Wood's proposal was not technologically complex. It's based on the idea, well-proven by atmospheric scientists, that volcano eruptions alter the climate for months by loading the skies with tiny particles that act as mini-reflectors, shading out sunlight and cooling the Earth. Why not apply the same principles to saving the Arctic? Getting the particles into the stratosphere wouldn't be a problem -- you could generate them easily enough by burning sulfur, then dumping the particles out of high-flying 747s, spraying them into the sky with long hoses or even shooting them up there with naval artillery. They'd be invisible to the naked eye, Wood argued, and harmless to the environment. Depending on the number of particles you injected, you could not only stabilize Greenland's polar ice -- you could actually grow it. Results would be quick: If you started spraying particles into the stratosphere tomorrow, you'd see changes in the ice within a few months. And if it worked over the Arctic, it would be simple enough to expand the program to encompass the rest of the planet. In effect, you could create a global thermostat, one that people could dial up or down to suit their needs (or the needs of polar bears). [...]

To his colleagues, Crutzen's willingness to consider deliberate intervention with the planet's climate is a sign that the debate over global warming has changed. "Here is a guy who knows more about the Earth's atmosphere than anyone else alive, and he's telling us that the situation is so dire we need to think about intervening with the atmosphere on a planetary scale," one climate scientist told me. "That's frightening, of course -- but from a purely scientific point of view, it's also very interesting." [...]

"In effect, we're already engineering the climate," says Ken Caldeira, "We just don't want to admit it. You can argue that the only real difference between what we're doing today and what geoengineering advocates are proposing is a matter of intention. And frankly, the atmosphere doesn't care about what's going on in our heads."