But what action should be taken, based on this knowledge? That was one of the knotty questions he and other experts wrestled with at a two-day conference that ended here on Friday.
Dr. Keith, an organizer of the conference, said that at one time he thought scientists should not talk in public about “geoengineering” remedies for global warming — like injecting chemicals into the upper atmosphere to cool the poles, or blocking sunlight by making clouds more reflective or stationing mirrors in space.
Like many other researchers, he explained, he worried that the potential for a climate fix, even an imperfect one, would only encourage people to continue the profligate burning of fossil fuels that got the planet into trouble in the first place.
As a result, though geoengineering is much talked about in the blogosphere, “it has been too little discussed in the broad science community,” said Daniel Schrag, a climate scientist at Harvard who was another organizer of the conference. It was held at the American Academy of Arts and Sciences and attended by dozens of experts from around the world in chemistry, economics, human behavior, oceanography and other fields.
Developing artificial techniques to cool the earth “is going to dampen the fervor for mitigation to a certain extent for some people,” said Thomas Homer-Dixon of the University of Toronto, an expert on how societies adapt to economic and ecological change.
On the other hand, he said in an interview, a serious discussion of geoengineering might finally sound an audible alarm for others. “When people in the general public realize that serious scientists are thinking seriously about doing this,” he said, “they are going to say, ‘O.K., we really need to look for alternatives.’”
Another issue, as several conferees put it, are the “unknown unknowns” — consequences of planetary engineering that cannot be anticipated but may be serious or even devastating.
“Will there be some consequences you don’t like?” Dr. Keith said in an interview. “Of course.”
Also, as several researchers noted, engineering a cooler earth would do nothing for other climate-related problems, like the way accumulating carbon is making the oceans more acidic.
And who should decide what action should be taken or when?
“I have no idea,” Dr. Keith replied. But just as international organizations were formed to regulate the use of radio frequencies, organize air traffic control, track space debris and deal with other problems, it might be possible to create an international organization to deal with these questions, he said.
“We are backing our way into global governance, very slowly,” he said.
On one subject, though, there was wide agreement: interest in geoengineering is no longer merely theoretical. The participants in the conference noted that global emissions of greenhouse gases were already moving above the upper limits predicted by many climate models. As a result, several said, the projected arrival of ice-free summers in the Arctic Ocean has shifted, in a few years, from 2100 to 2040 to 2013. And survival estimates are changing for the Greenland ice sheet, whose melting would cause a potentially devastating rise in sea levels. Once couched in terms of millenniums, they are now expressed in centuries or even decades.
In any case, many of the scientists noted, humans are already geoengineering — with the greenhouse gases they are pumping into the atmosphere.
One way or another, Dr. Keith said, in 200 years the earth will be “an artifact,” a product of human design.
