How to Cool the Planet, Manually

Eli is a contributing correspondent for Science magazine.

Over the last 3 years, interest has been growing among climate scientists in radical new schemes to tinker with the planet's temperature or the make-up of the atmosphere. Now, in a new paper, scientists have estimated just how effective these schemes would be.

In a study published today in the journal Atmospheric Chemistry and Physics Discussions, earth systems scientist Tim Lenton of the University of East Anglia in the United Kingdom and a graduate student analyzed 17 schemes for cooling the planet. Roughly half involve changing the reflectivity of the atmosphere or the ground, employing humanmade pollution, machines to alter clouds, or schemes to lighten deserts or city roofs with plastic sheets or white paint. The other half would involve altering Earth's carbon cycle to draw in CO2, either by growing massive amounts of new trees, boosting the growth of carbon-sucking algae at sea, or creating machines that draw down the atmospheric carbon and store it underground.

The most effective cooling technique proposed to date, the researchers found, would be blocking sunlight by spewing billions of tons of sulfur aerosols into the upper atmosphere each year, where they would reflect roughly 1% more energy back into space than the atmosphere already does. Aerosols would be more than twice as effective as another approach: a wild idea to cover every desert with white plastic sheets. Lenton also determined that a scheme to cover cities with white paint to reflect energy back into space would be less than 1/350 as effective as aerosols. And planting trees and plants would have seven times less impact. (Lenton didn't examine the possible side effects of geoengineering schemes, though he will in another paper.)

Lenton's close colleague Andrew Watson of the University of East Anglia, who did not assist on the calculations or writing involved with the research, welcomes the analysis. He notes that several of the proposals might have positive local effects, even if they only battle global warming on a small scale. White roofs, for example, can lower heating bills and offer cooler cities, and a technique to turn carbon crops into charcoal and bury them in the soil can assist farmers by making agricultural land more arable.

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