Wetland sedimentation from Hurricanes Katrina and Rita
Science
REPORTS
8. C. Hilty, T. J. Lowery, D. E. Wemmer, A. Pines, Angew.
sors represents an additional dimension of The HYPER-CEST technique is amenable to
Chem. Int. Ed. 45, 70 (2006).
sensitivity and specificity for molecular imag- any type of MRI image acquisition methodology.
9. S.-I. Han et al., Anal. Chem. 77, 4008 (2005).
ing. The depletion process generating the image We demonstrated CSI here, but faster acquisition 10. M. M. Spence et al., J. Am. Chem. Soc. 126, 15287 (2004).
contrast depends on several parameters, includ- techniques that incorporate a frequency encoding 11. A. Bifone et al., Proc. Natl. Acad. Sci. U.S.A. 93, 12932
ing saturation power and time, sensor concen- domain such as FLASH (fast low angle shot) (1996).
12. K. M. Ward, A. H. Aletras, R. S. Balaban, J. Magn. Res.
tration, and ambient temperature. The latter have been successfully used to acquire in vivo Xe
143, 79 (2000).
parameter provides another promising approach tissue images (17).
13. A. A. Maudsley, S. K. Hilal, W. H. Perman, H. E. Simon,
to increase sensitivity even further, because the The modular setup of the biosensor (i.e., the J. Magn. Res. 51, 147 (1983).
exchange rate increases considerably when ap- nuclei that are detected are not covalently bound 14. N. Goffeney, J. W. M. Bulte, J. Duyn, L. H. Bryant, P. C. M.
proaching 37-C (10). Characterization of the to the targeting molecule) allows accumulation van Zijl, J. Am. Chem. Soc. 123, 8628 (2001).
15. K. Knagge, J. Prange, D. Raftery, Chem. Phys. Lett. 397,
saturation dynamics is currently under way and of the biosensor in the tissue for minutes to hours
11 (2004).
will reveal optimized parameters for future before delivery of the hyperpolarized xenon 16. J. L. Mynar, T. J. Lowery, D. E. Wemmer, A. Pines,
applications. nuclei, which have much higher diffusivity. In J. M. Frechet, J. Am. Chem. Soc. 128, 6334 (2006).
The technique is also quite promising for combination with the long spin-lattice relaxation 17. B. M. Goodson et al., Proc. Natl. Acad. Sci. U.S.A. 94,
14725 (1997).
biomedical imaging in vivo. A typical surface time of Xe, this two-step process optimally pre-
18. G. Huber et al., J. Am. Chem. Soc. 128, 6239 (2006).
coil of 20 cm diameter detects a volume of ca. serves the hyperpolarization before signal acqui- 19. J. P. Mugler et al., Magn. Res. Med. 37, 809 (1997).
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voxel by a factor of 27.2 compared with our frequencies allow for multiplexing to detect T. E. Chupp, Magn. Res. Med. 42, 1137 (1999).
21. This work was supported by the Director, Office of
setup. This loss is less than 50% of the gain for simultaneously several different targets (18).
Science, Office of Basic Energy Sciences, Materials
an optimized system using 945% polarized Also the serum- and tissue-specific Xe NMR
Sciences and Engineering Division, of the U.S.
isotopically enriched 129Xe. An isotropic reso- signals (19, 20) arising after injection of the Department of Energy under contract no. DE-AC03-
lution of 2 to 3 mm is feasible without signal carrier medium can be used for perfusion studies 76SF00098. L.S. acknowledges support from the
averaging for a concentration of pure polarized (Fig. 3B) in living tissue, making Xe-CSI a Deutsche Forschungsgemeinschaft (SCHR 995/1-1)
through an Emmy Noether Fellowship. T.J.L.
129Xe that is È2 mM in tissue. This minimum multimodal imaging technique.
acknowledges the Graduate Research and Education in
value is below those observed for direct Adaptive bio-Technology (GREAT) Training Program of the
injection of Xe-carrying lipid solutions into rat UC Systemwide Biotechnology Research and Education
References and Notes
muscle (70 mM) or for inhalation delivery for Program (no. 2005-264), and C.H. acknowledges support
1. J. M. Tyszka, S. E. Fraser, R. E. Jacobs, Curr. Opin.
from the Schweizerischer Nationalfonds through a
Biotechnol. 16, 93 (2005).
brain tissue (8 mM) used in previous studies
postdoctoral fellowship.
2. A. Y. Louie et al., Nat. Biotechnol. 18, 321 (2000).
that demonstrated Xe tissue imaging in vivo
3. S. Aime, C. Carrera, D. Delli Castelli, S. Geninatti,
(17). Sensitive molecular imaging of the bio- Supporting Online Material
E. Terreno, Angew. Chem. Int. Ed. 44, 1813 (2005).
sensor is therefore possible as long as the www.sciencemag.org/cgi/content/full/314/5798/446/DC1
4. M. S. Albert et al., Nature 370, 199 (1994).
Materials and Methods
distribution of dissolved xenon can be imaged 5. M. M. Spence et al., Proc. Natl. Acad. Sci. U.S.A. 98,
Fig. S1
10654 (2001).
with sufficient S/N and the biosensor target is References
6. T. J. Lowery et al., Magn. Reson. Imaging 21, 1235
not too dilute, because HYPER-CEST is based (2003).
on the detection of the free Xe resonance, not 7. K. Bartik, M. Luhmer, J. P. Dutasta, A. Collet, J. Reisse, 28 June 2006; accepted 29 August 2006
direct detection of the biosensor resonance. J. Am. Chem. Soc. 120, 784 (1998). 10.1126/science.1131847
Wetland Sedimentation from September, 2005, respectively, leaving behind a
devastated urban and rural landscape. Massive
amounts of water, salt, and sediments were re-
Hurricanes Katrina and Rita distributed across the coastal zone within a few
hours as a storm surge of up to 5 m propagated in
R. Eugene Turner,1,2* Joseph J. Baustian,1,2 Erick M. Swenson,1,2 Jennifer S. Spicer2 a northerly direction at the coastline south of New
Orleans, LA (Katrina), and near Sabine Pass,
Texas (TX) (Rita), inundating coastal wetlands in
More than 131 Â 106 metric tons (MT) of inorganic sediments accumulated in coastal wetlands
the region. A thick deposit of mud remained in
when Hurricanes Katrina and Rita crossed the Louisiana coast in 2005, plus another 281 Â 106 MT
these coastal wetlands after the storm waters re-
when accumulation was prorated for open water area. The annualized combined amount of
ceded (Fig. 1). We used this post-storm remnant
inorganic sediments per hurricane equals (i) 12% of the Mississippi River’s suspended load, (ii)
to learn about how coastal systems work.
