through mechanisms not yet understood. Changes in thermohaline circulation, which may be related to the surface freshwater balance associated with growth and transport of sea ice, can alter surface volume (i.e., gyre characteristics) in subtropical regions.
What are the history and current global budget of land-locked ice and snow and what are the primary mechanisms controlling this budget? Given the direct impact of this budget on sea level, we must better quantify the mass balance of continental ice sheets, alpine glaciers, and permanent snow fields. In particular, the balance at the base of floating ice shelves is in considerable question, and whether the Greenland and Antarctic ice sheets are gaining or losing mass is still uncertain. These questions must be resolved. Knowledge of how the land-locked ice and snow budget has varied over time will give some indication of the range, rate, and rapidity of change experienced through natural variability. Several other phenomena are critical in this budget and also must be better understood: the melt and growth rates and dependencies at the base of floating ice sheets, as well as ice sheet drainage rate, as a function of sea level (which alters the grounding line and thus frictional retarding force to the flow), and the response of precipitation to cold-region climatic changes.
Observations critical for addressing these issues include long-term monitoring of surface salinity with SST, since salinity is the principal control on the density of seawater in high-latitude regions. Also, the sea ice fields themselves, including the motion fields and ice thickness, are required to determine the freshwater transports and buoyancy fluxes associated with the ice fields. Finally, consistent monitoring of iceberg calving and an observational system to determine basal melt or growth (e.g., temperature/salinity moorings across the floating ice shelves) must be established to measure the land-locked ice and snow budget.
Model parameterizations must be improved to better represent certain phenomena: (1) ice-albedo feedbacks, (2) snow-climate feedbacks, (3) ice-cloud feedbacks, (4) ice-ocean feedbacks, and (5) ice sheet-ocean feedbacks and ice sheet instabilities. Improved simulation of sea ice and snow distributions and related impacts also must be achieved.
Land and vegetation influence climate through many means. Most notably, their geographic distributions relative to the oceans help define the nature of the climate pattern because their low heat capacity relative to the high heat capacity of the oceans leads to alternating cycles of surface response to the same atmospheric forcing. However, on decade to century timescales, the main influences of land and vegetation are through their influences on the carbon (and methane)