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+DOI: 10.1126/science.1129116
, 449 (2006); 314Science
et al.R. Eugene Turner,
Rita
Wetland Sedimentation from Hurricanes Katrina and
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+sors represents an additional dimension of
sensitivity and specificity for m olecular imag-
ing. The depletion process generating the image
contrast depends on several parameters, inc lud-
ing saturation power and time, sensor concen-
tration, and ambient temperature. The latter
parameter provides another promising approach
to increase sensitivity even further, because the
exchange rate increases considerably when ap-
proaching 37-C(10). Characterization of the
saturation dynamics is currently under way and
will reveal optimized parameters for future
applications.
The technique is also quite promising for
biomedical imaging in vivo. A typical surface
coil of 20 cm diameter detects a volume of ca.
2.1 liters, thus decreasing S/N for a (2.8 mm)
3
voxel by a factor of 27.2 compared with our
setup. This loss is less than 50% of the gain for
an optimized system using 945% polarized
isotopical ly enri ched
129
Xe. An isotropic reso-
lution of 2 to 3 mm is feasible without signal
averaging for a concentration of pure polarized
129
Xe that is È2 mM in tissue. This minimum
value is below those observed for direct
injection of Xe-carrying lipid solutions into rat
muscle (70 mM) or for inhalation delivery for
brain tissue (8 mM) used in previous studies
that demonstrated Xe tissue imaging in vivo
(17). Sensitive molecular imaging of the bio-
sensor is therefore possible as long as the
distribution of dissolved xenon can be imaged
with sufficie nt S/N and the biosensor target is
not too dilute, because HYPER-CEST is based
on the detection of the free Xe resonance, not
direct detection of the biosensor resonance.
The HYPER-CEST technique is amenable to
any type of M RI image acquisition methodology.
We demo nstrat ed CSI here, but faster acquisiti on
techniques that incorporate a frequency encoding
domain such as FLASH (fast low angle shot)
have been successfully used to acquire in vivo Xe
tissue images (17).
The modular setup of the biosensor (i.e., the
nuclei t hat are detected are not covalently bound
to the targeting molecule) allows accumulation
of the biosensor in the t issue for minutes to h ours
before delivery of the hyperpolarized xenon
nuclei, which have much higher diffusivity. In
combination with the long spin-lattice relaxation
time of Xe, this two-step process optimally pre-
serves the hyp erpolarization before signal a cqui-
sition. Biosensor cages that yield distinct xenon
frequencies allow for multiplexing to detect
simultaneously several different targets (18).
Also the serum- and tissue-specific Xe NMR
signals (19, 20) arising after injection of the
carrier medium can be used f or perfusion studies
(Fig. 3B) in living tissue, making Xe-CSI a
multimodal imaging technique.
References and Notes
1. J. M. Tyszka, S. E. Fraser, R. E. Jacobs, Curr. Opin.
Biotechnol. 16, 93 (2005).
2. A. Y. Louie et al., Nat. Biotechnol. 18 , 321 (2000).
3. S. Aime, C. Carrera, D. Delli Castelli, S. Geninatti,
E. Terreno, Angew. Chem. Int. Ed. 44, 1813 (2005).
4. M. S. Albert et al., Nature 370, 199 (1994).
5. M. M. Spence et al., Proc. Natl. Acad. Sci. U.S.A. 98,
10654 (2001).
6. T. J. Lowery et al., Magn. Reson. Imaging 21, 1235
(2003).
7. K. Bartik, M. Luhmer, J. P. Dutasta, A. Collet, J. Reisse,
J. Am. Chem. Soc. 120, 784 (1998).
8. C. Hilty, T. J. Lowery, D. E. Wemmer, A. Pines, Angew.
Chem. Int. Ed. 45, 70 (2006).
9. S.-I. Han et al., Anal. Chem. 77, 4008 (2005).
10. M. M. Spence et al., J. Am. Chem. Soc. 126, 15287 (2004).
11. A. Bifone et al., Proc. Natl. Acad. Sci. U.S.A. 93, 12932
(1996).