5.5 times the inorganic load delivered by overbank flooding before flood protection levees were
The loss of LA_s coastal wetlands peaked be-
constructed, and (iii) 227 times the amount introduced by a river diversion built for wetland
tween 1955 and 1978 at 11,114 ha year –1 (1)
restoration. The accumulation from hurricanes is sufficient to account for all the inorganic
and declined to 2591 ha year –1 from 1990 to
sediments in healthy saltmarsh wetlands.
2000 (2). Coastal wetlands, barrier islands, and
norganic sediments accumulating in coastal sions. They may also arrive from offshore during shallow waters are thought to provide some pro-
I wetlands may be delivered from inland tidal inundation or storm events. It is important to tection from hurricanes, by increasing resistance
sources via (i) unconstrained overbank flood- know the quantities delivered by each pathway to to storm surge propagation and by lowering hur-
ing, (ii) explosive releases through unintentional understand how inorganic sediments contribute ricane storm surge height (3). Restoring LA_s
breaks in constructed levees, and (iii) river diver- to wetland stability and to spend wetland res- wetlands has become a political priority, in part
toration funds effectively. Here we estimate the because of this perceived wetland/storm surge
amount of inorganic sediments deposited on connection. A major part of LA_s restoration
1
Coastal Ecology Institute, 2Department of Oceanography
wetlands of the microtidal Louisiana coast effort is to divert part of the Mississippi River
and Coastal Sciences, Louisiana State University, Baton
into wetlands, and at considerable cost Eref.
during Hurricanes Katrina and Rita.
Rouge, LA 70803, USA.
Hurricanes Katrina and Rita passed through (S1) in supporting online material (SOM)^.
*To whom correspondence should be addressed. E-mail:
the Louisiana (LA) coast on 29 August and 24 Widely adopted assumptions supporting this
euturne@lsu.edu
449
www.sciencemag.org SCIENCE VOL 314 20 OCTOBER 2006
REPORTS
2.23 g cm–2 (T1 SD 0 T 3.4; range, 0 to 28.6 g
diversion are that flood protection levees have where water was withdrawn in a southerly
cm–2; n 0 169) and 2.25 g cm–2 in the deltaic
eliminated overbank flooding, which has caused direction out of the wetlands (Fig. 2, D to F).
diminished sediment accumulation and eventual plain. The thickness of the newly deposited The greatest deposition in the Deltatic Plain was
mud was 5.18 cm (T1 SD 0 T 7.7; range, 0 to
wetland loss, and introducing sediments into in the Breton Sound estuary, on the east side of
68 g cm; n 0 186). The thickest newly de-
estuaries via river diversions will enhance wet- the Mississippi River. The marshes within the
land restoration. posited sediments were observed inland of the 4- longitude distance between the two hurri-
If increasing inorganic sediment loading to area of maximum bulk density in eastern LA canes (approximately 300 km) had intermediate
coastal wetlands is important for their restora- but were coincidental with the sediments of rates of deposition. The peak water level, but
tion, then it is important to quantify the major highest bulk density in western LA (Fig. 2C). not sediment accumulation, was higher in west-
sediment pathways. Hurricanes Katrina and The annualized average sediment accumulation ern Lake Pontchartrain during Hurricane Rita
Rita obviously brought some inorganic sedi- from one hurricane was 89% of the average than during Hurricane Katrina because of these
ment into the coastal wetlands, but how much, accumulation in healthy saltmarsh wetlands in differences in wind fields (11). The peak in bulk
the deltaic plain E0.166 g cm–2 year –1 (10)^.
and where was it distributed? A few measure- density was highest on the eastern side of the
Sediment deposition (in grams per cm2) was
ments of the inorganic sediments accumulating center of the storm track.
from a hurricane have been made at specific greatest near the center of the storm track (Fig. There were peaks in sediment deposition
sites along the coast (4–8) Eref. (S7) in SOM^, 2D). The highest values in the Chenier Plain where navigation channels confined the incom-
but until this study there were no coastwide were on the east side of the hurricane path, ing storm surge to a narrow area at Sabine Pass,
data on the inorganic sediments accumulating where counterclockwise winds brought a storm TX, and in the industrial canal by Paris Road,
from hurricanes. Here we show that the dom- surge inland, and were least on the western side, New Orleans, where the Intracoastal Waterway
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
cm3) 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 (T1 SD 0
T 0.35; range, 0 to 1.78 g cm–3; n 0 170
samples); 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 Fig. 1. Examples of sediments deposited by Hurricanes Katrina and Rita. (A) Sand overwash on the
came from where the bottom resistance to the former location of the coastal community of Holly Beach, LA (photo taken 18 November 2005 by
hurricane winds before landfall was the R.E.T.). (B) Mud on the lawn of a St. Bernard Parish subdivision home (photo taken 27 September
greatest: in the shallow water zone immedi- 2005 by M. Collins). (C) Mud on the marsh surface brought by Hurricanes Katrina and Rita (photo
ately offshore of the deposition site. taken 16 November 2005 by J.B.). (D) Recent mud deposit (10.5 cm) accumulated over a root mass
The average dry weight accumulation of the 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).
deposition layer at all sampled locations was
450 20 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org
REPORTS
meets the Mississippi River Gulf Outlet. These annualized deposition from one hurricane (Table 1) (13). The more frequent smaller storms
would be 8.3 Â 106 MT year–1 if all hurricanes
observations are consistent with results from not included in this analysis may also trans-
modeled storm surge velocities (12). brought an equal amount of sediments to these port substantial amounts of inorganic material
The total amount of recently deposited wet- wetlands. If sediments from these hurricanes are (14, 15).
land sediments on the LA coast was calculated deposited in open-water areas at the same rate as The amount of sediments delivered to coast-
using information on the average sediment in wetlands (9), then the pro rata deposition for al wetlands by Hurricanes Katrina and Rita was
accretion and wetland area for each of four to open-water areas is proportional to the open greater than the estimated amounts once flow-
water/wetland area (1) and is equal to 17.8 Â
six subunits of four coastal regions. The min- ing through or over Mississippi River banks.
