are small and manageable, cooling the planet by deflecting sunlight would not reduce the carbon dioxide in the atmosphere, and elevated levels of that gas have consequences beyond raising the temperature. One is that the ocean absorbs more carbon dioxide and becomes more acidic as a result. That harms shellfish and some forms of plankton, a key source of food for fish and whales. The fishing industry could be devastated. What’s more, carbon dioxide levels will continue to rise if we don’t address them directly, so any sunlight-reducing technology would have to be continually ratcheted up to compensate for their warming effects.
And if the geoengineering had to stop–say, for environmental or economic reasons–the higher levels of greenhouse gases would cause an abrupt warm-up. “Even if the geoengineering worked perfectly,” says Raymond Pierrehumbert, a professor of geophysical sciences at the University of Chicago, “you’re still in the situation where the whole planet is just one global war or depression away from being hit with maybe a hundred years’ worth of global warming in under a decade, which is certainly catastrophic. Geoengineering, if it were carried out, would put the earth in an extremely precarious state.”
Figuring out the consequences of various geoengineering plans and developing strategies to make them safer and more effective will take years, or even decades, of research. “For every dollar we spend figuring out how to actually do geoengineering,” says Schrag, “we need to be spending 10 dollars learning what the impacts will be.”
Space Shades: Trillions of disks launched into space could reflect incoming sunlight. Pros: Space-based systems don’t pollute the atmosphere. Once in place, they would cool the earth quickly. Cons: The technology could take decades to develop. And launching trillions of disks is fantastically expensive.
To begin with, scientists aren’t even sure that sulfates delivered over the course of decades, rather than in one short volcanic blast, will work to cool the planet down. One key question is how microscopic particles interact in the stratosphere. It’s possible that sulfate particles added repeatedly to the same area over time would clump together. If that happened, the particles could start to interact with longer-wave radiation than just the wavelengths of electromagnetic energy in visible light. This would trap some of the heat that naturally escapes into space, causing a net heating effect rather than a cooling effect. Or the larger particles could fall out of the sky before they had a chance to deflect the sun’s heat. To study such phenomena, David Keith, the director of the Energy and Environmental Systems Group at the University of Calgary, envisions experiments in which a plane would spray a gas at low vapor pressure over an area of 100 square kilometers. The gas would condense into particles in the stratosphere, and the plane would fly back through the particle cloud totake measurements. Systematically altering the size of the particles, the quantity of particles in a given area, the timing of their release, and other variables could reveal key details about their microscale interactions.
Yet even if the behavior of sulfate particles can be understood and managed, it’s far from clear how injecting them into the stratosphere would affect vast, complex climate systems. So far, most models have been crude; only recently, for example, did they start taking into account the movement of ice and ocean currents. Sulfates would cool the planet during the day, but they’d make no difference when the sun isn’t shining. As a result, nights would probably be warmer relative to days, but scientists have done little to model this effect and study how it could affect ecosystems. “Similarly, you could affect the seasons,” Schrag says: the sulfates would lower temperatures less during the winter (when there’s less daylight) and more during the summer. And scientists have done little to understand how stratospheric circulation patterns would change with the addition of sulfates, or precisely how any of these things could affect where and when we might experience droughts, floods, and other disasters.
If scientists could learn more about the effects of sulfates in the stratosphere, it could raise the intriguing possibility of “smart” geoengineering, Schrag says. Volcanic eruptions are crude tools, releasing a lot of sulfur in the course of a few days,