Pulling the shade.
Some scientists want to combat global warming by reflecting more of the sun's light back into space, perhaps by making the atmosphere hazier.

No Easy Fix for Global Warming

Some scientists argue that the best way to combat global warming is to tinker with Earth's atmosphere so it reflects more sunlight and keeps the planet cooler. But a new study suggests that although such "geoengineering" would be feasible, it would also scramble rainfall patterns and lead to a far more rapid increase in temperatures if the effort were ever halted.

For the past century or more, average atmospheric temperatures have been rising, largely because of the buildup of heat-trapping carbon dioxide from the burning of fossil fuels. Air temperatures already are 0.74 degrees Celsius higher than they were a century ago and likely will climb another 1.7 to 4.0 degrees by the end of this century, according to a recent report by the Intergovernmental Panel on Climate Change (Science, 9 February, p. 754). That could lead to rising seas, droughts, and myriad other environmental problems, so researchers and politicians are scrambling to find ways to slow atmospheric warming, primarily by limiting CO2 emissions.

Others have suggested another option: geoengineering the atmosphere to make it more reflective (Science, 20 October 2006, p. 401). Proposals include seeding the clouds with tiny sulfate particles and even placing arrays of large mirrors in orbit. Such measures might be easier and quicker than restricting emissions, proponents argue.

Geoengineering could indeed cool the atmosphere, ecologist Ken Caldeira of the Carnegie Institution in Stanford, California, and colleagues conclude in their new analysis. The team examined the impact of 11 possible projects over the next century using computer simulations and assuming trends in greenhouse-gas emissions will continue unchecked. The good news is such measures would be effective even if undertaken decades from now, the researchers report online this week in the Proceedings of the National Academy of Sciences. The bad news is that in all cases studied, reducing solar radiation would also shift global rainfall patterns, potentially drenching some areas and parching formerly productive agricultural land. Worse, the simulations predict that if the atmospheric fiddling suddenly stopped, the warming would accelerate dramatically--possibly to 20 times the current rate--because CO2 would still be accumulating.

The study underscores the fact that "we may face new surprises if we try to engineer the climate,” says Uma Bhatt, a geophysicist at the University of Alaska, Fairbanks. Robert Charlson, an atmospheric scientist at the University of Washington, Seattle, points to another hazard in current geoengineering proposals: The schemes would cool the planet uniformly, but the local warming from greenhouse gases varies depending on the distribution of surface temperatures, clouds, and humidity. So, even if geoengineering works as planned, Charlson says, "you could end up keeping the average temperature 'right' and still melt all the ice on Greenland."

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