106 MT year –1, for a combined sediment
imum amount of inorganic sediment brought The suspended sediments overflowing uncon-
deposition of 26.1 Â 106 MT year –1.
in by these two hurricanes was estimated to be fined (natural) levees of the Mississippi River
131 Â 106 metric tons (MT) (9) (Table 1). The in the past century were 4.8 Â 106 MT year –1,
The sediment accumulations in wetlands and
and 1.7 Â 106 MT year –1 in a confined levee
average occurrence of a Category 3 or larger open water were 4.0 and 8.5%, respectively, of
hurricane on this coast was every 7.88 years the average annual suspended sediment load of system with occasional crevasses (Table 1).
the Mississippi River (210 Â 106 MT year –1)
from 1879 to 2005 (9) (table S1). The The Caernarvon Diversion, a restoration project
located downstream from New Orleans, LA,
delivered a 2-year average sediment load of
Fig. 2. Location of re-
0.115 Â 106 MT year –1 (16). One conclusion
cent sediment samples
to be drawn from these numbers is that the
and data arranged by
amount of sediment deposited on these wet-
longitude. (A) Sample lo-
lands from an average Category 3 or larger
cations (red dots) and
hurricane is 1.7 times the amount potentially
the distribution of coast-
available through unconfined overbank flood-
al wetlands in southern
ing, 4.6 times more than through crevasses in
LA (black background).
the unconfined channel, and 72 times more than
The vertical gray arrow
from this river diversion. The combined sedi-
is the crossing location
ment accumulation in wetlands and open water
of Hurricanes Rita (west-
ern LA) and Katrina (east- resulting from an average Category 3 or larger
ern LA). (B) All samples hurricane is 5.5 times larger than the material
(open circles) and sam- delivered by unconfined flow in the Mississippi
ples with a bulk density River and 227 times larger than that delivered
value 91.0 g cm–3 (red by the Caernarvon Diversion.
dots). (C) All samples However, these comparisons are conserva-
(open circles) and sam- tive estimates. Not all of the inorganic sediment
ples with a vertical ac- flowing from rivers and over or through levees
cretion 93 cm (red dots). is deposited onto a wetland. Levees are higher
(D) Accumulation rela- than the surrounding land because inorganics
tive to the longitude of
settle out onto the levees or within the nearby
sample collection (black
marshes. Also, the peak in river heights, and
circles). (E) Bulk density
hence in discharge, occurs in the spring when
relative to the longitude
water levels in the estuary are at their seasonal
of sample collection. (F)
low and wetlands are infrequently flooded (17).
Vertical accretion relative
Sediments that do accumulate are deposited
to the longitude of sam-
close to the diversion. For example, the
ple collection.
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-
Table 1. Estimates of the sediment source pathways for the Mississippi River deltaic plain in
crotidal coastal wetlands, is that most of the
Louisiana.
inorganics accumulating in them went down
the Mississippi_s birdfoot delta before they
Amount (106 MT year –1 )
Sediment source pathways
were deposited during large storms. The es-
Mississippi River discharge into ocean (13) 210
timates indicate that the amount of storm-
One hurricane every 7.88 years (table S1)
transported material is much greater than that
Onto wetland only 8.3
introduced to wetlands from the historical over-
Onto wetland and into open water (9) 25.9
bank flow, from crevasses, or from river diver-
Overbank flooding (before flood protection levees) and into open water (22) 4.79
sions. In particular, hurricanes appear to be the
Crevasses through levees and into open water (22) 1.81
overwhelming pathway for depositing new in-
Caernarvon Diversion
organic sediments in coastal wetlands in west-
Into the estuary (16) 0.115
ern LA, because the few riverine sources bring
Onto wetland (18) 0.06
relatively trivial amounts of inorganic sedi-
451
www.sciencemag.org SCIENCE VOL 314 20 OCTOBER 2006
REPORTS
(contribution no. 58-3, Coastal Studies Institute, Louisiana 20. R. H. Kesel, Environ. Geol. Water Res. 11, 271 (1988).
ments into the marsh. Because hurricanes are so
State Univ., Baton Rouge, LA, 1958). 21. C. M. Belt Jr., Science 189, 681 (1975).
important to the inorganic sediment budget, other
8. J. A. Nyman, R. D. DeLaune, H. H. Roberts, W. H. Patrick 22. Kesel (20) estimated the sediment load in the spring
factors must be considered to understand how to Jr., Mar. Ecol. Prog. Ser. 96, 269 (1992). floods using water records from 1950 to 1983 and
reduce wetland losses and further their restora- 9. Materials and methods are available as supporting determined the amount of sediment that would be
tion. Changes in the in situ accumulation of material on Science Online. available from unconfined overbank flooding if the levees
10. R. E. Turner, E. M. Swenson, C. S. Milan, in Concepts and were not there. The estimate is actually an overestimate
organics, rather than the reduction of inorganic
Controversies in Tidal Marsh Ecology, M. Weinstein, of the amount available, because the artificial constric-
sediments arriving via overbank flooding, are D. A. Kreeger, Eds. (Kluwer, Dordrecht, Netherlands, tion of the river channel throughout the basin raised the
implicated as a causal agent of wetland losses on 2001), pp. 583–595. flood stage for the same-sized discharge event (21).
this coast. This is illustrated by the fact that the 11. The peak water measured by U.S. Geological Survey 23. Supported by the NSF Division of Geomorphology and
water-level gage 30174809020900, at Pass Manchac Land-Use Dynamics (award EAR-061250), the National
soil volume occupied by organic sediments plus
Turtle Cove near Ponchatoula, LA, in the western Lake Oceanic and Atmospheric Administration Coastal Ocean
water in healthy saltmarsh wetlands is 990% Pontchartrain watershed, was about 6 cm higher during Program MULTISTRESS (award no. NA16OP2670), and a
(10) and is certainly the same or higher in Hurricane Rita than during Hurricane Katrina, even Louisiana Board of Regents Fellowship (J.S.S.). We thank
wetlands of lower salinity. This organic portion though the hurricane path was over 300 km further away. J. Gore for assistance in sampling from an airboat; E. Babin
12. Storm surge model results for Hurricanes Katrina and Rita for assistance in finding photographs; H. Hampp, our deft,
plays a major role in wetland soil stability and
are available at the Louisiana State University Hurricane safe, and considerate helicopter pilot; and B. Sen Gupta,
hence in wetland ecosystem health (19).
