Critical cryosphere-related observations for climate patterns on decadal to centennial timescales include long-term monitoring of surface salinity along with SST, since salinity represents the dominant control on the density of seawater in high-latitude regions. Also, measurements of the sea ice fields themselves, including motion fields and ice thickness, are required to determine the freshwater transports and buoyancy fluxes associated with the ice fields. This freshwater transport has been implicated in driving major changes, even mode shifts in the global thermohaline circulation. Finally, consistent monitoring of iceberg calving and an observational system for determining ice basal melt or growth (e.g., through temperature/salinity moorings across the floating ice shelves) must be established to better determine the freshwater budget. Both field and satellite studies are needed to refine the mass budgets of the Greenland and Antarctic ice sheets. Onsite studies that have focused on ice flow, melting, and calving should be continued and extended. Water vapor flux divergence observations will help pin down the source of the ice sheets' mass. A laser altimeter on a polar-orbiting satellite is needed to augment existing radar altimetry. These satellite data will provide accurate estimates of ice sheet volume and give early warning of possible ice sheet collapse. As in the case of ice, the distribution of snow fields, including thickness and spatial extent, must be monitored. The response of snow distribution 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.

Observations of changes in land surface characteristics, including surface vegetation, are essential for research goals in both ecosystems and dec-cen climate. Observational requirements are discussed in detail in the section on ecosystems earlier in this chapter. Changes in land surface properties 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 is also needed; these distributions may induce stationary changes in the surface radiation bal-