tion to climate change has been hypothesized as being important in surface-climate feedbacks as well as in climate change diagnostics.

Finally, the ocean-atmosphere-ice interaction, particularly the ice or snow surface energy balance (including surface albedo and ocean-ice, ice-cloud, and snow-cloud feedbacks), must be addressed through detailed process studies to improve parameterizations of these processes in climate models.

Land and Vegetation Observations

As explained in sections above, it is also essential to monitor changes in land surface characteristics, including surface vegetation. These changes alter not only the distribution of surface reservoirs and the surface-atmosphere exchange of radiatively active gases but also albedo and even surface stress and evapotranspiration efficiency —and the last two both influence the hydrological cycle. This serves as an external forcing to the planet that cannot be predicted and must be introduced into the models as they occur to properly maintain the models' surface forcing conditions.

Long-term monitoring of near-surface aerosol distributions also is needed. These distributions may induce stationary changes in the surface radiation balance, which may lead to large-scale circulation moderation through stable gradient perturbations.

Hydrological Observations

Precipitation is the key hydrological variable. For most studies of dec-cen variability and its effects, global fields of precipitation over timescaies of 10 to 100 years are essential. We have no such global instrumental records currently. The National Aeronautics and Space Administration's Tropical Rainfall Monitoring Mission is an important first step, but global data are needed. To relate precipitation to global boundary conditions, SST, vegetative ground cover and soil moisture, and sea and land ice and snow must be simultaneously measured. Nearly every theory of anthropogenic warming finds an increased rate of the hydrological cycle and possible alteration of atmospheric distributions of moisture and of the frequency, intensity, and distribution of rainfall (including severe rainfall events). Thus, monitoring of the surface distribution of precipitation and evaporation must begin. This monitoring includes that over the oceans, where changes in the precipitation minus evaporation balance alter the surface salinity budget, which in high latitudes has been implicated in altering the thermohaline circulation (and driving internal oscillations on dec-cen timescaies in ocean models).

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