Geoengineering, a once-taboo concept that was thought of as a last-ditch approach, is starting to enter mainstream discussions about how to counter the threat of global climate change.

In "Engineering a Cooler Earth," a daylong symposium at MIT last fall, more than a dozen researchers described potential benefits and pitfalls of massive projects to remove greenhouse gases from the atmosphere or offset the warming they cause. Possible approaches include pulling carbon dioxide right out of the air or blocking some percentage of incoming sunlight to reduce temperatures.

While most of the talks dealt with the technical issues, Judith Layzer, an associate professor of environmental policy, outlined the social and political obstacles the world faces in trying to change deeply entrenched habits, systems, and interests.

"There's a fundamental disagreement over whether the risks of geoengineering exceed the risks of climate change," Layzer said. Virtually all the symposium's presenters agreed that methods based on reducing sunlight, such as injecting huge amounts of sulfur into the upper atmosphere to mimic the cooling effects of volcanic eruptions, are too risky. (The sulfur approach would require an essentially permanent commitment to releasing two volcanoes' worth of the material into the stratosphere every year; stopping could cause global temperatures to rise even faster than they would with no intervention.) But schemes to remove carbon, whether by enhancing natural biological processes or by developing technological substitutes such as "artificial trees," are worth researching.

Layzer, like most of the symposium's speakers, framed geoengineering as something that might turn out to be necessary if other measures don't take effect in time, or if climate change turns out to be happening faster than expected. In short, she said, it's something that should be studied just in case.

Increasingly, she said, big businesses that for many years were pressuring leaders to delay action on controlling carbon emissions now see a clean-energy future as an opportunity. They have "changed the image, from sacrifice to business gains."

Nobody thinks mitigating climate change will be easy. Layzer pointed out that any such efforts involve going up against the fossil-fuel industry, which she calls "the biggest industry in the history of mankind." But "the political momentum does seem to be real," she said, "and the collapse of coalitions that have opposed it is the best evidence of that."