12. K. M. Ward, A. H. Aletras, R. S. Balaban, J. Magn. Res.
143, 79 (2000).
13. A. A. Maudsley, S. K. Hilal, W. H. Perman, H. E. Simon,
J. Magn. Res. 51, 147 (1983).
14. N. Goffeney, J. W. M. Bulte, J. Duyn, L. H. Bryant, P. C. M.
van Zijl, J. Am. Chem. Soc. 123, 8628 (2001).
15. K. Knagge, J. Prange, D. Raftery, Chem. Phys. Lett. 397,
11 (2004).
16. J. L. Mynar, T. J. Lowery, D. E. Wemmer, A. Pines,
J. M. Frechet, J. Am. Chem. Soc. 128, 6334 (2006).
17. B. M. Goodson et al., Proc. Natl. Acad. Sci. U.S.A. 94,
14725 (1997).
18. G. Huber et al., J. Am. Chem. Soc. 128, 6239 (2006).
19. J. P. Mugler et al., Magn. Res. Med. 37, 809 (1997).
20. S. D. Swanson, M. S. Rosen, K. P. Coulter, R. C. Welsh,
T. E. Chupp, Magn. Res. Med. 42, 1137 (1999).
21. This work was supported by the Director, Office of
Science, Office of Basic Energy Sciences, Materials
Sciences and Engineering Division, of the U.S.
Department of Energy under contract no. DE-AC03-
76SF00098. L.S. acknowledges support from the
Deutsche Forschungsgemeinschaft (SCHR 995/1-1)
through an Emmy Noether Fellowship. T.J.L.
acknowledges the Graduate Research and Education in
Adaptive bio-Technology (GREAT) Training Program of the
UC Systemwide Biotechnology Research and Education
Program (no. 2005-264), and C.H. acknowledges support
from the Schweizerischer Nationalfonds through a
postdoctoral fellowship.
Supporting Online Material
www.sciencemag.org/cgi/content/full/314/5798/446/DC1
Materials and Methods
Fig. S1
References
28 June 2006; accepted 29 August 2006
10.1126/science.1131847
Wetland Sedimentation from
Hurricanes Katrina and Rita
R. Eugene Turner,
1,2
*
Joseph J. Baustian,
1,2
Erick M. Swenson,
1,2
Jennifer S. Spicer
2
More than 131 10
6
metric tons (MT) of inorganic sediments accumulated in coastal wetlands
when Hurricanes Katrina and Rita crossed the Louisiana coast in 2005, plus another 281 10
6
MT
when accumulation was prorated for open water area. The annualized combined amount of
inorganic sediments per hurricane equals (i) 12% of the Mississippi River’s suspended load, (ii)
5.5 times the inorganic load delivered by overbank flooding before flood protection levees were
constructed, and (iii) 227 times the amount introduced by a river diversion built for wetland
restoration. The accumulation from hurricanes is sufficient to account for all the inorganic
sediments in healthy saltmarsh wetlands.
I
norganic sediments accumulating in coastal
wetlands may be delivered from inland
sources via (i) unconstrained overbank flood-
ing, (ii) explosive releases through unintentional
breaks in constructed levees, and (iii) river diver-
sions. They may also arrive from o ffshore during
tidal inundation or storm events. It is important to
know the quantities delivered by each pathway to
understand how inorganic sediments contribute
to wetland stability and to spend wetland res-
toration funds effectively. Here we estimate the
amount of inorganic sediments deposited on
wetlands of the microtidal Louisiana coast
during Hurricanes Katrina and Rita.
Hurricanes Katrina and Rita passed through
the Louisiana (LA) coast on 29 August and 24
September, 2005, respectively, leaving behind a
devastated urban and rural landscape. Massive
amounts of water, salt, and sediments were re-
distributed across the coastal zone within a few
hours as a storm surge of up to 5 m propagated in
a northerly direction at the coastline south of New
Orleans, LA (Katrina), and near Sabine Pass,
Texas (TX) (Rita), inundating coastal wetlands in
the region. A thick deposit of mud remained in
these coastal wetlands after the storm waters re-
ceded (Fig. 1). We used this post-storm remnant
to learn about how coastal systems work.
