That method would allow a single region of the Earth to be cooled and, if something went wrong, it could be halted more quickly, Russell said, adding that it would be less effective at cooling the entire planet.
A 2010 Government Accountability Office report showed how little research has been conducted on solar radiation management, noting that all but one analysis of climate impacts relies on computer models. Only a 2009 study by Russia injected particles into the atmosphere to study their reflective quality.
That leaves scientists uncertain about how a major solar radiation management effort would affect rainfall, crop growth and ocean life, among other things. How quickly would the planet heat up again when the effort ended, or would it go on indefinitely? How badly would oceans acidify when sulfur fell from the skies as acid rain?
There is an “underlying feeling that any time we toy with complex systems that unexpected things will happen,” Caldeira said.
It may be more difficult for nations to come to an agreement on an effort that could affect them in different ways. Would India stand by if the United States was readying a project that could affect the monsoon rains on which Indian agriculture depends? Would the United States allow China to try something that it believed would harm North America?
Chris Field, director of Carnegie’s global ecology department, said there is no agreed-upon institution to oversee such an effort, and no rules to govern it. A wealthy individual or corporation could try something over the ocean with no regulation, he said.
“Whose hand would be on the thermostat of the globe?” Robock asked. “How do you decide what temperature you want the planet to be?”
Some people may have ethical qualms about any attempt to alter natural patterns on a global scale, the scientists said. But Russell noted that we are already doing that.
“We’ve spent the last 100 years putting carbon dioxide into the atmosphere and seeing how much we can warm the planet,” she said. “It’s a huge geoengineering experiment, because we’re changing the climate.”