The irony is that the more respectable geo-engineering option, carbon capture, is also by far the more expensive and less likely to counteract a steep rise in temperatures. Each year roughly 30 billion metric tonnes of carbon dioxide are released by the world's industries and autos. If converted to liquid form, it would take less than four years to fill an underground space with the volume of Lake Geneva. And that doesn't take into account the 1.8 percent yearly rise in emissions, or the billions of tonnes of carbon dioxide that have already accumulated in the atmosphere for the past 100 years. Scientists still think there’s enough porous rock deep beneath the earth’s surface to accommodate all the liquid carbon dioxide we can pump, but getting it there would take many years and cost billions. Assuming the cost of removing carbon eventually falls to $50 a tonne (it now costs $200 per tonne), the bill for removing only the current year’s emissions would reach $150 billion.

Geo-engineering, say critics, would create many nasty side effects. One of the drawbacks to SO2 is that it destroys the ozone layer, exposing people in the Southern Hemisphere to deadly ultraviolet radiation. One way around the problem is to release SO2 a bit at a time, study how the climate responds and try a bit more.

David Keith, a physicist at the University of Calgary, is working on designing particles that are more efficient at cooling than sulphates, but without the side effects. Because sulphates tend to settle on the ground after a few months, they’d need to be replenished regularly. Keith’s engineered particle would absorb the sun’s energy unevenly, causing one side to heat more quickly than the other and to drift upward. Such a particle might be released on the ground and rise up on its own accord. It might be built in such a way that it rises higher than the ozone layer—to the mesosphere, 100 km up—where it would reflect light but leave the ozone intact. For safety reasons, these particles could also have preordained lifetimes. “It’s something we’re developing,” he says. “It may not work. But it’s almost certain that if engineers put their minds to this, something could be made to work better than sulphates.”
_FRED GUTERL —with Michael Levitin & Sangwon Yoon, Newsweek