and 1970 and decreased by a similar amount from 1970 to 1990. The daily surface air temperature maxima under both clear sky and overcast conditions cooled by about 0.9°C from 1950 to 1970 and warmed by a similar amount thereafter. The results are consistent with the expected direct, and also possible the indirect impact of SOx emissions on surface air temperature …. The possible interference of factors affecting aerosol formation and transport, interaction with clouds, as well as the role of natural variations of atmospheric/ocean circulation require further assessment.

IPCC (1995). The effect of tropospheric aerosols. Excerpted from Radiative Forcing of Climate Change. The 1994 Report of the Scientific Assessment Working Group of IPCC, Summary for Policymakers, Intergovernmental Panel on Climate Change:

New estimates of both the direct and indirect effect of anthropogenic aerosols in the troposphere have become available since IPCC [Intergovernmental Panel on Climate Change] 1992. In order to compare different aerosol effects it is useful to express them in terms of globally averaged values of radiative forcing. The direct radiative forcing due to increases in sulphate aerosol since 1850, averaged globally, is estimated to lie in the range -0.25 to -0.9 W m-2. The direct effect of aerosol from biomass burning is estimated to lie in the range -0.5 to -0.6 W m-2. Calculations of the indirect effect of aerosols are at an early stage. Preliminary results suggest that the radiative effect of aerosols on cloud radiative properties is probably a negative forcing and may be of similar magnitude to the direct effect. Note that in the global average the total aerosol induced radiative forcing is negative, but the absorption of solar radiation by carbonaceous aerosols may cause local positive radiative forcing. It is interesting to compare these estimates with the direct radiative forcing due to increases in greenhouse gases since pre-industrial times (+2.1 to +2.8 W m-2), although it is unlikely to be appropriate to add the negative global radiative forcing of aerosols to the positive global radiative forcing of greenhouse gases.

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