Leading environmental writer and activist Bill McKibben talks about how climate science and climate politics are quickly evolving - and how we now have a much more specific idea both of the peril we face and the steps (large and difficult) necessary to solve it.Even two years ago, scientists could offer only vague ideas of how much carbon in the atmosphere was too much. But in the wake of the rapid melt of Arctic sea ice in 2007, it’s become clear that this is a problem not for the future but very much for the present.In addition, McKibben describes the swelling grassroots global movement, 350.org, which looks set to coordinate the largest day of global environmental action ever, with actions from high in the Himalayas to underwater on the Great Barrier Reef.