The 1991 volcanic eruption of Mount Pinatubo in the Philippines blasted enough fine particles and sulfur dioxide gas into the atmosphere to envelop the Earth in a high-altitude cloud for the better part of two months. When scientists checked in 1992, they determined that the cloud had deflected enough sunlight to cool the planet by about 1 degree.
Now, with the planet warming inexorably and the threat of long-term climate change looming, some experts are wondering whether the time may have come to deliberately attempt such "solar radiation management." The idea is being investigated by, among others, the National Academy of Sciences, which is conducting research funded by the CIA, NASA and the National Oceanic and Atmospheric Administration. The academy has invited experts to discuss the idea Tuesday.
But even considering such an endeavor raises many practical, economical, political and ethical questions, experts said, including what the affects on global and regional climates would be.
"I'm not in favor of doing it today. I'm agnostic about whether we should ever do it," said Alan Robock, a distinguished professor of environmental sciences at Rutgers University. "We don't have enough information and, in any case, we don't have the technology."
But Robock said additional research should be conducted despite concerns that determining the feasibility of such "geoengineering" might encourage a government or wealthy individual to try it, and could lessen efforts to curb greenhouse gases such as carbon dioxide.
"I don't think you can just ignore it and pretend it doesn't exist, and sometime in the future, when someone becomes scared and wants to do it, they don't even know whether they can do it," he said.
Mount Pinatubo's eruption sent an estimated 20 million tons of sulfur dioxide and ash into the stratosphere, where the sulfur dioxide formed sulfate particles that reflected sunlight back into space.
Some believe this could be done effectively and at relatively low cost by injecting an aerosol of sulfur dioxide or some other gas into the atmosphere over a period of time, either from aircraft or powerful missiles.
Ken Caldeira, a senior investigator for the Carnegie Institution for Science, told a congressional committee in 2009 that such methods are inexpensive, can be deployed quickly, and probably would cool the Earth effectively. He stressed, however, that it is more important to address the root cause of global warming by reducing the production of greenhouse gases. Carnegie will host a panel discussion on the subject Tuesday evening.
But Robock, in an interview, said the effort is "not feasible. No technology exists to do solar radiation management. There are no airplanes, or hoses or missiles that exist to get sulfur up into the stratosphere."
A more limited variation of the idea is to spray sea salt into clouds over the ocean, probably from ships, to form more water droplets in the clouds and make them whiter, said Lynn Russell, a professor of atmospheric chemistry at the Scripps Institute of Oceanography, which is part of the University of California at San Diego. The brighter clouds would reflect more sunlight.
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."