality, amount or internal 
consistency of individual lines of evidence, and the second being 
the level of agreement between different lines of evidence. Very 
high confidence findings must either be supported by a high level 
of agreement across multiple lines of mutually independent and 
individually robust lines of evidence or, if only a single line of evidence 
is available, by a very high level of understanding underlying that 
evidence. Findings of low or very low confidence are presented only 
if they address a topic of major concern.
Likelihood:  A calibrated language scale is used to communicate 
assessed probabilities of outcomes, ranging from exceptionally 
unlikely  (<1%), extremely unlikely  (<5%), very unlikely  (<10%), 
unlikely  (<33%), about as likely as not  (33–66%),  likely  (>66%),  very 
likely  (>90%), extremely likely  (>95%) to virtually certain  (>99%). 
These terms are normally only applied to findings associated with 
high or very high confidence. Frequency of occurrence within a model 
ensemble does not correspond to actual assessed probability of 
outcome unless the ensemble is judged to capture and represent the 
full range of relevant uncertainties. 
Three specific challenges arise in the treatment of uncertainty and 
risk in this report. First, the current state of the scientific literature on 
1.5°C means that findings based on multiple lines of robust evidence 
for which quantitative probabilistic results can be expressed may be 
few in number, and those that do exist may not be the most policy-
relevant. Hence many key findings are expressed using confidence 
qualifiers alone.
Second, many of the most important findings of this report are 
conditional because they refer to ambitious mitigation scenarios, 
potentially involving large-scale technological or societal 
transformation. Conditional probabilities often depend strongly on 
how conditions are specified, such as whether temperature goals 
are met through early emission reductions, reliance on negative 
emissions, or through a low climate response. Whether a certain 
risk is considered high at 1.5°C may therefore depend strongly on 
how 1.5°C is specified, whereas a statement that a certain risk may 
be substantially higher at 2°C relative to 1.5°C may be much more 
robust. 
Third, achieving ambitious mitigation goals will require active, 
goal-directed efforts aiming explicitly for specific outcomes and 
incorporating new information as it becomes available (Otto et 
al., 2015). This shifts the focus of uncertainty from the climate 
outcome itself to the level of mitigation effort that may be required 
to achieve it. Probabilistic statements about human decisions are always problematic, but in the context of robust decision-making, 
many near-term policies that are needed to keep open the option of 
limiting warming to 1.5°C may be the same, regardless of the actual 
probability that the goal will be met (Knutti et al., 2015).
1.7 Storyline of the Report
The storyline of this report (Figure 1.6) includes a set of interconnected 
components. The report consists of five chapters (plus Supplementary 
Material for Chapters 1 through 4), a Technical Summary and a 
Summary for Policymakers. It also includes a set of boxes to elucidate 
specific or cross-cutting themes, as well as Frequently Asked 
Questions for each chapter, a Glossary, and several other Annexes.
At a time of unequivocal and rapid global warming, this report 
emerges from the long-term temperature goal of the Paris Agreement 
– strengthening the global response to the threat of climate change 
by pursuing efforts to limit warming to 1.5°C through reducing 
emissions to achieve a balance between anthropogenic emissions by 
sources and removals by sinks of greenhouse gases. The assessment 
focuses first, in Chapter 1, on how 1.5°C is defined and understood, 
what is the current level of warming to date, and the present 
trajectory of change. The framing presented in Chapter 1 provides the 
basis through which to understand the enabling conditions of a 1.5°C 
warmer world and connections to the SDGs, poverty eradication, and 
equity and ethics.
In Chapter 2, scenarios of a 1.5°C warmer world and the associated 
pathways are assessed. The pathways assessment builds upon 
the AR5 with a greater emphasis on sustainable development in 
mitigation pathways. All pathways begin now and involve rapid 
and unprecedented societal transformation. An important framing 
device for this report is the recognition that choices that determine 
emissions pathways, whether ambitious mitigation or ‘no policy’ 
scenarios, do not occur independently of these other changes and 
are, in fact, highly interdependent. 
Projected impacts that emerge in a 1.5°C warmer world and beyond 
are dominant narrative threads of the report and are assessed in 
Chapter 3. The chapter focuses on observed and attributable global 
and regional climate changes and impacts and vulnerabilities. The 
projected impacts have diverse and uneven spatial, temporal, human, 
economic, and ecological system-level manifestations. Central to the 
assessment is the reporting of impacts at 1.5°C and 2°C, potential 
impacts avoided through limiting warming to 1.5°C, and, where 
possible, adaptation potential and limits to adaptive capacity.
Response options and associated enabling conditions emerge next, in 
Chapter 4. Attention is directed to exploring questions of adaptation 
and mitigation implementation, integration, and transformation in 
a highly interdependent world, with consideration of synergies and 
trade-offs. Emission pathways, in particular, are broken down into 
policy options and instruments. The role of technological choices, 
institutional capacity and global-scale trends like urbanization and 
changes in ecosystems are assessed. 