The loss of LA_s coastal wetlands peaked be-
tween 1955 and 1978 at 11,114 ha year
–1
(1)
and declined to 2591 ha year
–1
from 1990 to
2000 (2). Coastal wetlands, barrier islands, and
shallow waters are thought to provide some pro-
tection from hurricanes, by increasing resistance
to storm surge propagation and by lowering hur-
ricane storm surge height (3). Restoring LA_s
wetlands has become a political priority, in part
because of this perceived wetland/storm surge
connection. A major part of LA_srestoration
effort is to divert part of the Mississippi River
into wetlands, and at considerable cost Eref.
(S1) in supporting online material (SOM)^.
Widely adopted assumptions supporting this
1
Coastal Ecology Institute,
2
Department of Oceanography
and Coastal Sciences, Louisiana State University, Baton
Rouge, LA 70803, USA.
*To whom correspondence should be addressed. E-mail:
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+diversion are that flood protection levees have
eliminated overbank flooding, which has caused
diminished sediment accumulation and eventual
wetland loss, and introducing sediments into
estuaries via river diversi ons will enhance wet-
land restoration.
If increasing inorganic sediment loading to
coastal wetlands is important for their restora-
tion, then it is important to quantify the major
sediment pathways. Hurricanes Katrina and
Rita obviously brought some inorganic sedi-
ment into the coastal wetlands, but how much,
and where was it distributed? A few measure-
ments of the inorganic sediments accumulating
from a hurricane have been made at specific
sites along the coast (4–8) Eref. (S7)inSOM^,
but until this study there were n o coastwide
data on the inorganic sediments accumulating
from hurricanes. Here we show that the dom-
inant pathway of inorganic sediments into the
microtidal LA coastal wetlands is from offshore
to inshore during hurricanes, and not from over-
bank flooding along the main channel of the
Mississippi River, from smaller storm events, or
from tidal inundations.
We sampled from the shoreline to the on-
shore limit of storm sediment deposition in
wetlands (9) (Fig. 1A). We collected samples
from all coastal watersheds in LA and at seven
sites in eastern TX, using a helicopter and air-
boat (145 samples) or by walking out 920 m
into the wetland from access points reached by
boat or car (53 samples). Freshly deposited mud
was easily identifiable from the layers beneath
on the basis of color, texture, and density, and
by the absence of plant debris (Fig. 1E). At least
one preliminary sampling was done at each
location (often three or four) before the final
sample was taken. Samples for sediment depth
(in centimeters) and density (in grams per
cm
3
) were taken only over the vegetation zone
and not in rivulets between clumps of wetland
plants.
The average bulk density of the newly
deposited material was 0.37 g cm
–3
(T1SD0
T 0.35; range, 0 to 1.78 g cm
–3
; n 0 170
sam p l e s) ; it was highest near the coastline and
decreased inland (Fig. 2B). The bulk density of
material in these wetlands was determined by
the amount of inorganic materials, not the
organic content, and so bulk density multiplied
by deposition height is an estimate of inorganic
sediment deposition (9, 10). Sediment with a
bulk density 91 was largely composed of sand
(Fig. 1A) (9). There was sparse plant debris
(stems, leaves, and roots) in the newly
deposited sediments within a few kilometers
from the shoreline. The unconfirmed hypoth-
esis is that the source materials from offshore
came from where the bottom resistance to the
hurricane winds before landfall was the
greatest: in the shallow water zone immedi-
ately offshore of the deposition site.
The average dry weight accumulation of the
deposition layer at all sampled locations was
2.23 g cm
–2
(T1SD0T3.4; range, 0 to 28.6 g
cm
–2
; n 0 169) and 2.25 g cm
–2
in the deltaic
plain. The thickness of the newly deposited
mud was 5.18 cm (T1SD0T7.7; range, 0 to
68 g cm; n 0 186). The thickest newly de-
posited sediments were observed inland of the
area of maximum bulk density in eastern LA
but were coincidental with the sediments of
highest bulk density in western LA (Fig. 2C).
