Geoengineering is no magic bullet, Cicerone said. But done correctly, he added, it will act like an insurance policy if the world one day faces a crisis of overheating, with repercussions like melting icecaps, droughts, famines, rising sea levels and coastal flooding.
Many scientists still deride geoengineering as an irresponsible dream with more risks and potential bad side effects than benefits; they call its extreme remedies a good reason to redouble efforts at reducing heat-trapping gases like carbon dioxide. And skeptics of human-induced global warming dismiss geoengineering as a costly effort to battle a mirage.
Even so, many analysts say the prominence of its new advocates is giving the field greater visibility and credibility and adding to the likelihood that global leaders may one day consider taking such emergency steps.
"People used to say, 'Shut up, the world isn't ready for this,'" said Wallace Broecker, a geoengineering pioneer at Columbia University in New York.
"Maybe the world has changed."
Michael MacCracken, chief scientist of the Climate Institute, a private research group in Washington, said he was resigned to the need to take geoengineering seriously.
"It's really too bad," MacCracken said, "that the United States and the world cannot do much more so that it's not necessary to consider getting addicted to one of these approaches."
Martin Apple, president of the Washington-based Council of Scientific Society Presidents, said of geoengineering at a recent meeting in Washington, "Let's talk about research funding with enough zeroes on it so we can make a dent."
The study of futuristic countermeasures began quietly in the 1960s, as scientists theorized that global warming caused by human-generated emissions might one day pose a serious threat. But little happened until the 1980s, when global temperatures started to rise.
Some scientists noted that the earth reflected about 30 percent of incoming sunlight back into space and absorbed the rest. Slight increases of reflectivity, they reasoned, could easily counteract heat-trapping gases, thereby cooling the planet.
Broecker of Columbia proposed doing so by lacing the stratosphere with tons of sulfur dioxide, as erupting volcanoes occasionally do.
The injections, he calculated in the '80s, would require a fleet of hundreds of jumbo jets and, as a byproduct, would increase acid rain.
Critics of geoengineering argue that it makes more sense to avoid global warming than to gamble on risky fixes.
But international efforts like the Kyoto Protocol have so far failed to diminish the threat.
Geoengineering's advocates say humankind is already vastly altering the global environment and simply needs to do so more intelligently.
Such visionary plans have their critics. James Hansen of the NASA Goddard Institute for Space Studies in New York, who attended the talk and strongly advocates curbing emissions, belittled the orbital sunshade as "incredibly difficult and impractical."
Crutzen, the Nobel laureate from the Max Planck Institute, has also drawn fire for his paper about injecting sulfur into the stratosphere.
"Climatic engineering, such as presented here, is the only option available to rapidly reduce temperature rises" if international efforts fail Crutzen wrote. "So far," he added, "there is little reason to be optimistic."
Andrew C. Revkin contributed reporting for this article.
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