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6.6.3. Total Anthropogenic Radiative Forcing: Greenhouse Gases and Aerosols

Table 6.7 shows the estimate (based on IPCC, 1995) of the radiative forcing due to both the anthropogenic increase of atmospheric greenhouse gases and aerosols since pre-industrial times. Each estimate of forcing is accompanied by an uncertainty range to express the level of confidence associated with the "best guess" value. Direct greenhouse forcing has been estimated with a fairly high level of confidence, whilst levels of aerosol forcing and indirect ozone forcing are poorly constrained. An estimate of the net global-mean radiative forcing due to all anthropogenic activity since pre-industrial times is not presented, as the usefulness of combining estimates of global-mean radiative forcings of different signs, and resulting from different temporal and spatial patterns, is not currently understood. Nevertheless, it is qualitatively possible to recognise that much of the positive radiative forcing due to increases in greenhouse gases has been partially offset by the negative forcing associated with increases in atmospheric aerosols.

Table 6.7. Estimates of globally averaged radiative forcing (Wm-2) due to increases in atmospheric greenhouse gas and aerosol concentrations

Radiative forcing Wm-2

Uncertainty range Wm-2

Direct greenhouse forcing

2.45

0.4

Stratospheric ozone forcing

-0.15

0.075

Tropospheric ozone forcing

0.4

0.25

Direct aerosol forcing

-0.9

0.6

Indirect aerosol forcing

uncertain (negative)

0.75

Solar forcing

0.3

0.2

�Indirect greenhouse forcing