The annualized average sediment accumulation
from one hurricane was 89% of the average
accumulation in healthy saltmarsh wetlands in
the deltaic plain E0.166 g cm
–2
year
–1
(10)^.
Sediment deposition (in grams per cm
2
)was
greatest near the center of the storm track (Fig.
2D). The highest values in the Chenier Plain
were on the east side of the hurricane path,
where counterclockwise winds brought a storm
surge inland, and were least on the western side,
where water was withdrawn in a southerly
direction out of the wetlands (Fig. 2, D to F).
The greatest deposition in the Deltatic Plain was
in the Breton Sound estuary, on the east side of
the Mississippi River. The marshes within the
4- longitude distance between the two hurri-
canes (approximately 300 km) had intermediate
rates of deposition. The peak water level, but
not sediment accumulation, was higher in west-
ern Lake Pontchartrain during Hurricane Rita
than during Hurricane Katrina because of these
differences in wind fields (11).Thepeakinbulk
density was highest on the eastern side of the
center of the storm track.
There were peaks in sediment deposition
where navigation channels confined the incom-
ing storm surge to a narrow area at Sabine Pass,
TX, and in the industrial canal by Paris Road,
New Orleans, where the Intracoastal Waterway
Fig. 1. Examples of sediments deposited by Hurricanes Katrina and Rita. (A) Sand overwash on the
former location of the coastal community of Holly Beach, LA (photo taken 18 November 2005 by
R.E.T.). (B) Mud on the lawn of a St. Bernard Parish subdivision home (photo taken 27 September
2005 by M. Collins). (C) Mud on the marsh surface brought by Hurricanes Katrina and Rita (photo
taken 16 November 2005 by J.B.). (D) Recent mud deposit (10.5 cm) accumulated over a root mass
in the St. Bernard estuary (photo taken 16 November 2005 by J.B.). (E) Dried mud on the lawn of a
Chalmette, LA, subdivision home, September 2005 (photo taken by R. Richards).
REPORTS
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+meets the Mississippi River Gulf Outlet. These
observations are consistent with results from
modeled storm surge velocities (12).
The total amount of recently deposited wet-
land sediments on the LA coast was calculated
using information on the average sediment
accretion and wetland area for each of four to
six subunits of four coastal regions. The min -
imum amount of inorganic sediment brought
in by these two hurricanes was estimated to be
131 10
6
metric tons (MT) (9) (Table 1). The
aver ag e occurrence of a Category 3 or larger
hurricane on this coast was every 7.88 years
from 1879 to 2005 (9) (table S1). The
annualized deposition from one hurricane
would be 8.3 10
6
MT year
–1
if all hurricanes
brought an equal amount of sediments to these
wetlands. If sediments from these hurricanes are
deposited in open-water areas at the same rate as
in wetlands (9), then the pro rata deposition for
open-water areas is proportional to the open
water/wetland area (1) and is equal to 17.8
10
6
MT year
–1
, for a combined sediment
deposition of 26.1 10
6
MT year
–1
.
The sediment accumulations in wetlands and
open water were 4.0 and 8.5%, respectively, of
the average annual suspended sediment load of
the Mississippi River (210 10
6
MT year
–1
)
(Table 1) (13). The more frequent smaller storms
not included in this analysis may also trans-
port substantial amounts of inorganic material
(14, 15).
The amount of sediments delivered to coast-
al wetlands by Hurricanes Katrina and Rita was
greater than the estimated amounts once flow-
ing through or over Mississippi River banks.
The suspended sediments overflowing uncon-
fined (natural) levees of the Mississippi River
in the past century were 4.8 10
6
MT year
–1
,
and 1.7 10
6
MT year
–1
in a confined levee
system with occasional crevasses (Table 1).
