Solar geoengineering: weighing costs of blocking the sun’s rays

Basic physics shows that warming from sunlight boosts the planet's water cycle more than warming from carbon dioxide. This is because sunlight adds more energy to the system, like turning up the heat on a stove under a pot of water, while carbon dioxide simply puts a lid on the pot. So counteracting greenhouse warming by reducing sunlight would likely make the planet drier - models predict a 1 percent reduction in rainfall for every degree Celsius of warming counteracted, says Axel Kleidon of the Max Planck Institute for Biogeochemistry in Jena, Germany. "When you try to fix one problem you create other problems," says Kleidon, who opposes pursuing such techniques.

The Geoengineering Model Intercomparison Project (GEOMIP) recently looked at how solar alterations might affect regional climate patterns. It concluded that the tropics would be over-cooled and the poles under-cooled. While the project concluded that solar radiation management would likely protect more than 90 percent of the globe from the serious changes in rainfall predicted from climate change, summer monsoons might start to dry up, requiring a change in agriculture or water storage systems to adapt to the new climate.

The upshot is that things would get better for some people and worse for others, though the details are hard to predict. Rice production might go down in China because of water cycle changes for example, but could increase because of increased carbon dioxide to feed the plants, says Caldeira. Despite the drop in sunshine, crop productivity would probably increase worldwide because of higher atmospheric concentrations of CO2.

Because sulfates only remain in the atmosphere for a few years, a geoengineering program could be stopped at any time if unanticipated disaster ensues. But then the temperature would race upward as the planet readjusts to the amount of carbon dioxide in the air. If geoengineering were used to counteract 2 degrees C of warming over 50 years, for example, that 2 degrees of warming would come back quickly once the geoengineering stops. And there is no governance system at present to oversee if and how a program should start or stop. A group called The Solar Radiation Management Governance Initiative has held a few workshops in recent years to discuss these questions.

Such concerns have led some people to take a hard-line stance against any kind of geoengineering. This started with environmental organizations likeETC Group protesting against iron fertilization of the seas - an idea meant to stimulate phytoplankton growth and so suck up carbon dioxide from the air, which controversially interferes with the base of the ocean's food chain. This led to a 2008 Convention on Biological Diversity moratorium against iron fertilization, which in 2010 was expanded to any geoengineering.

These agreements are non-binding, but still have influence, even on apparently harmless experimentation. When the Stratospheric Particle Injection for Climate Engineering (SPICE) research group attempted to run a small field experiment in 2011 to spray water into the air from balloons as a way of testing a stratospheric delivery system, protestors forced the group to stop. This irritates Caldeira. "I think it's very dangerous to tell scientists that an experiment with no risk in itself cannot be performed because we don't like what it might lead to," he says.

To date, no solar geoengineering field trials have taken place, aside from a study or two looking at the idea of seeding more reflective clouds. Keith argues that some experiments would be good to test the risks and efficacy of such a strategy, and he has proposed a meeting of researchers to hash out a list of suitable work that should be done. He and his colleagues are currently planning experiments that would inject less than a hundred kilograms of aerosol material into the stratosphere in order to investigate some of the ensuing chlorine chemistry. They haven't yet gotten to the stage of an official proposal.

But Robock argues that while modeling and indoor experiments should be pursued, outdoor field trials are problematic. "You can't see a climate response unless an experiment is so large as to actually be geoengineering," Robock says.

Keith concludes that it "makes sense to move with deliberate haste towards deployment of geoengineering," so long as early work supports the theoretical promise of the technique. Caldeira is less bullish, saying, "Climate change is not going to extinguish us as a species. Geoengineering will always be a decision, not a necessity."