Center Web sites (http://hurricane.lsu.edu/floodprediction/ N. N. Rabalais, and three anonymous reviewers for
katrina/deadly_funnel1.jpg and http://hurricane.lsu.edu/ constructive reviews of the manuscript.
References and Notes floodprediction/rita24/images/adv24_SurgeTX_LA.jpg).
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15, 189 (1990). 14. R. H. Baumann, thesis, Louisiana State Univ., Baton www.sciencemag.org/cgi/content/full/1129116/DC1
2. T. A. Morton, J. C. Bernier, J. A. Barras, N. F. Ferina, USGS Rouge, LA (1980). Materials and Methods
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40 (2005). 16. G. A. Snedden, J. E. Cable, C. Swarzenski, E. Swenson, Table S1
4. J. M. Rybczyk, D. R. Cahoon, Estuaries 25, 985 (2002). Estuar. Coastal Shelf Sci., in press. References and Notes
5. M. L. Parsons, J. Coastal Res. 14, 939 (1998). 17. R. E. Turner, Estuaries 14, 139 (1991).
6. D. R. Cahoon et al., J. Coastal Res. 21 (special issue), 280 18. K. W. Wheelock, thesis, Louisiana State Univ., Baton 24 April 2006; accepted 1 September 2006
(1995). Rouge, LA (2003). Published online 21 September 2006;
7. J. P. Morgan, L. G. Nichols, M. Wright, Morphological 19. R. E. Turner, E. M. Swenson, C. S. Milan, J. M. Lee, 10.1126/science.1129116
Effect of Hurricane Audrey on the Louisiana Coast T. A. Oswald, Ecol. Res. 19, 29 (2004). Include this information when citing this paper.
A Combined Mitigation/Geoengineering Increased sulfate aerosol loading of the strato-
sphere may present other risks, such as through its
influence on stratospheric ozone. This particular
Approach to Climate Stabilization risk, however, is likely to be small. The effect of
sulfate aerosols depends on the chlorine loading
(22–24). With current elevated chlorine loadings,
T. M. L. Wigley
ozone loss would be enhanced. This result would
Projected anthropogenic warming and increases in CO2 concentration present a twofold threat, both delay the recovery of stratospheric ozone slightly
from climate changes and from CO2 directly through increasing the acidity of the oceans. Future climate but only until anthropogenic chlorine loadings
change may be reduced through mitigation (reductions in greenhouse gas emissions) or through returned to levels of the 1980s (which are ex-
geoengineering. Most geoengineering approaches, however, do not address the problem of increasing pected to be reached by the late 2040s).
ocean acidity. A combined mitigation/geoengineering strategy could remove this deficiency. Here we Figure 1 shows the projected effect of mul-
consider the deliberate injection of sulfate aerosol precursors into the stratosphere. This action could tiple sequential eruptions of Mount Pinatubo
substantially offset future warming and provide additional time to reduce human dependence on fossil every year, every 2 years, and every 4 years.
fuels and stabilize CO2 concentrations cost-effectively at an acceptable level. The Pinatubo eruption–associated forcing that
was used had a peak annual mean value of
–2.97 W/m2 (20, 21). The climate simulations
n the absence of policies to reduce the mag- technological challenges faced by a mitigation-
I nitude of future climate change, the globe is only approach. were carried out using an upwelling-diffusion
expected to warm by È1- to 6-C over the energy balance model EModel for the Assessment
The geoengineering strategy examined here is
the injection of aerosol or aerosol precursors Esuch
21st century (1, 2). Estimated CO2 concentra- of Greenhouse gas–Induced Climate Change
tions in 2100 lie in the range from 540 to 970 as sulfur dioxide (SO2)^ into the stratosphere to (MAGICC) (2, 25, 26)^ with a chosen climate
parts per million, which is sufficient to cause provide a negative forcing of the climate system sensitivity of 3-C equilibrium warming for a CO2
doubling (2 Â CO2). Figure 1 suggests that a
substantial increases in ocean acidity (3–6). and consequently offset part of the positive
sustained stratospheric forcing of È–3 W/m2 (the
Mitigation directed toward stabilizing CO2 forcing due to increasing greenhouse gas con-
concentrations (7) addresses both problems but centrations (18). Volcanic eruptions provide ideal average asymptotic forcing for the biennial
presents considerable economic and technologi- experiments that can be used to assess the effects eruption case) would be sufficient to offset much
cal challenges (8, 9). Geoengineering (10–17) of large anthropogenic emissions of SO2 on of the anthropogenic warming expected over the
could help reduce the future extent of climate stratospheric aerosols and climate. We know, for next century. Figure 1 also shows how rapidly the
example, that the Mount Pinatubo eruption EJune
change due to warming but does not address the aerosol-induced cooling disappears once the in-
problem of ocean acidity. Mitigation is therefore 1991 (19, 20)^ caused detectable short-term cool- jection of material into the stratosphere stops, as
necessary, but geoengineering could provide ing (19–21) but did not seriously disrupt the might become necessary should unexpected envi-
additional time to address the economic and climate system. Deliberately adding aerosols or ronmental damages arise.
aerosol precursors to the stratosphere, so that the Three cases are considered to illustrate pos-
loading is similar to the maximum loading from sible options for the timing and duration of aerosol
National Center for Atmospheric Research, Post Office Box
the Mount Pinatubo eruption, should therefore injections. In each case, the loading of the strato-
3000, Boulder, CO 80307–3000, USA. E-mail: wigley@
present minimal climate risks. sphere begins in 2010 and increases linearly to
ucar.edu
452 20 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org
8. C. Hilty, T. J. Lowery, D. E. Wemmer, A. Pines, Angew.
sors represents an additional dimension of The HYPER-CEST technique is amenable to
Chem. Int. Ed. 45, 70 (2006).
sensitivity and specificity for molecular imag- any type of MRI image acquisition methodology.