The Caernarvon Diversion, a restoration project
located downstream from New Orleans, LA,
delivered a 2-year average sediment load of
0.115 10
6
MT year
–1
(16). One conclusion
to be drawn from these numbers is that the
amount of sediment deposited on these wet-
lands from an average Category 3 or larger
hurricane is 1.7 times the amount potentially
available through unconfined overbank flood-
ing, 4.6 times more than through crevasses in
the unconfined channel, and 72 times more than
from this river diversion. The combined sedi-
ment accumulation in wetlands and open water
resulting from an average Category 3 or larger
hurricane is 5.5 times larger than the material
delivered by unconfined flow in the Mississippi
River and 227 times larger than that delivered
by the Caernarvon Diversion.
However, these comparisons are conserva-
tive estimates. Not all of the ino rganic sediment
flowing from rivers and over or through levees
is deposited onto a wetland. Levees are higher
than the surrounding land because inorganics
settle out onto the levees or within the nearby
marshes. Also, the peak in river heights, and
hence in discharge, occurs in the spring whe n
water levels in the estuary are at their seasonal
low and wetlands are infrequently flooded (17).
Sediments that do accumulate are deposited
close to the diversion. For example, the
Caernarvon Diversion distributes about 50%
of its sediments into the wetlands for a
maximum distance of about 6 km, covering
about 15% of a direct path to the coastline
(Table 1) (18). Hurricanes, in contrast, are
much more democratic in that they flood the
entire coastal landscape with new sediments.
A coastwide perspective on sediment load-
ing to these wetlands, and perhaps to other mi-
crotidal coastal wetlands, is that most of the
inorganics accumula ting in them went down
the Mississippi_s birdfoot delta before they
were deposited during large storms. The es-
timates indicate that the amount of storm-
transported material is much greater than that
introduced to wetlands from the historical over-
ba n k f l o w, from crevasses, or from river diver-
sions. In particular, hurricanes appear to be the
overwhelming pathway for depositing new in-
organic sediments in coastal wetlands in west-
ern LA, because the few riverine sources bring
relatively trivial amounts of inorganic sedi-
Table 1. Estimates of the sediment source pathways for the Mississippi River deltaic plain in
Louisiana.
Sediment source pathways Amount (10
6
MT year
–1
)
Mississippi River discharge into ocean (13) 210
One hurricane every 7.88 years (table S1)
Onto wetland only 8.3
Onto wetland and into open water (9) 25.9
Overbank flooding (before flood protection levees) and into open water (22 ) 4.79
Crevasses through levees and into open water (22) 1.81
Caernarvon Diversion
Into the estuary (16) 0.115
Onto wetland (18) 0.06
Fig. 2. Location of re-
cent sediment samples
and data arranged by
longitude. (A)Samplelo-
cations (red dots) and
the distribution of coast-
al wetlands in southern
LA (black background).
The vertical gray arrow
is the crossing location
of Hurricanes Rita (west-
ern LA) and Katrina (east-
ern LA). (B)Allsamples
(open circles) and sam-
pl e s with a bulk density
value 91.0 g cm
–3
(red
dots). (C) All samples
(open circles) and sam-
ples with a vertical ac-
cretion 93cm(reddots).
(D) Accumulation rela-
tive to the long itude of
sample collection (black
circles). (E)Bulkdensity
relative to the longitude
of sample collection. (F)
Vertical accretion relative
to the longitude of sam-
ple collection.
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+ments into the marsh. Because hurricanes are so
important to the inorganic sediment budget, other
factors must be considered to understand how to
reduce wetland losses and further their restora-
tion. Changes in the in situ accumulation of
organics, rather than the reduction of inorganic
sediments arriving via overbank flooding, are
implicated as a causal agent of wetland losses on
this coast. This is illustrated by the fact that the
soil volume occupied by organic sediments plus
water in healthy saltmarsh wetlands is 990%
(10) and is certainly the same or higher in
wetlands of lower salinity. This organic portion
plays a major role in wetland soil stability and
hence in wetland ecosystem health (19).
References and Notes
1. R. H. Baumann, R. E. Turner, Environ. Geol. Water Res.
15, 189 (1990).
2. T. A. Morton, J. C. Bernier, J. A. Barras, N. F. Ferina, USGS
Open-File Rep. 2005-1215 (2005).
