Figure 1: Observations over land and oceans indicate that the global average surface temperature has increased by ~0.5°C (~1°F) since the mid-19th century (annual average temperature departures are shown by the solid, irregular line). The smoother curves show results from simplified climate models that include the effects of the increasing concentrations of greenhouse gases and the changing influences of sulfate aerosols and solar forcing. The central curve is for the IPCC "best estimate" climate sensitivity of 2.5°C for a doubling of the CO2concentration; the outer curves are for higher and lower sensitivities. The results suggest that the temperature increase during this period is due mainly to the warming influence of the increased concentrations of CO2 and other greenhouse gases, but that the global average temperature is also affected by the cooling influence of sulfate aerosols and the cyclic effects of variations in solar radiation. The general agreement of observations and model results suggests that estimates of climate sensitivity are in the range predicted by comprehensive climate models (see Appendix D for further information). Source: T.M.L. Wigley, P.D. Jones, and S.C.B. Raper, "The observed global warming record: What does it tell us?" Proceedings of the National Academy of Sciences, in press.

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