Science

An Emergency Cooling System for the Planet

Can geoengineering save us from global warming?

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Last week, the American Enterprise Institute (AEI) held a conference that asked if geoengineering was a feasible solution to lower our planet's temperature, at least temporarily. The question is what to do if man-made global warming turns out to be a serious problem? At AEI, climatologist Tom Wigley from the National Center for Atmospheric Research in Colorado defined geoengineering as the deliberate modification of the earth's short wave radiation budget in order to reduce the magnitude of climate change. In his presentation, Wigley looked mostly at two possible approaches to geoengineering: injecting sulfate or other aerosols into the stratosphere, and changing the reflectivity of clouds.

Why consider geoengineering in the first place? As Columbia University economist Jeffrey Sachs wrote in Scientific American in April: "[O]ur current technologies cannot support both a decline in carbon dioxide emissions and an expanding global economy. If we try to restrain emissions without a fundamentally new set of technologies, we will end up stifling economic growth, including the development prospects for billions of people."

So if we don't want to perpetuate poverty in the name of preventing climate change, geoengineering may be our way out. Why? Because geoengineering would provide more time for the world's economy to grow while inventors and entrepreneurs develop and deploy new carbon neutral energy sources to replace fossil fuels. Wigley also noted that cutting greenhouse gas emissions is a tremendous global collective action problem. It seems unlikely that fast-growing poor countries like India and China will agree cut back on their use of fossil fuels any time soon. If that's the case, then emissions reductions in rich countries would have almost no effect on future temperature trends. Geoengineering could give humanity more time to resolve this collective action problem, too.

So let's take Wigley's second proposal first—changing the reflectivity of clouds. Researchers know that this can be done because it already happens with ship tracks. Ship exhaust over the oceans injects particles into the atmosphere that serve as cloud condensation nuclei, creating clouds in the wakes of ships. Ship exhaust produces and brightens clouds so that they cool the planet by reflecting sunlight back into space, but only by a little bit. However, recent modeling research by University of Edinburgh engineer Stephen Salter and his colleagues calculates that doubling the number of cloud condensation nuclei would more than compensate for any warming associated with a doubling of carbon dioxide in the atmosphere. This could be accomplished by having ships deliberately inject seawater into the atmosphere where salt particles would serve as extra cloud condensation nuclei.

In 2006, Chemistry Nobelist Paul Crutzen proposed injecting sulfate particles into the stratosphere to reflect some sunlight back into space (an idea discussed by reason contributor Gregory Benford more than ten years ago). This might be done with giant cannons. Crutzen argues that it would cost between $25 and $50 billion per year to shoot enough sulfate particles into the stratosphere to reduce incoming sunlight by 1.8 percent. This would be enough to counter the predicted warming produced by doubling atmospheric carbon dioxide. An earlier study by Yale University economist William Nordhaus estimated that the sulfate injection proposal would cost about $8 billion per year. This compares nicely with the $125 billion per year Nordhaus calculated it would have cost the U.S. to implement the Kyoto Protocol.

Wigley spent most of his time at AEI discussing the possible risks involved with the sulfate injection proposal. Wigley argued that sulfates injected into the stratosphere would be equal to only about 10 percent of those humanity already injects into the lower atmosphere, so this wouldn't greatly boost acid rain. In April, a study by some of Wigley's National Center for Atmospheric Research colleagues found that injecting sulfates would further deplete the ozone layer that shields the earth's surface from damaging ultraviolet light. Wigley simply noted in passing that even more recent research suggests that the damage to the ozone layer will be less than the April study estimated.

Stratospheric sulfate injection might also change rainfall patterns, perhaps reducing precipitation from the monsoons on which millions of Asian farmers are dependent. In response to these worries, Wigley noted that stratospheric sulfates might reduce the intensity of monsoons by two to three percent which contrasts with a current monsoon variability of 30 percent.

But one big problem that sulfate injection would not solve is the continuing acidification of the ocean that is occurring as extra carbon dioxide from the atmosphere dissolves into the seas. This acidification could eventually pose problems for creatures such as mollusks and corals that use calcium carbonate to grow their shells and skeletons.

What is the safe level at which to stabilize carbon dioxide? The current greenhouse gas concentrations are equivalent to 385 parts per million (ppm) of carbon dioxide, up 100 ppm over pre-industrial levels. In the past some researchers suggested that stabilizing concentrations at 550 ppm would avoid the most serious effects of global warming. Now other researchers are arguing that we have to get back to 350 ppm. Wigley sees no signs that humanity is on a track to stabilize carbon dioxide concentrations at 550 ppm. Consequently, he believes that we will have to resort to geoengineering as a way to buy the time humanity needs to figure out how to cut carbon dioxide emissions. He foresees an effort to ramp up stratospheric sulfate injection over 75 years to counter the climatic effects of rising carbon dioxide concentrations.

Stabilization can only be achieved by cutting current carbon dioxide emissions by 80 percent. This means implementing highly unpopular policies of carbon rationing and higher energy prices. So some climate change researchers and environmental activists worry that the public and policymakers will see geoengineering as way to avoid making hard decisions. "If humans perceive an easy technological fix to global warming that allows for 'business as usual,' gathering the national (particularly in the United States and China) and international will to change consumption patterns and energy infrastructure will be even more difficult," writes Rutgers University environmental scientist Alan Robock.

Perhaps. But that is not an argument against pushing ahead with a vigorous research program on geoengineering responses to climate change. Insisting on cuts in carbon dioxide emissions is like trying to require a healthy diet and exercise regimen to prevent heart disease. But when you have a heart attack, you are happy to have a bypass surgeon handy.

Ronald Bailey is reason's science correspondent. His book Liberation Biology: The Scientific and Moral Case for the Biotech Revolution is now available from Prometheus Books.