From The Guardian: Race against time:

But two years ago, almost to the week, scientists noticed that the Arctic was losing ice at an almost unbelievable pace, outstripping the climate models by decades. Clearly we’d passed a threshold, and global warming had gone from future threat to present crisis. It wasn’t just Arctic ice; at about the same time methane levels in the atmosphere began to spike, apparently as a result of thawing permafrost. Surveys of high altitude glaciers showed they were uniformly melting, and much faster than expected. Oceanographers reported – incredulously – that we’d managed to make the oceans 30% more acidic.

Those observations changed everything – and they produced what is almost certainly the most important number in the world. A Nasa team headed by James Hansen reported that the maximum amount of carbon the atmosphere can safely hold is 350ppm, at least if we want a planet “similar to the one on which civilization developed and to which life on earth is adapted.” Since we’re already at 390ppm, the message was clear: we don’t need to buy an insurance policy to reduce the threat of future warming. We need a fire extinguisher, and we need it now.

Scientists have heard that message – in March they gathered by the thousands at an emergency conference to declare that the five-year-old findings of the Intergovernmental Panel on Climate Change were dangerously out of date.

But politicians haven’t caught up. As we head toward the crucial Copenhagen talks slated for December, Obama and the rest of the world’s political class are still using the dated science and its now stale conclusions.