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 infrared radiation flux in the troposphere. However the climate change ascribed to such increases, not to mention predictions of future climate change in response to prospective changes in the earth's radiation budget, is based virtually entirely on model simulations of climate response to changes in radiation rather than on empirically established relationships. There is thus an urgent need to evaluate the performance of climate models to ascertain the accuracy with which they represent the changes in temperature and other indicia of climate that have been observed over the industrial period. Such an evaluation, however, requires an accurate assessment of the totality of changes in the earth's radiation budget in both the longwave (thermal infrared) and shortwave (solar) spectral regions, not just of changes in the longwave due to increased concentrations of greenhouse gases. This requires concerted measurement and modeling efforts to characterize shortwave forcing by aerosols. Measurements permit characterization of aerosol properties including the spatial and temporal variation of all of these quantities, together with that of other pertinent variables sufficiently well to permit evaluation of aerosol direct forcing. However such measurements at best permit evaluation only of forcing at the present time. They do not permit identification of the responsible substances: sulfates and nitrates from fossil fuel combustion, anthropogenic organics from photochemical smog, organics from biomass burning, biogenic organics, mineral dust, sea salt. It is therefore essential to carry out a commensurate effort in modeling aerosol loading and properties. Such models would need to represent the key components of the aerosol life cycle: emissions of aerosols and precursors, chemical transformations responsible for gas-to-particle conversion, new particle formation, aerosol microphysical evolution, three-dimensional transport, and wet and dry removal processes of aerosols and precursors. The performance of these models also would need to be evaluated, by comparison with measurements, so the models certainly do not supplant the measurements. Once the models have been evaluated over a wide range of conditions, they can be used with known confidence to evaluate aerosol loading, and in turn forcing, at present, and for past times and for future emissions scenarios.
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