FIGURE 2.1 Probability distribution of the predicted increase in global mean surface temperature due to a 3 Wm−2 radiative forcing from increases in greenhouse gases from preindustrial times to 2005. The probability density of the expected warming adopts the IPCC (2007a) climate sensitivity of 3°C warming due to a doubling of CO2, with a 90 percent confidence level of 2°C to 4.5°C warming. The realized warming is the warming from 1750 to 2005 that has been attributed to greenhouse forcing. Because of the small amount of warming that has been realized to date and the presence of strong cooling by aerosols, temperature increases above 2°C are likely not imminent but could be very large before the end of the century. The temperature thresholds for various climate tipping points are marked by the blue words. The ranges, taken from Lenton et al. (2008), are not shown, but are 0.5°C to 2°C for the melting of Arctic summer sea ice; 1°C to 2°C for radical shrinkage of the Greenland Ice Sheet and 3°C to 5°C for shrinkage of the West Antarctic Ice Sheet; 3°C to 4°C for the dieback of the Amazon rain forest due to drastic reductions in precipitation; 3°C to 6°C for persistent El Niño conditions; and 3°C to 5°C for a shutoff in the North Atlantic deep water formation and the associated thermohaline circulation. The tipping point of Himalayan-Tibetan glaciers is based on the IPCC (2007a) finding that these glaciers may suffer drastic melting when warming exceeds 1°C to 2°C above preindustrial levels. SOURCE: Ramanathan and Feng (2008).