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forcing. The distribution of all these aerosols can be expected to vary on dec-cen time scales in response to climatic and human influences.

Processes, Parameterizations, and Observations

Changes in land-surface characteristics—including surface vegetation, topsoil extent, and soil moisture—must be monitored on a long-term basis. Not only do these changes alter the distribution of surface reservoirs of radiatively active gases and the surface-atmosphere exchange of those gases, they also influence albedo and, through stress effects on plant evapotranspiration efficiency, the hydrologic cycle.

Long-term monitoring of near-surface aerosol distributions will be required to assess whether perturbations of stable gradients of these aerosols could induce stationary changes in the surface radiation balance, which could lead to large-scale alteration of circulation.

In order to improve models' abilities to predict dec-cen-scale variability, we need to more realistically parameterize many land-surface processes, such as: interactions between soil and vegetation under various conditions (including frozen soils); surface-atmosphere gas exchange and net uptake (including biogeochemical and physical feedbacks); and the effect of land-surface processes on atmospheric conditions, (including evaporation and precipitation). Clearly our understanding of most of these processes must be improved first.

Land-surface characteristics and radiatively active atmospheric constituents are vital sets of climate-model parameters, and are generally not prognostic variables that can be used interactively by models. At present, because changes in these factors cannot yet be adequately predicted, they are considered to be an external forcing in most models, and their characteristics must be specified in advance. Even in the absence of any significant skill in predicting land-cover change, however, we can usefully run different vegetation scenarios in physical global-change models. This approach would at least yield some insight into likely climatic and environmental consequences of those scenarios, and provide some guidance for setting environmental-policy goals pertaining to land cover. In addition, as with greenhouse gases, the transient evolution of land cover (including wetlands) under a slowly changing climate and rapidly exploding population must be monitored to provide the boundary conditions needed for model simulations and assessment of plausible future trends.