Scientists weigh geoengineering in global warming battle

Not every crazy idea, say dropping out of Harvard to start a software firm, is a bad one. But you don't have to be Bill Gates to place your bets that way.

Consider atmospheric geoengineering — pumping reflective particles into the stratosphere to reflect sunlight — seen as a way to cut the effects of global warming. In 1991, the eruption of Mt. Pinatubo in the Philippines cooled the atmosphere's average temperature worldwide almost one degree Fahrenheit, a kind of "global dimming," serving as an inspiration for the idea. Such high-altitude aerosols, different from the ones found in spray cans, can play a big role in climate.

A 2006 paper in the journal Science, for example, written by the eminent atmospheric scientist Tom Wigley of the National Center for Atmospheric Research, suggested that annually blasting roughly 500,000 tons of sulfur (about 7% of yearly sulfur production) into the stratosphere every year for three decades would prevent global warming. But there is that acid rain issue.

Earlier this month, White House science adviser John Holdren found himself at the center of a brouhaha over remarks to the Associated Press that geoengineering of all sorts was "mentioned" as the administration pondered means of limiting global warming. Holdren later downplayed geoengineering schemes, after news stories appeared linking atmospheric geoengineering to drought, ozone depletion and acid rain, among other concerns.

A pair of recent papers point to some unintended consequences of atmospheric geoengineering, ones that add to the sense that it might not be such a good idea.

In a study in the journal Environmental Science and Technology, federal scientist Daniel Murphy of the National Oceanic and Atmospheric Administration looked at what stratospheric aerosols would do for solar cells and mirrored solar power collectors. He turned to 1991 data from the Mt. Pinatubo eruption for an answer.

In the study, calculations of sunlight scattering combined with records from Hawaii's Mauna Loa observatory showed that for every one watt's worth of sunlight reflected away from Earth by stratospheric aerosols, another four watts were converted from direct sunlight to diffuse sunlight. Such sunlight is bad news for the large power-generating solar collectors that rely on mirrors to concentrate power. Even though total direct sunlight fell only 3% in 1991, power generated by these collectors dropped by 20%. "It turns out that any systems using mirrors to concentrate direct sunlight are much more sensitive than one-for-one," Murphy says, by e-mail.

"Among all of the possible side effects of geoengineering, the effect on solar power is probably not the most important. It is one of the most certain," Murphy adds.

A second paper, out Sunday in the journal Nature Geoscience, points to another problem with stratospheric aerosols. "We are really uncertain about their role in the climate system," says study lead author Dan Cziczo of Pacific Northwest National Laboratories in Richland, Wash. Ice condenses around aerosol particles, a process that scientists know leads to high-flying cirrus clouds. Those clouds in turn reflect sunlight, a cooling effect in the global warming equation.

But how this happens exactly isn't clear, Cziczo says. "So, what we were really trying to do was look at what particles helped to form the clouds," he says. "What really cropped up was lead."

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