350ppm is the CO2 Line in the Sand, says James Han

ExecutorElassus writes: For about twenty years, scientists and policymakers have speculated at what the limit for atmospheric carbon would have to be to avoid a feedback loop that would cause widespread environmental change, mostly negative for humans. At the time, the guess was double pre-Industrial Revolution levels, which would make the limit 550ppm. Now, though, NASA Climate Expert James Hansen (whom you might remember for claiming that political appointees at NASA were censoring him) has pegged the limit at 350ppm, a much lower number. That's the limit, he argues, at which global carbon dioxide levels cause an accelerating warming trend, leading to the continuously revised estimates for when the ice caps, glaciers, etc., will melt completely, and all those scary animations Al Gore made come to pass. The catch? We're already at 383ppm, meaning that if we want to avoid the worst, Hansen argues that we should ban all new coal plants, phase out the old ones, and instate a carbon tax so high that all those tar sands and oil shale deposits stay in the ground.