Anthropogenically induced climate change is of great current interest because of increases in atmospheric loading of infrared active (greenhouse) gases over the past 150 years and the inferred resultant increase in the flux of infrared radiation in the troposphere. In parallel with the increase in greenhouse gas loadings there has been an increase in loadings of tropospheric aerosols derived from industrial activities. These aerosols scatter shortwave (solar) radiation leading to a decrease in the shortwave radiation absorbed by the earth-atmosphere system, thereby exerting a cooling influence on climate, the direct aerosol effect. These aerosols are thought also to modify the microphysical and radiative properties of clouds, enhancing their albedo and perhaps suppressing precipitation, thereby enhancing cloud lifetimes, both of which effects would also exert a cooling influence, the indirect aerosol effect.
Present estimates suggest that the magnitude of the global average climate forcing by these aerosol effects is comparable to the longwave (thermal infrared) forcing by greenhouse gases, that is one to a few watts per square meter. However the estimates, which are based entirely on model calculations of aerosol loading, are considered quite uncertain. If the magnitude of aerosol forcing is at the low end of the currently estimated uncertainty range, aerosols negate only a small fraction of the greenhouse forcing, but at the high end of the uncertainty range, aerosols could be negating virtually all of the present greenhouse forcing. There is thus great uncertainty in net climate forcing over the industrial period, mainly because of uncertainty in aerosol forcing. This situation urgently requires resolution in view of the implications of this overall uncertainty in climate sensitivity to greenhouse gas forcing and the necessity for decision making on possible strategies to limit greenhouse gas forcing.
Improving estimates of aerosol forcing requires global atmospheric transport models to estimate the loading and pertinent microphysical properties as a function of location and time. Ultimately it will be necessary to represent aerosol processes and forcing "on-line" in climate models in order to capture the feedbacks of aerosols on the climate system. These requirements place strong demands on the aerosol research community, first to develop process-level understanding necessary for accurate representation of aerosol evolution and second to develop means of representing these processes with sufficient economy to permit their inclusion in climate models.
It is essential to carry out a major effort in modeling aerosol loading and properties and in evaluating model performance by comparison with observation. This places high demands on the aerosol research community. However the stakes are high and we must rise to the challenge.
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