Figure 1.Global mean radiative forcing of the climate system for the year 2000, relative to 1750. As determined by the Intergovernmental Panel on Climate Change, the estimated changes in the radiative forcing since preindustrial times are due to (1) increases in concentrations of greenhouse gases such as carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), chlorofluorocarbons and other halogenated gases (halocarbons), and lower atmospheric ozone (tropospheric ozone), and a decrease of upper atmospheric ozone (stratospheric ozone), (2) the direct effect of aerosols on climate, including sulfate, soot (black carbon), organics (organic carbon), biomass-burning-generated particles, and mineral dust; and (3) the indirect effect of aerosols on clouds. The height of the rectangle represents the best estimate of the change in the radiative forcing, and the extent of the vertical lines represent error estimates. No estimate is made for the indirect effect and for the mineral dust direct effect because of large uncertainties. Note that errors associated with aerosol effects are often larger than those associated with greenhouse gas effects. (Adapted from J. T. Houghton et al., eds., Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, 2001)
