MEASURING GLOBAL MEAN SEA-LEVEL VARIATIONS USING TOPEX POSEIDON ALTIMETER DATA/

The variations of global mean sea level are an important indicator of global climate change, and their measurement can provide important inf ormation for determining the socioeconomic impact of sea level change on coastal land use. The analysis of historical tide gauge records gen erally indicates that sea level has risen at a rate of about 2 mm/yr d uring the last 100 years; however, this estimate is somewhat uncertain due to the effects of regional crustal motion, lack of uniform tempor al coverage, and the limited spatial sampling of tide gauges. The pros pect of measuring variations in global mean sea level has been assesse d using approximately 2.5 years of satellite altimeter data from the T OPEX/POSEIDON (T/P) mission, where synoptic mapping of the geocentric height of the ocean surface is routinely achieved with a point-to-poin t accuracy of better than 5 cm. The global mean sea level variations m easured by T/P every 10 days have an rms of 6 mm (4 mm after detrendin g), some of which is shown to be correlated with sea surface temperatu re variations. The rate of change of global mean sea level derived fro m 2.5 years of data is +5.8 mm/yr with a scatter of 0.7 mm/yr. Current ly, it is impossible to accurately estimate the error in the measured rate of sea level rise, since little is known about the long-term beha vior of the measurement errors at the millimeter level. In addition, t here is evidence from the sea surface temperature record that the meas ured rate of sea level rise is associated with a relatively short-term (interannual) variation unrelated to the long-term signal expected fr om global warming. Nevertheless, these results suggest that T/P is ach ieving the necessary repeatability to measure global sea level variati ons caused by climate change, and a longer time series will significan tly improve the sea level rise estimate by averaging measurement error and real sea level variations. A longer time series will also reduce the errors in estimates of the altimeter calibration, providing an imp ortant constraint on any long-term instrument drift. Future research w ill focus on establishing a realistic error budget for these measureme nts of global mean sea level, so that they can be put in the proper co ntext with other observations of global climate change.