Climate science is famously complicated, but one useful number to keep in mind is "climate sensitivity". This measures the amount of warming that can eventually be expected to follow a doubling in the atmospheric concentration of carbon dioxide. The Intergovernmental Panel on Climate Change, in its most recent summary of the science behind its predictions, published in 2007, estimated that, in present conditions, a doubling of CO_2 would cause warming of about 3°C, with uncertainty of about a degree and a half in either direction. But it also says there is a small probability that the true number is much higher. Some recent studies have suggested that it could be as high as 10°C.
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