PREDICTIONS AND OBSERVATIONS OF GLOBAL SEA LEVEL FROM SHORELINES, TIDE GAUGES AND SATELLITES

Sea-level changes are mapped on differing time scales and differing spatial scales. Because a dominant cause of sea-level change is ice-sheet growth or retreat, sea-level studies provide information regarding changes in ice sheet volume. Unloading of the earth as ice sheets retreat results in deformation of the earth's solid surface and its geoid as ice, water, and mantle mass redistribute through glacial-isostatic adjustment. Classically sea level has been recorded through measurement of ancient shoreline elevations or long tide gauge records. These sea-level changes are only measured on coastlines which are also the regions subject to greatest tilting and tidal complications. Since 1992 the TOPEX/Poseidon satellite has used radar altimetry to measure ocean surface height, covering the entire ocean (between 66N and 66S latitude) every 10 days. These data have been used to determine global mean sea-level changes possibly related to modern glacier melting and global warming. We used a numerical calculation that successfully modeled glacial-isostatic effects upon past sea levels to predict modern rates of sea-level change as recorded by both tide gauges and satellites. Because a tide gauge measures sea level relative to the land surface it will record a different sea-level change than a satellite which measures sea level relative to the earth's center. Both tide gauge and satellite observations are predicted to be spatially non-uniform although past ice-sheet fluctuations and glacial-isostatic adjustment contaminate the satellite record less than the tide gauge record. Our model also predicts the sea-level signature expected from contemporary melting of ice sheets. Both satellite and tide gauge data are predicted to reflect a strong spatial gradient pointing towards the source of any modern meltwater. But it is difficult to extract this signal from sea-level effects resulting from the spatial distribution of thermally induced density variations of ocean water. An inverse calculation, which can reconstruct past ice sheets based upon relative sea-level curves, is being modified to include satellite data and to search for modern sources of meltwater.