The best-studied proposal [for climate geoengineering], to pump sulfate aerosols into the upper atmosphere to block sunlight, would cause its own troubles. The sulfates would slow or reverse the recovery of the ozone layer; they might also reduce global rainfall, and the rain that did fall would be more acidic. And those are just the foreseeable effects. Aerosols are the least understood aspect of the climate system.

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The possibility that international collective action might not be entirely reliable brings up the fourth and perhaps most intractable barrier to geoengineering: the geopolitics. Imagine if, say, Chinese-produced clouds of sulfuric acid blew across the Pacific or if American efforts to reduce flooding on our shores triggered drought in Central Asia. How would nations respond to such provocations as anything but an act of war?

High cost, unintended consequences, uncertainty, short attention spans, international bickering: if these problems sound familiar, it is because climate skeptics have made the very same criticisms of plans to cut emissions, such as the Kyoto Protocol. The difference is that geoengineering is even worse. Emissions cuts may be challenging, but the science is well established, most of the technology already exists, the costs can be spread over the natural capital-replacement cycle, public awareness is high, and international institutions such as carbon markets are taking root. The time to act is now.