"Further global warming of 1 °C defines a critical threshold. Beyond that we will likely see changes that make Earth a different planet than the one we know."

So says Jim Hansen, director of NASA's Goddard Institute for Space Studies in New York. Hansen and colleagues have analysed global temperature records and found that surface temperatures have been increasing by an average of 0.2 °C every decade for the past 30 years. Warming is greatest in the high latitudes of the northern hemisphere, particularly in the sub-Arctic boreal forests of Siberia and North America. Here the melting of ice and snow is exposing darker surfaces that absorb more sunlight and increase warming, creating a positive feedback.

Earth is already as warm as at any time in the last 10,000 years, and is within 1 °C of being its hottest for a million years, says Hansen's team. Another decade of business-as-usual carbon emissions will probably make it too late to prevent the ecosystems of the north from triggering runaway climate change, the study concludes (Proceedings of the National Academy of Sciences, vol 103, p 14288).

Whole story: http://www.newscientist.com/article/mg19125713.300-one-degree-and-were-done-for.html