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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
Processes, Parameterizations, and
Changes in land-surface characteristicsincluding surface
vegetation, topsoil extent, and soil moisturemust 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
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.