3.1.6 Global Mean Sea Level

The use of pre-EOS and EOS radar altimeter sensors to obtain a definitive measurement of long-term global sea level variations is one of the primary goals of our investigation. During the last Ice Age 18,000 years ago, the global sea level was more than 100 meters lower than at the present time. Recent IPCC studies of global sea level change [Warrick and Oerlemans, 1990] have concluded that the average rate of rise during the last century has been 1-3 mm/yr and that the sea level by the year 2070 A.D. may be 20-70 cm higher than today. The sea level rise is a consequence of a number of geophysical, oceanographic, meteorological, cryospheric, and human-induced processes, whose signals cover a wide spectrum. An example is the recent El Niño/Southern Oscillation events which led to a larger than normal interannual fluctuation of the ocean. Another example is the global redistribution of fresh water fluxes into the oceans, causing a volume change of the global ocean (see also Section 3.1.4). The increasing concentration of greenhouse gasses present in the atmosphere can lead to a global warming trend, which can be detected by the rise of the global sea level. Our investigation intends to model and separate mass redistribution from other geophysical and oceanic circulation signals using the long-term global sea level measurements from satellite altimetry. Other necessary measurements for this study include the knowledge of the mass balance of the ice sheets, which will be measured by GLAS (EOS laser altimeter), and are currently being coarsely measured by radar altimeters. Altimetric Observation of Mean Sea Level Variations

Recent studies using over 50 years of tide gauge data indicate that the global mean sea level over the last century may be rising at a rate of approximately 1-4 mm/yr [Peltier, 1988; Douglas, 1991], with a suggestion of possible acceleration since the middle of the last century [Warrick and Oerlemans, 1990]. The availability of satellite altimetry missions in this decade (ERS-1, TOPEX/Poseidon, ERS-2, GFO-1, Envisat, and EOS-ALT) presents an opportunity to establish a more accurate observational system for global sea level measurements with of 1 mm/yr accuracy or better [Shum et al., 1995]. The unprecedented accuracy of the TOPEX/Poseidon enables a number of studies based primarily on the technique reported by Born et al. [1986].

During our investigation period, we have studied the use of Geosat altimeter (1986\(en1990) and the currently flying TOPEX/Poseidon and ERS-1 altimeters for the potential monitoring of mean sea level rise. Tapley et al. [1992] used Geosat altimetry to attempt an observation of the global mean sea level variation and obtained a value of 0±5 mm/yr. Orbit error, uncertainties in Geosat's instrument bias and its drift, and errors in media and geophysical corrections were among the major error sources inhibiting Geosat's use to measure absolute sea level. More recently, the unprecedented accuracy of the pre-EOS sensor, TOPEX/Poseidon, has demonstrated the potential for achieving a mm/yr measurement accuracy level for the global sea level e.g, [Shum et al., 1994c]. Analysis of almost three years of TOPEX/Poseidon data indicates that a global mean sea level rise of approximately 3.6 mm/yr, with regional trends at much larger values, for example, reaching 80 mm/yr in the South Indian Ocean and the western tropical Pacific (Figure 8). The sea level trends may be partially attributed to annual and interannual thermal expansion of the ocean during the observational time period, and not necessarily global warming due to the greenhouse effect. The observed global mean sea level rise is 3.6 mm/yr, after correcting for the internal drift of the TOPEX altimeter instrument bias, is shown in Figure 9. The data points are determined for successive 10-day TOPEX/Poseidon ground track repeat intervals. Several studies have pointed out the current altimetric determination of sea level variations have significant error sources at the 1 mm/yr level, including the uncertainties in the drift of the altimeter instrument, and errors in the altimeter media and geophysical corrections. Consequently, a much longer observational data span, which will be provided by TOPEX/Poseidon and EOS-ALT, along with the associated external altimeter calibration systems, is needed to provide a definitive measurement of the absolute sea level.