Costs, benefits, and harms associated with geoengineering must be assessed before it is used to mitigate climate change.

Whither Geoengineering?

Alan Robock

According to the Inter-governmental Panel on Climate Change (IPCC) (1), global warming will soon have severe consequences for our planet. The IPCC also estimates (2) that mitigation would only cost ~0.1% of the global gross national product per year for the next 30 years, a price far smaller than the damage that would occur. As a potential route to mitigation, the old idea of "geoengineering" has gotten much attention in the last 2 years (3, 4). On page 1201 of this issue, Tilmes et al. (5) quantify the effects of one geoengineering approach- the introduction of additional aerosols into Earth's strato-sphere, akin to a volcanic eruption-on high-latitude stratospheric ozone concentrations.

Geoengineering involves trying to reduce the amount of sunlight reaching Earth's surface to compensate for the additional long-wave infrared radiation from greenhouse

gases, thereby reducing or reversing global warming (6). Even if it works, there are prob-

lems with this approach (7). If perceived to be a possible remedy for global warming, it

would reduce societal pressure to reduce greenhouse gas emissions. It could reduce

overall precipitation, particularly Asian and African summer monsoon rainfall, threaten-

ing the food supply of billions. It would allow continued ocean acidification, because some

of the carbon dioxide humans put into the atmosphere continues to accumulate in the

ocean. Weather modification could be used as a weapon (8), thus violating the 1977 U.N.

Convention on the Prohibition of Military or Any Other Hostile Use of Environmental

Modification Techniques. There would be rapid warming if geoengineering stopped sud-

denly. If geoengineering worked, whose hand would be on the thermostat? How could the

world agree on an optimal climate?

Nevertheless, for some schemes, the benefits may outweigh the problems, especially if

used on a temporary basis. To date, only some schemes have been investigated in detail.

Furthermore, proponents of geoengineering, especially the fossil fuel industry, will con-

tinue to push for its use. Sunshades in orbit around Earth (9) or cloud seeding to brighten them (10) have been proposed, but most geoengineering ideas focus on emulating explosive volcanic eruptions by injecting SO2 or H2S into the stratosphere, producing a sulfuric acid cloud to scatter solar radiation back to space and cool the planet.

Deciding whether this is a good idea or not requires detailed analysis of the costs, benefits,

and harm to the planet that such a strategy would entail, and comparison to the same met-

rics for mitigation and sequestration. Given the need for rapid mitigation, these ideas need

rapid and thorough investigation. It has been suggested (3, 4) that the cooling of the global climate for a couple years after large volcanic eruptions-like the 1991 Mount Pinatubo eruption-serves as an innocuous model for what humans could do by creating a permanent stratospheric aerosol layer. However, volcanic eruptions actually serve as a warning about geoengineering: They produce drought (11), hazy skies, much less direct solar radiation for use as solar power, and ozone depletion (12).

We now have an ozone hole over Antarctica every spring because the polar stratospheric clouds that form there serve as surfaces for heterogeneous chemistry that releases chlorine, which then catalytically destroys ozone. Polar stratospheric clouds only form when the

temperature falls below ~195 K, but additional sulfate aerosols provided by geoengineering or volcanic eruptions alter these temperature restrictions and provide more surface

area for the chemistry, allowing more chlorine to be activated and more ozone to be destroyed.

Advocates of geoengineering suggest that this ozone problem would not be important,

because the stratospheric concentration of chlorine is slowly decreasing as a result

of global environmental agreements (13). However, Tilmes et al. show that even with

the projected chlorine declines, ozone depletion (and increased ultraviolet flux) would be

prolonged for decades by geoengineering of the stratospheric sulfate layer. In their model,

the effects would occur every spring in the Southern Hemisphere and in most springs in

the warmer Northern Hemisphere. The presence of sulfate aerosols would raise the tem-

perature needed for chlorine activation over 200 K, expanding both vertically and hori-

zontally the regions of polar ozone depletion.

A U.S. Department of Energy white paper (14) in October 2001 recommended a $13

million/year national geoengineering research effort, but the paper was never released.

According to the paper, "any effort to deliberately moderate or ameliorate threats that may

arise or become more likely as a result of climate change should be undertaken only in

extraordinary circumstances.... In view of the risk of significant consequences to society

and the environment from either inaction or ....?....poorly understood actions, research should be initiated now to examine possible options to moderate adverse climate threats; to ensure that these options are effective, affordable, reversible and sustainable."

It is not too late to make up for lost time, but further delay must be avoided. A research program, more generously funded than that proposed in 2001, supported by the U.S. federal government with international cooperation, will allow us to compare the efficacy, costs, and consequences of the various options of responding to global warming-mitigation, sequestration, geoengineering, or doing nothing-so that an

informed public can agree on the best courses of action.

References and Notes

1. IPCC, Climate Change 2007: The Physical Science Basis.

Contribution of Working Group I to the Fourth

Assessment Report of the Intergovernmental Panel on

Climate Change, S. Solomon et al., Eds. (Cambridge

Univ. Press, Cambridge, UK, and New York, NY, 2007).

2. IPCC, Climate Change 2007: Mitigation. Contribution of

Working Group III to the Fourth Assessment Report of the

Intergovernmental Panel on Climate Change, B. Metz, O.

R. Davidson, P. R. Bosch, R. Dave, L. A. Meyer, Eds.

(Cambridge Univ. Press, Cambridge, UK, 2007).

3. P. J. Crutzen, Climatic Change 77, 211 (2006).

4. T. M. L. Wigley, Science314, 452 (2006).

5. S. Tilmes, R. Müller, R. Salawitch, Science320, 1201

(2008).

6. I use "geoengineering" to refer to schemes designed to

reduce solar radiation input to the climate system; I

exclude the broader meaning that includes sequestra-

tion of atmospheric carbon dioxide, for example, by

iron fertilization of the oceans [an idea that has been

shown to be premature (15)], afforestation, and

reforestation.

7. A. Robock, Bull. Atomic Scientists64(2), 14 (2008).

8. J. R. Fleming, Wilson Q.2007, 46 (spring 2007).

9. R. Angel, Proc. Nat. Acad. Sci. U.S.A.103, 17184

(2006).

10. K. Bower, T. Choularton, J. Latham, J. Sahraei, S. Salter,

Atm. Res. 82, 328 (2006).

11. K. Trenberth, A. Dai, Geophys. Res. Lett. 34, L15702,

10.1029/2007GL030524 (2007).

12. S. Solomon, Rev. Geophys. 37, 275 (1999).

13. L. Lane, K. Caldeira, R. Chatfield, S. Langhoff, Eds.,

Workshop Report on Managing Solar Radiation,

NASA/CP-2007-214558(NASA, Ames Research Center,

Moffett Field, CA, 2007).

14. E. Khan et al., Response Options to Limit Rapid or Severe

Climate Change(Department of Energy, Washington, DC,

2001).

15. K. O. Buesseler et al., Science319, 162 (2008).

16. I thank R. Salawitch, S. Tilmes, G. Stenchikov, and A.

Marquardt for valuable suggestions. Supported by NSF

grant ATM-0730452.10.1126/science.11592801167CREDIT: NASA/CXC/MIT/UNIV. OF MASSACHUSETTS AMHERST/M. D. STAGE ET AL.

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