Sea ice can take many forms, as seen in this image of Arctic sea ice from a recent Operation IceBridge aerial survey. Varying thicknesses of sea ice are shown here, from thin, nearly transparent layers to thicker, older sea ice covered with snow. NASA via nsidc.org

By Michael Marshall
21 October 2012 If we have to hack the planet, we could at least do it with some finesse. Some of the problems with geoengineering could be fixed by targeting specific regions of the planet, rather than cooling everywhere equally. A rough modelling study published in Nature Climate Change offers a crude blueprint for how to save the Arctic ice cap, but raises questions about who decides which areas to save. Devised to stop climate change if attempts to cut greenhouse gas emissions fail or are insufficient to cool the planet, geoengineering schemes range from sucking carbon dioxide out of the air to “sunshades” that block incoming sunlight. Sunshades produced by injecting aerosols into the stratosphere have received the most attention as they could be a cheap way to cool the planet. But they are far from perfect. Models show that no sunshade can restore both temperature and rainfall to their preindustrial levels. What’s more, while average global temperatures could be restored, there would be regional differences. As a result, nations would probably disagree over how much geoengineering to do. In a bid to reduce these trade-offs, Douglas MacMartin of the California Institute of Technology in Pasadena and colleagues simulated a more regional approach. They set up a simple climate model in which carbon dioxide levels were doubled, which is expected to happen in the second half of this century. Next they modelled three different geoengineering scenarios: a uniform global sunshade, one sunshade over each pole, and a whole-planet sunshade that was thickest over the North Pole and gradually thinned as it spread over the equator and southern hemisphere. They also varied the time of year the sunshades were up. The team found that the regional geoengineering approaches gave better results. A uniform global sunshade left some regions sweltering while others were too cold. But with the regional solutions, these disparities were reduced. To see if it was possible to restore the Arctic sea ice, which is rapidly melting, MacMartin tried switching a sunshade on above the Arctic during summers only. He found this could restore the Arctic sea ice to its preindustrial extent, while also bringing average global temperatures and rainfall close to their original levels. […]

Localised sunshade could stop Arctic melting