9. S.-I. Han et al., Anal. Chem. 77, 4008 (2005).
ing. The depletion process generating the image We demonstrated CSI here, but faster acquisition 10. M. M. Spence et al., J. Am. Chem. Soc. 126, 15287 (2004).
contrast depends on several parameters, includ- techniques that incorporate a frequency encoding 11. A. Bifone et al., Proc. Natl. Acad. Sci. U.S.A. 93, 12932
ing saturation power and time, sensor concen- domain such as FLASH (fast low angle shot) (1996).
12. K. M. Ward, A. H. Aletras, R. S. Balaban, J. Magn. Res.
tration, and ambient temperature. The latter have been successfully used to acquire in vivo Xe
143, 79 (2000).
parameter provides another promising approach tissue images (17).
13. A. A. Maudsley, S. K. Hilal, W. H. Perman, H. E. Simon,
to increase sensitivity even further, because the The modular setup of the biosensor (i.e., the J. Magn. Res. 51, 147 (1983).
exchange rate increases considerably when ap- nuclei that are detected are not covalently bound 14. N. Goffeney, J. W. M. Bulte, J. Duyn, L. H. Bryant, P. C. M.
proaching 37-C (10). Characterization of the to the targeting molecule) allows accumulation van Zijl, J. Am. Chem. Soc. 123, 8628 (2001).
15. K. Knagge, J. Prange, D. Raftery, Chem. Phys. Lett. 397,
saturation dynamics is currently under way and of the biosensor in the tissue for minutes to hours
11 (2004).
will reveal optimized parameters for future before delivery of the hyperpolarized xenon 16. J. L. Mynar, T. J. Lowery, D. E. Wemmer, A. Pines,
applications. nuclei, which have much higher diffusivity. In J. M. Frechet, J. Am. Chem. Soc. 128, 6334 (2006).
The technique is also quite promising for combination with the long spin-lattice relaxation 17. B. M. Goodson et al., Proc. Natl. Acad. Sci. U.S.A. 94,
14725 (1997).
biomedical imaging in vivo. A typical surface time of Xe, this two-step process optimally pre-
18. G. Huber et al., J. Am. Chem. Soc. 128, 6239 (2006).
coil of 20 cm diameter detects a volume of ca. serves the hyperpolarization before signal acqui- 19. J. P. Mugler et al., Magn. Res. Med. 37, 809 (1997).
2.1 liters, thus decreasing S/N for a (2.8 mm)3 sition. Biosensor cages that yield distinct xenon 20. S. D. Swanson, M. S. Rosen, K. P. Coulter, R. C. Welsh,
voxel by a factor of 27.2 compared with our frequencies allow for multiplexing to detect T. E. Chupp, Magn. Res. Med. 42, 1137 (1999).
21. This work was supported by the Director, Office of
setup. This loss is less than 50% of the gain for simultaneously several different targets (18).
Science, Office of Basic Energy Sciences, Materials
an optimized system using 945% polarized Also the serum- and tissue-specific Xe NMR
Sciences and Engineering Division, of the U.S.
isotopically enriched 129Xe. An isotropic reso- signals (19, 20) arising after injection of the Department of Energy under contract no. DE-AC03-
lution of 2 to 3 mm is feasible without signal carrier medium can be used for perfusion studies 76SF00098. L.S. acknowledges support from the
averaging for a concentration of pure polarized (Fig. 3B) in living tissue, making Xe-CSI a Deutsche Forschungsgemeinschaft (SCHR 995/1-1)
through an Emmy Noether Fellowship. T.J.L.
129Xe that is È2 mM in tissue. This minimum multimodal imaging technique.
acknowledges the Graduate Research and Education in
value is below those observed for direct Adaptive bio-Technology (GREAT) Training Program of the
injection of Xe-carrying lipid solutions into rat UC Systemwide Biotechnology Research and Education
References and Notes
muscle (70 mM) or for inhalation delivery for Program (no. 2005-264), and C.H. acknowledges support
1. J. M. Tyszka, S. E. Fraser, R. E. Jacobs, Curr. Opin.
from the Schweizerischer Nationalfonds through a
Biotechnol. 16, 93 (2005).
brain tissue (8 mM) used in previous studies
postdoctoral fellowship.
2. A. Y. Louie et al., Nat. Biotechnol. 18, 321 (2000).
that demonstrated Xe tissue imaging in vivo
3. S. Aime, C. Carrera, D. Delli Castelli, S. Geninatti,
(17). Sensitive molecular imaging of the bio- Supporting Online Material
E. Terreno, Angew. Chem. Int. Ed. 44, 1813 (2005).
sensor is therefore possible as long as the www.sciencemag.org/cgi/content/full/314/5798/446/DC1
4. M. S. Albert et al., Nature 370, 199 (1994).
Materials and Methods
distribution of dissolved xenon can be imaged 5. M. M. Spence et al., Proc. Natl. Acad. Sci. U.S.A. 98,
Fig. S1
10654 (2001).
with sufficient S/N and the biosensor target is References
6. T. J. Lowery et al., Magn. Reson. Imaging 21, 1235
not too dilute, because HYPER-CEST is based (2003).
on the detection of the free Xe resonance, not 7. K. Bartik, M. Luhmer, J. P. Dutasta, A. Collet, J. Reisse, 28 June 2006; accepted 29 August 2006
direct detection of the biosensor resonance. J. Am. Chem. Soc. 120, 784 (1998). 10.1126/science.1131847
Wetland Sedimentation from September, 2005, respectively, leaving behind a
devastated urban and rural landscape. Massive
amounts of water, salt, and sediments were re-
Hurricanes Katrina and Rita distributed across the coastal zone within a few
hours as a storm surge of up to 5 m propagated in
R. Eugene Turner,1,2* Joseph J. Baustian,1,2 Erick M. Swenson,1,2 Jennifer S. Spicer2 a northerly direction at the coastline south of New
Orleans, LA (Katrina), and near Sabine Pass,
Texas (TX) (Rita), inundating coastal wetlands in
More than 131 Â 106 metric tons (MT) of inorganic sediments accumulated in coastal wetlands
the region. A thick deposit of mud remained in
when Hurricanes Katrina and Rita crossed the Louisiana coast in 2005, plus another 281 Â 106 MT
these coastal wetlands after the storm waters re-
when accumulation was prorated for open water area. The annualized combined amount of
ceded (Fig. 1). We used this post-storm remnant
inorganic sediments per hurricane equals (i) 12% of the Mississippi River’s suspended load, (ii)
to learn about how coastal systems work.