78
Chapter 1 Framing and Context
1Chapter 5 covers linkages between achieving the SDGs and a 1.5°C 
warmer world and turns toward identifying opportunities and 
challenges of transformation. This is assessed within a transition to 
climate-resilient development pathways and connection between the 
evolution towards 1.5°C, associated impacts, and emission pathways. 
Positive and negative effects of adaptation and mitigation response 
measures and pathways for a 1.5°C warmer world are examined. Progress along these pathways involves inclusive processes, 
institutional integration, adequate finance and technology, and 
attention to issues of power, values, and inequalities to maximize 
the benefits of pursuing climate stabilisation at 1.5°C and the goals 
of sustainable development at multiple scales of human and natural 
systems from global, regional, national to local and community 
levels.
Figure 1.6 |  Schematic of report storyline.


79
1Framing and Context Chapter 1Frequently Asked Questions 
FAQ 1.1 | Why are we Talking about 1.5°C?
Summary:  Climate change represents an urgent and potentially irreversible threat to human societies and the 
planet. In recognition of this, the overwhelming majority of countries around the world adopted the Paris Agree -
ment in December 2015, the central aim of which includes pursuing efforts to limit global temperature rise 
to 1.5°C. In doing so, these countries, through the United Nations Framework Convention on Climate Change 
(UNFCCC), also invited the IPCC to provide a Special Report on the impacts of global warming of 1.5°C above pre-
industrial levels and related global greenhouse gas emissions pathways. 
At the 21st Conference of the Parties (COP21) in December 2015, 195 nations adopted the Paris Agreement2. The 
first instrument of its kind, the landmark agreement includes the aim to strengthen the global response to the 
threat of climate change by ‘holding the increase in the global average temperature to well below 2°C above 
pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels’. 
The first UNFCCC document to mention a limit to global warming of 1.5°C was the Cancun Agreement, adopted 
at the sixteenth COP (COP16) in 2010. The Cancun Agreement established a process to periodically review the 
‘adequacy of the long-term global goal (LTGG) in the light of the ultimate objective of the Convention and the 
overall progress made towards achieving the LTGG, including a consideration of the implementation of the 
commitments under the Convention’. The definition of LTGG in the Cancun Agreement was ‘to hold the increase 
in global average temperature below 2°C above pre-industrial levels’. The agreement also recognised the need 
to consider ‘strengthening the long-term global goal on the basis of the best available scientific knowledge…to 
a global average temperature rise of 1.5°C’. 
Beginning in 2013 and ending at the COP21 in Paris in 2015, the first review period of the long-term global goal 
largely consisted of the Structured Expert Dialogue (SED). This was a fact-finding, face-to-face exchange of views 
between invited experts and UNFCCC delegates. The final report of the SED3 concluded that ‘in some regions and 
vulnerable ecosystems, high risks are projected even for warming above 1.5°C’. The SED report also suggested 
that Parties would profit from restating the temperature limit of the long-term global goal as a ‘defence line’ 
or ‘buffer zone’, instead of a ‘guardrail’ up to which all would be safe, adding that this new understanding 
would ‘probably also favour emission pathways that will limit warming to a range of temperatures below 2°C’. 
Specifically on strengthening the temperature limit of 2°C, the SED’s key message was: ‘While science on the 
1.5°C warming limit is less robust, efforts should be made to push the defence line as low as possible’. The 
findings of the SED, in turn, fed into the draft decision adopted at COP21.
With the adoption of the Paris Agreement, the UNFCCC invited the IPCC to provide a Special Report in 2018 on 
‘the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emissions 
pathways’. The request was that the report, known as SR1.5, should not only assess what a 1.5°C warmer world 
would look like but also the different pathways by which global temperature rise could be limited to 1.5°C. In 
2016, the IPCC accepted the invitation, adding that the Special Report would also look at these issues in the 
context of strengthening the global response to the threat of climate change, sustainable development and 
efforts to eradicate poverty.
The combination of rising exposure to climate change and the fact that there is a limited capacity to adapt to its 
impacts amplifies the risks posed by warming of 1.5°C and 2°C. This is particularly true for developing and island 
countries in the tropics and other vulnerable countries and areas. The risks posed by global warming of 1.5°C are 
greater than for present-day conditions but lower than at 2°C.
(continued on next page)
2 Paris Agreement FCCC/CP/2015/10/Add.1 https://unfccc.int/documents/9097
3 Structured Expert Dialogue (SED) final report FCCC/SB/2015/INF .1 https://unfccc.int/documents/8707

