Localised sunshade could stop Arctic melting

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 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.

Friendly local geoengineer

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.

"People complain about setting a global thermostat," says MacMartin. "That's not the right way to think about it. It's an engineering problem."

Tim Lenton of the University of Exeter, UK, points out that current climate models are nowhere near good enough to make such conclusions. While they do a reasonable job of predicting temperature changes, they give wildly differing predictions for rainfall. As a result, it's not clear what geoengineering needs to achieve in terms of precipitation.

Lenton is particularly sceptical about the claims for restoring Arctic sea ice, as the model MacMartin used distorts the geography of the North Atlantic to simplify its calculations.

Geopolitical nightmare

MacMartin admits that his simulations are "idealised". In the model, he simply turned down the sunlight over different regions of the Earth. In reality, aerosols move around so this level of control may not be possible.

Nevertheless, his results open the possibility of a more sophisticated form of geoengineering. By combining different techniques, it may be possible to tailor the future climates of different continents, although MacMartin cautions that any change in one region will have knock-on effects elsewhere.

This will make it harder for countries to agree what to do. Lenton points out that nations would have to decide between them which aspects of the global climate to preserve. "Is protecting the sea ice more important than protecting the Indian monsoon?" he says. "Who decides? It inevitably turns into a power game."

In an attempt to reduce the inequalities caused by geoengineering, Lenton argues, MacMartin has uncovered even more complicated inequalities that, if anything, make the geopolitical problem worse.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate1722

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