5.5 times the inorganic load delivered by overbank flooding before flood protection levees were
The loss of LA_s coastal wetlands peaked be-
constructed, and (iii) 227 times the amount introduced by a river diversion built for wetland
tween 1955 and 1978 at 11,114 ha year –1 (1)
restoration. The accumulation from hurricanes is sufficient to account for all the inorganic
and declined to 2591 ha year –1 from 1990 to
sediments in healthy saltmarsh wetlands.
2000 (2). Coastal wetlands, barrier islands, and
norganic sediments accumulating in coastal sions. They may also arrive from offshore during shallow waters are thought to provide some pro-
I wetlands may be delivered from inland tidal inundation or storm events. It is important to tection from hurricanes, by increasing resistance
sources via (i) unconstrained overbank flood- know the quantities delivered by each pathway to to storm surge propagation and by lowering hur-
ing, (ii) explosive releases through unintentional understand how inorganic sediments contribute ricane storm surge height (3). Restoring LA_s
breaks in constructed levees, and (iii) river diver- to wetland stability and to spend wetland res- wetlands has become a political priority, in part
toration funds effectively. Here we estimate the because of this perceived wetland/storm surge
amount of inorganic sediments deposited on connection. A major part of LA_s restoration
1
Coastal Ecology Institute, 2Department of Oceanography
wetlands of the microtidal Louisiana coast effort is to divert part of the Mississippi River
and Coastal Sciences, Louisiana State University, Baton
into wetlands, and at considerable cost Eref.
during Hurricanes Katrina and Rita.
Rouge, LA 70803, USA.
Hurricanes Katrina and Rita passed through (S1) in supporting online material (SOM)^.
*To whom correspondence should be addressed. E-mail:
the Louisiana (LA) coast on 29 August and 24 Widely adopted assumptions supporting this
euturne@lsu.edu
449
www.sciencemag.org SCIENCE VOL 314 20 OCTOBER 2006
REPORTS
2.23 g cm–2 (T1 SD 0 T 3.4; range, 0 to 28.6 g
diversion are that flood protection levees have where water was withdrawn in a southerly
cm–2; n 0 169) and 2.25 g cm–2 in the deltaic
eliminated overbank flooding, which has caused direction out of the wetlands (Fig. 2, D to F).
diminished sediment accumulation and eventual plain. The thickness of the newly deposited The greatest deposition in the Deltatic Plain was
mud was 5.18 cm (T1 SD 0 T 7.7; range, 0 to
wetland loss, and introducing sediments into in the Breton Sound estuary, on the east side of
68 g cm; n 0 186). The thickest newly de-
estuaries via river diversions will enhance wet- the Mississippi River. The marshes within the
land restoration. posited sediments were observed inland of the 4- longitude distance between the two hurri-
If increasing inorganic sediment loading to area of maximum bulk density in eastern LA canes (approximately 300 km) had intermediate
coastal wetlands is important for their restora- but were coincidental with the sediments of rates of deposition. The peak water level, but
tion, then it is important to quantify the major highest bulk density in western LA (Fig. 2C). not sediment accumulation, was higher in west-
sediment pathways. Hurricanes Katrina and The annualized average sediment accumulation ern Lake Pontchartrain during Hurricane Rita
Rita obviously brought some inorganic sedi- from one hurricane was 89% of the average than during Hurricane Katrina because of these
ment into the coastal wetlands, but how much, accumulation in healthy saltmarsh wetlands in differences in wind fields (11). The peak in bulk
the deltaic plain E0.166 g cm–2 year –1 (10)^.
and where was it distributed? A few measure- density was highest on the eastern side of the
Sediment deposition (in grams per cm2) was
ments of the inorganic sediments accumulating center of the storm track.
from a hurricane have been made at specific greatest near the center of the storm track (Fig. There were peaks in sediment deposition
sites along the coast (4–8) Eref. (S7) in SOM^, 2D). The highest values in the Chenier Plain where navigation channels confined the incom-
but until this study there were no coastwide were on the east side of the hurricane path, ing storm surge to a narrow area at Sabine Pass,
data on the inorganic sediments accumulating where counterclockwise winds brought a storm TX, and in the industrial canal by Paris Road,
from hurricanes. Here we show that the dom- surge inland, and were least on the western side, New Orleans, where the Intracoastal Waterway
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
cm3) 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 (T1 SD 0
T 0.35; range, 0 to 1.78 g cm–3; n 0 170
samples); 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 Fig. 1. Examples of sediments deposited by Hurricanes Katrina and Rita. (A) Sand overwash on the
came from where the bottom resistance to the former location of the coastal community of Holly Beach, LA (photo taken 18 November 2005 by
hurricane winds before landfall was the R.E.T.). (B) Mud on the lawn of a St. Bernard Parish subdivision home (photo taken 27 September
greatest: in the shallow water zone immedi- 2005 by M. Collins). (C) Mud on the marsh surface brought by Hurricanes Katrina and Rita (photo
ately offshore of the deposition site. taken 16 November 2005 by J.B.). (D) Recent mud deposit (10.5 cm) accumulated over a root mass
The average dry weight accumulation of the 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).
deposition layer at all sampled locations was
450 20 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org
REPORTS
meets the Mississippi River Gulf Outlet. These annualized deposition from one hurricane (Table 1) (13). The more frequent smaller storms
would be 8.3 Â 106 MT year–1 if all hurricanes
observations are consistent with results from not included in this analysis may also trans-
modeled storm surge velocities (12). brought an equal amount of sediments to these port substantial amounts of inorganic material
The total amount of recently deposited wet- wetlands. If sediments from these hurricanes are (14, 15).
land sediments on the LA coast was calculated deposited in open-water areas at the same rate as The amount of sediments delivered to coast-
using information on the average sediment in wetlands (9), then the pro rata deposition for al wetlands by Hurricanes Katrina and Rita was
accretion and wetland area for each of four to open-water areas is proportional to the open greater than the estimated amounts once flow-
water/wetland area (1) and is equal to 17.8 Â
six subunits of four coastal regions. The min- ing through or over Mississippi River banks.
