time series stations are the only series available that allow appropriate development, diagnosis, and improvement of these parameterizations.

Continued satellite data are needed for global coverage of sea surface height, SST, winds, and ocean color, but for these data to be useful, corresponding ground-truth ocean observations also are needed. Particular data of interest concern the heat budget. A concerted effort is required to improve estimates of heat flux divergence and heat storage and their variabilities from subsurface ocean data, eliminating disparities between those estimates and air-sea heat exchange estimates. Various subsurface floats and moorings are particularly helpful to supplement shipboard measurements for this study.

Sea level change is another important observational challenge. The IPCC (1996) estimates that in the year 2000 sea level will be 46 to 72 cm higher than today (36 to 53 cm, when the effects of sulfate aerosols are included). A range is given because each projection presumes a specific scenario for increase in greenhouse gasses. To validate these predictions, better monitoring of global sea level change and its components will be needed. The prospects for sea level monitoring are good. A global network of sea level stations (Global Sea Level Observing System) is being implemented. Land movements will be measured at some of these stations with satellite geodesy and gravimetric techniques. Satellite altimetry is another important tool coming into use to measure global sea level rise.

Cryosphere Observations

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 focus 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 distribu-



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