3. G. W. Stone, X. Zhang, A. Sheremet, J. Coastal Res. 44,
40 (2005).
4. J. M. Rybczyk, D. R. Cahoon, Estuaries 25, 985 (2002).
5. M. L. Parsons, J. C oastal Res. 14, 939 (1998).
6. D. R. Cahoon et al., J. Coastal Res. 21 (special issue), 280
(1995).
7. J. P. Morgan, L. G. Nichols, M. Wright, Morphological
Effect of Hurricane Audrey on the Louisiana Coast
(contribution no. 58-3, Coastal Studies Institute, Louisiana
State Univ., Baton Rouge, LA, 1958).
8. J. A. Nyman, R. D. DeLaune, H. H. Roberts, W. H. Patrick
Jr., Mar. Ecol. Prog. Ser. 96, 269 (1992).
9. Materials and methods are available as supporting
material on Science Online.
10. R. E. Turner, E. M. Swenson, C. S. Milan, in Concepts and
Controversies in Tidal Marsh Ecology, M. Weinstein,
D. A. Kreeger, Eds. (Kluwer, Dordrecht, Netherlands,
2001), pp. 583–595.
11. The peak water measured by U.S. Geological Survey
water-level gage 30174809020900, at Pass Manchac
Turtle Cove near Ponchatoula, LA, in the western Lake
Pontchartrain watershed, was about 6 cm higher during
Hurricane Rita than during Hurricane Katrina, even
though the hurricane path was over 300 km further away.
12. Storm surge model results for Hurricanes Katrina and Rita
are available at the Louisiana State University Hurricane
Center Web sites (http://hurricane.lsu.edu/floodprediction/
katrina/deadly_funnel1.jpg and http://hurricane.lsu.edu/
floodprediction/rita24/images/adv24_SurgeTX_LA.jpg).
13. G. J. Chakrapani, Curr. Sci. 88, 569 (2005).
14. R. H. Baumann, thesis, Louisiana State Univ., Baton
Rouge, LA (1980).
15. D. J. Reed, N. De Luca, A. L. Foote, Estuaries 20, 301
(1997).
16. G. A. Snedden, J. E. Cable, C. Swarzenski, E. Swenson,
Estuar. Coastal Shelf Sci., in press.
17. R. E. Turner, Estuaries 14, 139 (1991).
18. K. W. Wheelock, thesis, Louisiana State Univ., Baton
Rouge, LA (2003).
19. R. E. Turner, E. M. Swenson, C. S. Milan, J. M. Lee,
T. A. Oswald, Ecol. Res. 19, 29 (2004).
20. R. H. Kesel, Environ. Geol. Water Res. 11, 271 (1988).
21. C. M. Belt Jr., Science 189, 681 (1975).
22. Kesel (20) estimated the sediment load in the spring
floods using water records from 1950 to 1983 and
determined the amount of sediment that would be
available from unconfined overbank flooding if the levees
were not there. The estimate is actually an overestimate
of the amount available, because the artificial constric-
tion of the river channel throughout the basin raised the
flood stage for the same-sized discharge event (21).
23. Supported by the NSF Division of Geomorphology and
Land-Use Dynamics (award EAR-061250), the National
Oceanic and Atmospheric Administration Coastal Ocean
Program MULTISTRESS (award no. NA16OP2670), and a
Louisiana Board of Regents Fellowship (J.S.S.). We thank
J. Gore for assistance in sampling from an airboat; E. Babin
for assistance in finding photographs; H. Hampp, our deft,
safe, and considerate helicopter pilot; and B. Sen Gupta,
N. N. Rabalais, and three anonymous reviewers for
constructive reviews of the manuscript.
Supporting Online Material
www.sciencemag.org/cgi/content/full/1129116/DC1
Materials and Methods
SOM Text
Fig. S1
Table S1
References and Notes
24 April 2006; accepted 1 September 2006
Published online 21 September 2006;
10.1126/science.1129116
Include this information when citing this paper.