106 MT year –1, for a combined sediment
imum amount of inorganic sediment brought The suspended sediments overflowing uncon-
deposition of 26.1 Â 106 MT year –1.
in by these two hurricanes was estimated to be fined (natural) levees of the Mississippi River
131 Â 106 metric tons (MT) (9) (Table 1). The in the past century were 4.8 Â 106 MT year –1,
The sediment accumulations in wetlands and
and 1.7 Â 106 MT year –1 in a confined levee
average occurrence of a Category 3 or larger open water were 4.0 and 8.5%, respectively, of
hurricane on this coast was every 7.88 years the average annual suspended sediment load of system with occasional crevasses (Table 1).
the Mississippi River (210 Â 106 MT year –1)
from 1879 to 2005 (9) (table S1). The The Caernarvon Diversion, a restoration project
located downstream from New Orleans, LA,
delivered a 2-year average sediment load of
Fig. 2. Location of re-
0.115 Â 106 MT year –1 (16). One conclusion
cent sediment samples
to be drawn from these numbers is that the
and data arranged by
amount of sediment deposited on these wet-
longitude. (A) Sample lo-
lands from an average Category 3 or larger
cations (red dots) and
hurricane is 1.7 times the amount potentially
the distribution of coast-
available through unconfined overbank flood-
al wetlands in southern
ing, 4.6 times more than through crevasses in
LA (black background).
the unconfined channel, and 72 times more than
The vertical gray arrow
from this river diversion. The combined sedi-
is the crossing location
ment accumulation in wetlands and open water
of Hurricanes Rita (west-
ern LA) and Katrina (east- resulting from an average Category 3 or larger
ern LA). (B) All samples hurricane is 5.5 times larger than the material
(open circles) and sam- delivered by unconfined flow in the Mississippi
ples with a bulk density River and 227 times larger than that delivered
value 91.0 g cm–3 (red by the Caernarvon Diversion.
dots). (C) All samples However, these comparisons are conserva-
(open circles) and sam- tive estimates. Not all of the inorganic sediment
ples with a vertical ac- flowing from rivers and over or through levees
cretion 93 cm (red dots). is deposited onto a wetland. Levees are higher
(D) Accumulation rela- than the surrounding land because inorganics
tive to the longitude of
settle out onto the levees or within the nearby
sample collection (black
marshes. Also, the peak in river heights, and
circles). (E) Bulk density
hence in discharge, occurs in the spring when
relative to the longitude
water levels in the estuary are at their seasonal
of sample collection. (F)
low and wetlands are infrequently flooded (17).
Vertical accretion relative
Sediments that do accumulate are deposited
to the longitude of sam-
close to the diversion. For example, the
ple collection.
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-
Table 1. Estimates of the sediment source pathways for the Mississippi River deltaic plain in
crotidal coastal wetlands, is that most of the
Louisiana.
inorganics accumulating in them went down
the Mississippi_s birdfoot delta before they
Amount (106 MT year –1 )
Sediment source pathways
were deposited during large storms. The es-
Mississippi River discharge into ocean (13) 210
timates indicate that the amount of storm-
One hurricane every 7.88 years (table S1)
transported material is much greater than that
Onto wetland only 8.3
introduced to wetlands from the historical over-
Onto wetland and into open water (9) 25.9
bank flow, from crevasses, or from river diver-
Overbank flooding (before flood protection levees) and into open water (22) 4.79
sions. In particular, hurricanes appear to be the
Crevasses through levees and into open water (22) 1.81
overwhelming pathway for depositing new in-
Caernarvon Diversion
organic sediments in coastal wetlands in west-
Into the estuary (16) 0.115
ern LA, because the few riverine sources bring
Onto wetland (18) 0.06
relatively trivial amounts of inorganic sedi-
451
www.sciencemag.org SCIENCE VOL 314 20 OCTOBER 2006
REPORTS
(contribution no. 58-3, Coastal Studies Institute, Louisiana 20. R. H. Kesel, Environ. Geol. Water Res. 11, 271 (1988).
ments into the marsh. Because hurricanes are so
State Univ., Baton Rouge, LA, 1958). 21. C. M. Belt Jr., Science 189, 681 (1975).
important to the inorganic sediment budget, other
8. J. A. Nyman, R. D. DeLaune, H. H. Roberts, W. H. Patrick 22. Kesel (20) estimated the sediment load in the spring
factors must be considered to understand how to Jr., Mar. Ecol. Prog. Ser. 96, 269 (1992). floods using water records from 1950 to 1983 and
reduce wetland losses and further their restora- 9. Materials and methods are available as supporting determined the amount of sediment that would be
tion. Changes in the in situ accumulation of material on Science Online. available from unconfined overbank flooding if the levees
10. R. E. Turner, E. M. Swenson, C. S. Milan, in Concepts and were not there. The estimate is actually an overestimate
organics, rather than the reduction of inorganic
Controversies in Tidal Marsh Ecology, M. Weinstein, of the amount available, because the artificial constric-
sediments arriving via overbank flooding, are D. A. Kreeger, Eds. (Kluwer, Dordrecht, Netherlands, tion of the river channel throughout the basin raised the
implicated as a causal agent of wetland losses on 2001), pp. 583–595. flood stage for the same-sized discharge event (21).
this coast. This is illustrated by the fact that the 11. The peak water measured by U.S. Geological Survey 23. Supported by the NSF Division of Geomorphology and
water-level gage 30174809020900, at Pass Manchac Land-Use Dynamics (award EAR-061250), the National
soil volume occupied by organic sediments plus
Turtle Cove near Ponchatoula, LA, in the western Lake Oceanic and Atmospheric Administration Coastal Ocean
water in healthy saltmarsh wetlands is 990% Pontchartrain watershed, was about 6 cm higher during Program MULTISTRESS (award no. NA16OP2670), and a
(10) and is certainly the same or higher in Hurricane Rita than during Hurricane Katrina, even Louisiana Board of Regents Fellowship (J.S.S.). We thank
wetlands of lower salinity. This organic portion though the hurricane path was over 300 km further away. J. Gore for assistance in sampling from an airboat; E. Babin
12. Storm surge model results for Hurricanes Katrina and Rita for assistance in finding photographs; H. Hampp, our deft,
plays a major role in wetland soil stability and
are available at the Louisiana State University Hurricane safe, and considerate helicopter pilot; and B. Sen Gupta,
hence in wetland ecosystem health (19).
