Some climate experts are now willing to consider schemes for partly shielding the planet from the sun's rays, such as putting sulfer dioxide in the stratosphere to scatter sunlight; using 1,500 ships, each spraying eight gallons of sea water a second to whiten existing marine clouds (and reflect more light); and a space-based sunshade at L1, the inner Lagrangian point, using solar sails with tiny mirrors to deflect light.

Geoengineering schemes fall into two categories, corresponding to the two knobs you might imagine twiddling to adjust the earth's temperature.

One knob controls how much sunlight—or solar energy, to be more precise—reaches the planet's surface; the other controls how much heat escapes back into space, which depends on how much CO₂ is in the atmosphere.

Schemes for removing CO₂ from the atmosphere, say, by fertilizing the oceans with iron, would strike closer to the root of the problem. But they would inevitably take decades to have much of an effect.

In contrast, a sunshade could, in principle, stop global warming immediately—albeit only for as long as it was maintained. Sunshade ideas thus address what some scientists see as the extreme urgency of the climate problem.

The geoengineering scheme Crutzen and Wigley both defend is the cheapest and most certain to work; it was proposed as long ago as 1974 by the late Russian physicist Mikhail I. Budyko, then at the Main Geophysical Observatory in Leningrad. The idea is to inject several million tons a year of sulfur dioxide (SO₂) into the stratosphere.

There it would react with oxygen, water and other molecules to form minute sulfate droplets made up of water, sulfuric acid (H₂SO₄) and whatever dust, salt or other particles onto which the acid and water condense.

Clouds of sulfate droplets would scatter sunlight, making sunsets redder, the sky paler and the earth's surface, on average, cooler—everyone agrees on all that. In 1991 the volcanic erup­tion of Mount Pinatubo in the Philippines put 20 million tons of SO₂ into the stratosphere, and it had all those effects: it cooled the earth by nearly one degree Fahrenheit for about a year.

"So we basically know it works," Caldeira says. In fact, Caldeira started modeling the idea nearly a decade before Crutzen wrote about it.

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