These observed changes in climate have already had an influence on human and natural systems (IPCC, 2007b). The greatest warming over the past century occurred at high latitudes, with a large portion of the Arctic having experienced warming of more than 2 °C. The Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC, 2007a) concluded that a doubling of C02 concentrations from pre-industrial levels (when they were approximately 280 ppm) would likely lead to an increase of temperature somewhere between 2.0 °C and 4.5 °C7 (the so-called climate sensitivity8).