Hadley Centre warns of need for drastic cuts in ghg's from 2010

By comparison, an "early but slow" decline, beginning in 2010 but at a more modest level of 1% a year, would lead to likely temperature rises of 2.9C by the end of the century, and a "late and slow" decline beginning in 2030 would generate likely temperature increases of 4C by 2100.

At 2C, scientists have warned that at least one fifth of species are at risk of extinction and 1 to 2 billion people could suffer increased water stress; above 2C plants and soil significantly reduce the amount of carbon they absorb; and above 4C experts warn there is a danger of passing a "tipping point" at which methane release from permafrost and the collapse of big ice sheets accelerate the problems.