Comparison of Satellite Altimetry to Tide Gauge Measurement of Sea Level: Predictions of Glacio-Isostatic Adjustment.

Modern rates of sea level change are of interest because of concerns that global warming may be causing glacier retreat. Both tide gauges and satellite radar altimetry are used to measure the present rates of change in sea level. Tide gauges measure sea level relative to the ocean floor whereas the reference for satellite altimetry is the earth\textquoterights center. A numerical model of deformation of the earth\textquoterights solid surface and its geoid forced by melting ice sheets, both past and present, is used to predict the present rate of sea level change as measured by tide gauges and satellites. Sea level change observed by both tide gauges and satellites are predicted to be spatially nonuniform. Considering only past ice sheets tide gauges in glaciated regions would record a fall in sea level of 13 mm yr1, whereas satellite altimetry would record a rise in sea level of 0.7 mm yr1. In the region peripheral to the glaciated zone a tide gauge would record a rise in sea level (3 mm yr1) in contrast to a predicted fall in sea level (0.3 mm yr1) recorded with satellite altimetry. Near regions once glaciated, the rate of sea level change as recorded by satellite decays more rapidly than the tide gauge sea level signature and so isostatic adjustment caused by past ice sheets contributes proportionately less to the modern rate of sea level change as measured by satellites than by tide gauges. At regions distant from glaciated areas both tide gauges and satellites would record a slight (0.3 mm yr1) sea level fall. The immediate elastic sea level response from melting of present ice sheets yields a similar signature for data from both tide gauges and satellite altimetry. Recorded sea level would fall near the meltwater source and at great distance from the source would rise approximately 122\% of the oceanwide average sea level amount. Therefore glacio-isostatic processes will contaminate the sea level record measured by satellite less than the tide gauge record.