Others are beyond our powers today. To shade Earth with a swarm of space parasols would require an estimated 20 million rocket launches. Without some radical new technology, that would be astronomically expensive and fatally polluting. “This is complete science fiction,” says Tim Lenton of the University of Exeter in England. “We ought to stop talking about it.”
Many other schemes, such as painting roofs white, are certainly feasible, but can they actually fix the climate? The basic problem, of course, is that rising levels of greenhouse gases in the atmosphere are acting like a blanket around Earth, trapping heat. To stop the warming, any geoengineering scheme either has to block an equivalent amount of incoming heat from the sun or increase heat loss from the top of the atmosphere by that much. It also needs to work without drastically altering regional climates, while also preventing sea levels from rising. Ideally it should also stop the oceans from becoming so acidic that coral reefs vanish.
Does it make a difference?
But the first test is potency. In 2008, Lenton and Nem Vaughan of the University of East Anglia in England combined various model results with their own calculations to assess the potential cooling power of a couple of dozen proposals. “It was born of frustration,” Lenton says. “I had been at one too many workshops where people were advocating their pet technologies and arm-waving about ‘was this more effective than that?’ ”
They found that many schemes would make little difference. Take the idea of making roofs and roads whiter to reflect more sunlight. Even with optimistic assumptions, this could make at best only a minor contribution to restoring Earth’s heat balance.
A seemingly more promising plan is to fertilize the seas. Plankton consume CO2 as they grow, and sometimes their dead bodies sink to the sea floor and get buried, locking this carbon away. Adding nutrients that are in short supply, such as iron, could boost plankton growth. By the end of the century, this could improve the radiation balance by more than the white roofs, Lenton and Vaughan calculated, but that’s nothing like a game-changer — and, again that’s the top-end estimate, which could fall considerably as we learn more about this process.