GLOBAL RATES OF MODERN SEA LEVEL CHANGE FROM TOPEX/POSEIDON SATELLITE ALTIMETRY

Sea level rise is both an indicator and an effect of global warming. The Intergovernmental Panel on Climate Change predicts that a global sea level rise of 2 to 5 mm/yr should occur if global warming causes glaciers and ice sheets to begin melting. NASA's TOPEX/Poseidon satellite uses radar altimetry to measure the height of the ocean surface with an accuracy of a few centimeters for each individual measurement. In contrast to tide gauges which measure sea level change relative to the earth's solid surface and only at sparse coastal locations, satellite sea level measurements are relative to the earth's center and cover the entire ocean to within 24 degrees of the poles. Every 10 days the entire ocean is surveyed. Since 1992 the satellite has collected 30 gigabytes of data which we have analyzed for the global average rate of sea level change and for the spatial sea level signature. Results indicate that global sea level is rising at the rate of 1.3 mm/yr. However the rate is not uniform with a maximum rise of 28 mm/yr in some regions and a fall approaching 17 mm/yr in other areas. Furthermore the spatial variation in rate appears to be partially correlated with variations in ocean temperature changes. Because many processes affect sea level change we have calculated the Empirical Orthogonal Functions (EOF's) of the data set. EOF's isolate those spatial variations in rate that are mathematically independent. The first three EOF's explain 39% of the total variance in sea level change. The first and most significant EOF explains 27% of the total variance and its spatial signature indicates it may be related to melting ice on the Greenland Ice Sheet. Its global rate of sea level rise is 2.8 mm/yr. The second EOF (7% of the variance) is clearly related to a seasonal variation in sea level and has a global sea level fall of 0.2 mm/yr. An El Nino signature is obvious in the third EOF (4.9% of the variance) and exhibits a 0.7 mm/yr global fall in sea level. The TOPEX/Poseidon satellite gives critical sea level information which is useful in monitoring the effects of global warming and isolating sources of meltwater. The available data suggest that global warming may already be causing a slight rise in global sea level.