A Combined Mitigation/Geoengineering
Approach to Climate Stabilization
T. M. L. Wigley
Projected anthropogenic warming and increases in CO
2
concentration present a twofold threat, both
from climate changes and from CO
2
directly through increasing the acidity of the oceans. Future climate
change may be reduced through mitigation (reductions in greenhouse gas emissions) or through
geoengineering. Most geoengineering approaches, however, do not address the problem of increasing
ocean acidity. A combined mitigation/geoengineering strategy could remove this deficiency . Here we
consider the deliberate injection of sulfate aerosol precursors into the stratosphere. This action could
substantially offset future warming and provide additional time to reduce human dependence on fossil
fuels and stabilize CO
2
concentrations cost-effectively at an acceptable level.
I
n the absence of policies to reduce the mag-
nitude of future climate change, the globe is
expected to warm by È1- to 6-Coverthe
21st century (1, 2). Estimated CO
2
concentra-
tions in 2100 lie in the range from 540 to 970
parts per million, which is sufficient to cause
substantial increases in ocean acidity (3–6).
Mitigation directed toward stabilizing CO
2
concentrations (7) addresses both problems but
presents considerable economic and technologi-
cal challenges (8, 9). Geoengineering (10–17)
could help reduce the future extent of climate
change due to warming but does not address the
problem of ocean acidity. Mitigation is therefore
necessary, but geoengineering could provide
additional time to address the economic and
technological challenges faced by a mitigation-
only approach.
The geoengineering strategy examined here is
the injection of aerosol or aerosol precursors Esu ch
as sulfur dioxide (SO
2
)^ into the stratosphere to
provide a negative forcing of the climate system
and consequently offset part of the positive
forcing due to increasing greenhouse gas con-
centrations (18). Volcanic eruptions provide ideal
experiments that can be used to assess the effects
of large anthropogenic emissions of SO
2
on
stratospheric aerosols and climate. We know, for
example, that the Mount Pinatubo eruption EJune
1991 (19, 20)^ caused detectable short-term cool-
ing (19–21) but did not seriously disrupt the
climate system. Deliberately adding aerosols or
aerosol precursors to the stratosphere, so that the
loading is similar to the maximum loading from
the Mount Pinatubo eruption, should therefore
present minimal climate risks.
Increased sulfate aerosol loading of the strato-
sphere may present other risks, such as through its
influence on stratospheric ozone. This particular
risk, however, is likely to be small. The effect of
sulfate aerosols depends on the chlorine loading
(22–24). With current elevated chlorine loadings,
ozone loss would be enhanced. This result would
delay the recovery of stratospheric ozone slightly
bu t only unti l anthropogenic chlorine loadings
returned to levels of the 1980s (which are ex-
pected to be reached by the late 2040s).
Figure 1 shows the projected effect of m ul -
tiple sequential eruptions of Mount Pinatubo
every year, every 2 years, and every 4 years.
The Pinatubo eruption–associated forcing that
was used had a peak annual mean value of
–2.97 W/m
2
(20, 21). The climate simulations
were carried out using an upwelling-diffusion
energy balance model EModel for the Assessment
of Greenhouse gas–Induced Climate Change
(MAGICC) (2, 25, 26)^ with a chosen climate
sensitivity of 3-C equilibrium warming for a CO
2
doubling (2 CO
2
). Figure 1 suggests that a
sustained stratospheric forcing of È–3 W/m
2
(the
average asymptotic forcing for the biennial
eruption case) would be sufficient to offset much
of the anthropogen ic warming expected over the
next century. Figure 1 also shows how rapidly the
aerosol-induced cooling dis appears once the in-
jection of material into the stratosphere stops, as
might become necessary should unexpected envi-
ronmental damages arise.
Three cases are considered to illustrate pos-
sible options for the timing and d uration of aerosol
injections. In each case, the loading of the strato-
sphere begins in 2010 and increases linearly to
National Center for Atmospheric Research, Post Office Box
3000, Boulder, CO 80307–3000, USA. E-mail: wigley@
ucar.edu
REPORTS
20 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org
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