According to Chris Mooney in Wired, he is analyzing those risks, and appears now to be confident that filling the stratosphere with sulfur dioxide might actually work in reflecting sunlight back into space, just like it did after Mount Pinatubo blew 20 million tons of it 22 miles into the atmosphere. Caldeira recognizes that there are risks, but according to Wired,
"Caldeira's response is that it's hard to see how those consequences would be anywhere near as nasty as simply letting global warming go unchecked. But the more geoengineering becomes a matter of public debate and concern, the more the downsides of a remade world come under scrutiny. First, there's the fear that injecting sulfate into the stratosphere could destroy much-needed ozone, which also declined markedly after Pinatubo." More in ::Wired
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