The other, more publicly acceptable form of geo-engineering would focus on removing carbon from the atmosphere and storing it underground. Known as carbon capture and storage (CCS), this idea is behind today's experimental clean-power plants. But clean-coal plants will only reduce future emissions, which does not address the root of the problem. Among all the uncertainties that still surround climate change, one thing has become clear: the scary durability of carbon, which will hang in the air for a thousand years, continuing to warm the planet no matter how drastically future emissions are cut.

Geo-engineering laboured on the lunatic fringe of climate policy until recently. Experts shunned its ideas as mad science, and for fear that it would undermine the campaign to cut carbon emissions. Now, as more and more climate specialists come to believe that even current levels of carbon pollution are warming the globe more rapidly than previously thought, the case for developing an emergency earth-rescue plan is getting difficult to resist. Nobel laureates Paul Crutzen and Thomas Schelling have endorsed the need for a climate-engineering plan.

The most eloquent argument for geo-engineering as a Plan B is the failure of Plan A—emissions cuts. The Kyoto agreement calls for a 5.2 percent reduction of emissions below 1990s levels by 2012. Of the 40 countries that signed the agreement in 2001, 21 have seen carbon emissions increase since then. The campaign to expand and tighten Kyoto now looks likely to fall short, too. No one is certain how sharp the cuts in greenhouse-gas emissions need to be, but the best guess is a reduction of 80 per cent below current levels by 2050. That’s a radical goal.

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