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“By most accounts, the leading contender is stratospheric aerosol particles,” said climatologist John Shepherd of Britain’s Southampton University.

The particles would be sun-reflecting sulfates spewed into the lower stratosphere from aircraft, balloons or other devices _ much like the sulfur dioxide emitted by the eruption of the Philippines’ Mount Pinatubo in 1991, estimated to have cooled the world by 0.5 degrees C (0.9 degrees F) for a year or so.

Engineers from the University of Bristol, England, plan to test the feasibility of feeding sulfates into the atmosphere via a kilometers-long (miles-long) hose attached to a tethered balloon.

Shepherd and others stressed that any sun-blocking “SRM” technique _ for solar radiation management _ would have to be accompanied by sharp reductions in carbon dioxide emissions on the ground and some form of carbon dioxide removal, preferably via a chemical-mechanical process not yet perfected, to suck the gas out of the air and neutralize it.

Otherwise, they point out, the stratospheric sulfate layer would have to be built up indefinitely, to counter the growing greenhouse effect of accumulating carbon dioxide. And if that SRM operation shut down for any reason, temperatures on Earth would shoot upward.

The technique has other downsides: The sulfates would likely damage the ozone layer shielding Earth from damaging ultraviolet rays; they don’t stop atmospheric carbon dioxide from acidifying the oceans; and sudden cooling of the Earth would itself alter climate patterns in unknown ways.

“These scenarios create winners and losers,” said Shepherd, lead author of a pivotal 2009 Royal Society study of geoengineering. “Who is going to decide?”

Many here worried that someone, some group, some government would decide on its own to conduct large-scale atmospheric experiments, raising global concerns _ and resentment if it’s the U.S. that acts, since it has done the least among industrial nations to cut greenhouse emissions. They fear some in America might push for going straight to “Plan B,” rather than doing the hard work of emissions reductions.

In addition, “one of the challenges is identifying intentions, one of which could be offensive military use,” said Indian development specialist Arunabha Ghosh.

Experts point out, for example, that cloud experimentation or localized solar “dimming” could _ intentionally or unintentionally _ cause droughts or floods in neighboring areas, arousing suspicions and international disputes.

“In some plausible but unfortunate future you could have shooting wars between your country and mine over proposals on what to do on climate change,’ said the University of Michigan’s Ted Parson, an environmental policy expert.

The conferees worried, too, that a “geoengineering industrial complex” might emerge, pushing to profit from deployment of its technology. And Australian economist-ethicist Clive Hamilton saw other go-it-alone threats _ “cowboys” and “scientific heroes.”

“I’m queasy about some billionaire with a messiah complex having a major role in geoengineering research,” Hamilton said.

All discussions led to the central theme of how to oversee research.

Many environmentalists categorically oppose intentional fiddling with Earth’s atmosphere, or at least insist that such important decisions rest in the hands of the U.N., since every nation on Earth has a stake in the skies above.

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