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Chapter 1 Framing and Context
1FAQ 1.1, Figure 1 | Timeline of notable dates in preparing the IPCC Special Report on Global Warming of 1.5°C (blue) embedded within processes and milestones 
of the United Nations Framework Convention on Climate Change (UNFCCC; grey), including events that may be relevant for discussion of temperature limits.
FAQ 1.1 (continued)

81
1Framing and Context Chapter 1Frequently Asked Questions 
FAQ 1.2 | How Close are we to 1.5°C?
Summary:  Human-induced warming has already reached about 1°C above pre-industrial levels at the time of writ -
ing of this Special Report. By the decade 2006–2015, human activity had warmed the world by 0.87°C (±0.12°C) 
compared to pre-industrial times (1850–1900). If the current warming rate continues, the world would reach 
human-induced global warming of 1.5°C around 2040.
Under the 2015 Paris Agreement, countries agreed to cut greenhouse gas emissions with a view to ‘holding the 
increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to 
limit the temperature increase to 1.5°C above pre-industrial levels’. While the overall intention of strengthening 
the global response to climate change is clear, the Paris Agreement does not specify precisely what is meant by 
‘global average temperature’, or what period in history should be considered ‘pre-industrial’. To answer the 
question of how close are we to 1.5°C of warming, we need to first be clear about how both terms are defined 
in this Special Report.
The choice of pre-industrial reference period, along with the method used to calculate global average 
temperature, can alter scientists’ estimates of historical warming by a couple of tenths of a degree Celsius. Such 
differences become important in the context of a global temperature limit just half a degree above where we are 
now. But provided consistent definitions are used, they do not affect our understanding of how human activity 
is influencing the climate. 
In principle, ‘pre-industrial levels’ could refer to any period of time before the start of the industrial revolution. 
But the number of direct temperature measurements decreases as we go back in time. Defining a ‘pre-industrial’ 
reference period is, therefore, a compromise between the reliability of the temperature information and how 
representative it is of truly pre-industrial conditions. Some pre-industrial periods are cooler than others for 
purely natural reasons. This could be because of spontaneous climate variability or the response of the climate 
to natural perturbations, such as volcanic eruptions and variations in the sun’s activity. This IPCC Special Report 
on Global Warming of 1.5°C uses the reference period 1850–1900 to represent pre-industrial temperature. This 
is the earliest period with near-global observations and is the reference period used as an approximation of pre-
industrial temperatures in the IPCC Fifth Assessment Report.
Once scientists have defined ‘pre-industrial’, the next step is to calculate the amount of warming at any given 
time relative to that reference period. In this report, warming is defined as the increase in the 30-year global 
average of combined air temperature over land and water temperature at the ocean surface. The 30-year 
timespan accounts for the effect of natural variability, which can cause global temperatures to fluctuate from 
one year to the next. For example, 2015 and 2016 were both affected by a strong El Niño event, which amplified 
the underlying human-caused warming. 
In the decade 2006–2015, warming reached 0.87°C (±0.12°C) relative to 1850–1900, predominantly due to human 
activity increasing the amount of greenhouse gases in the atmosphere. Given that global temperature is currently 
rising by 0.2°C (±0.1°C) per decade, human-induced warming reached 1°C above pre-industrial levels around 
2017 and, if this pace of warming continues, would reach 1.5°C around 2040. 
While the change in global average temperature tells researchers about how the planet as a whole is changing, 
looking more closely at specific regions, countries and seasons reveals important details. Since the 1970s, most 
land regions have been warming faster than the global average, for example. This means that warming in 
many regions has already exceeded 1.5°C above pre-industrial levels. Over a fifth of the global population live 
in regions that have already experienced warming in at least one season that is greater than 1.5°C above pre-
industrial levels. 
(continued on next page)

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Chapter 1 Framing and Context
1FAQ 1.2, Figure 1 | Human-induced warming reached approximately 1°C above pre-industrial levels in 2017. At the present rate, global temperatures would 
reach 1.5°C around 2040. Stylized 1.5°C pathway shown here involves emission reductions beginning immediately, and CO2 emissions reaching zero by 2055.
Current 
warming rateFAQ1.2:How close are we to 1.5°C?
Climate uncertainty 
for 1.5°C pathwayHuman-induced 
warming2017
Observed 
warmingGlobal temperature change 
relative to 1850-1900 (°C)2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0.00
1960 1980 2000 2020 2040 2060 2080 2100Human-induced warming reached approximately 1°C above 
pre-industrial levels in 2017FAQ 1.2 (continued)

83
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