James E. Hansen, the NASA climate scientist who has long had a habit of pushing past where many colleagues dare go in describing the risks posed by global warming, has done it again. He and eight co-authors have drafted a fresh paper arguing that the world has already shot past a safe eventual atmospheric concentration of carbon dioxide, which they say would be around 350 parts per million, a level passed 20 years ago. (The atmosphere currently holds about 385 parts per million of the greenhouse gas.)
Looking at evidence from past climate swings and greenhouse-gas concentrations, he concludes that a sustained concentration of carbon dioxide at double the 280 parts per million that prevailed for hundreds of millenniums before the industrial revolution would — after a host of slowly-responding feedbacks kicked in to amplify the temperature rise — result in an enormous warming of some 11 degrees Fahrenheit (6 degrees Celsius). More >>>
