these observed changes in climate have already had an influence on human and natural systems ipcc b the greatest warming over the past century occurred at high latitudes with a large portion of the arctic having experienced warming of more than c the intergovernmental panel on climate change s fourth assessment report ipcc a concluded that a doubling of c concentrations from preindustrial levels when they were approximately ppm would likely lead to an increase of temperature somewhere between c and c the socalled climate sensitivity