Center Web sites (http://hurricane.lsu.edu/floodprediction/ N. N. Rabalais, and three anonymous reviewers for
katrina/deadly_funnel1.jpg and http://hurricane.lsu.edu/ constructive reviews of the manuscript.
References and Notes floodprediction/rita24/images/adv24_SurgeTX_LA.jpg).
1. R. H. Baumann, R. E. Turner, Environ. Geol. Water Res. 13. G. J. Chakrapani, Curr. Sci. 88, 569 (2005). Supporting Online Material
15, 189 (1990). 14. R. H. Baumann, thesis, Louisiana State Univ., Baton www.sciencemag.org/cgi/content/full/1129116/DC1
2. T. A. Morton, J. C. Bernier, J. A. Barras, N. F. Ferina, USGS Rouge, LA (1980). Materials and Methods
Open-File Rep. 2005-1215 (2005). 15. D. J. Reed, N. De Luca, A. L. Foote, Estuaries 20, 301 SOM Text
3. G. W. Stone, X. Zhang, A. Sheremet, J. Coastal Res. 44, (1997). Fig. S1
40 (2005). 16. G. A. Snedden, J. E. Cable, C. Swarzenski, E. Swenson, Table S1
4. J. M. Rybczyk, D. R. Cahoon, Estuaries 25, 985 (2002). Estuar. Coastal Shelf Sci., in press. References and Notes
5. M. L. Parsons, J. Coastal Res. 14, 939 (1998). 17. R. E. Turner, Estuaries 14, 139 (1991).
6. D. R. Cahoon et al., J. Coastal Res. 21 (special issue), 280 18. K. W. Wheelock, thesis, Louisiana State Univ., Baton 24 April 2006; accepted 1 September 2006
(1995). Rouge, LA (2003). Published online 21 September 2006;
7. J. P. Morgan, L. G. Nichols, M. Wright, Morphological 19. R. E. Turner, E. M. Swenson, C. S. Milan, J. M. Lee, 10.1126/science.1129116
Effect of Hurricane Audrey on the Louisiana Coast T. A. Oswald, Ecol. Res. 19, 29 (2004). Include this information when citing this paper.
A Combined Mitigation/Geoengineering Increased sulfate aerosol loading of the strato-
sphere may present other risks, such as through its
influence on stratospheric ozone. This particular
Approach to Climate Stabilization risk, however, is likely to be small. The effect of
sulfate aerosols depends on the chlorine loading
(22–24). With current elevated chlorine loadings,
T. M. L. Wigley
ozone loss would be enhanced. This result would
Projected anthropogenic warming and increases in CO2 concentration present a twofold threat, both delay the recovery of stratospheric ozone slightly
from climate changes and from CO2 directly through increasing the acidity of the oceans. Future climate but only until anthropogenic chlorine loadings
change may be reduced through mitigation (reductions in greenhouse gas emissions) or through returned to levels of the 1980s (which are ex-
geoengineering. Most geoengineering approaches, however, do not address the problem of increasing pected to be reached by the late 2040s).
ocean acidity. A combined mitigation/geoengineering strategy could remove this deficiency. Here we Figure 1 shows the projected effect of mul-
consider the deliberate injection of sulfate aerosol precursors into the stratosphere. This action could tiple sequential eruptions of Mount Pinatubo
substantially offset future warming and provide additional time to reduce human dependence on fossil every year, every 2 years, and every 4 years.
fuels and stabilize CO2 concentrations cost-effectively at an acceptable level. The Pinatubo eruption–associated forcing that
was used had a peak annual mean value of
–2.97 W/m2 (20, 21). The climate simulations
n the absence of policies to reduce the mag- technological challenges faced by a mitigation-
I nitude of future climate change, the globe is only approach. were carried out using an upwelling-diffusion
expected to warm by È1- to 6-C over the energy balance model EModel for the Assessment
The geoengineering strategy examined here is
the injection of aerosol or aerosol precursors Esuch
21st century (1, 2). Estimated CO2 concentra- of Greenhouse gas–Induced Climate Change
tions in 2100 lie in the range from 540 to 970 as sulfur dioxide (SO2)^ into the stratosphere to (MAGICC) (2, 25, 26)^ with a chosen climate
parts per million, which is sufficient to cause provide a negative forcing of the climate system sensitivity of 3-C equilibrium warming for a CO2
doubling (2 Â CO2). Figure 1 suggests that a
substantial increases in ocean acidity (3–6). and consequently offset part of the positive
sustained stratospheric forcing of È–3 W/m2 (the
Mitigation directed toward stabilizing CO2 forcing due to increasing greenhouse gas con-
concentrations (7) addresses both problems but centrations (18). Volcanic eruptions provide ideal average asymptotic forcing for the biennial
presents considerable economic and technologi- experiments that can be used to assess the effects eruption case) would be sufficient to offset much
cal challenges (8, 9). Geoengineering (10–17) of large anthropogenic emissions of SO2 on of the anthropogenic warming expected over the
could help reduce the future extent of climate stratospheric aerosols and climate. We know, for next century. Figure 1 also shows how rapidly the
example, that the Mount Pinatubo eruption EJune
change due to warming but does not address the aerosol-induced cooling disappears once the in-
problem of ocean acidity. Mitigation is therefore 1991 (19, 20)^ caused detectable short-term cool- jection of material into the stratosphere stops, as
necessary, but geoengineering could provide ing (19–21) but did not seriously disrupt the might become necessary should unexpected envi-
additional time to address the economic and climate system. Deliberately adding aerosols or ronmental damages arise.
aerosol precursors to the stratosphere, so that the Three cases are considered to illustrate pos-
loading is similar to the maximum loading from sible options for the timing and duration of aerosol
National Center for Atmospheric Research, Post Office Box
the Mount Pinatubo eruption, should therefore injections. In each case, the loading of the strato-
3000, Boulder, CO 80307–3000, USA. E-mail: wigley@
present minimal climate risks. sphere begins in 2010 and increases linearly to
ucar.edu
452 20 OCTOBER 2006 VOL 314 SCIENCE www.sciencemag.org