A new and potentially useful methodology involves the application of satellite altimetry and gravity data to monitoring the global water mass budget (Chen et al, 1998; Minister etal., 1999; Herring, 1998). Satellite radar altimeters, such as TOPEX/POSEIDON, can measure the ocean surface height to millimetric accuracy (Nerem, 1999). The global average sea level rise determined from TOPEX/POSEDON for 1993-1998 lies between 2.5 and 3.1 mm/yr, which is slightly higher than the range reported by Houghton et al. (1996), based on tide gauges. The TOPEX/POSEIDON satellite altimeter can also derive the ocean component of the global hydrologic cycle and help validate the global water mass budget (Minister et al, 1999; Chen et al., 1998). After the altimeter data are corrected for various orbital and atmospheric effects, the steric sea level variation, caused by temperature and salinity differences, is removed using historical in situ water density measurements. The residual sea level represents the true change in global ocean water mass. This value can be employed to constrain the contributions from the atmospheric and continental components of the global water budget, which are derived from various clima-tological data bases. The land component, dependent upon differences between precipitation, evaporation, and stream runoff, is the least well known, as demonstrated above. Future improved satellite observations will supplement ground-based measurements and hydrologic models, in order to detect anthropogenic changes, superimposed on natural climate variations.
Two satellites, to be launched within the next few years, will measure annual variations in the earth's gravity due to changes in mass as small as a layer of 1 cm of water over 250,000 km2 (Herring, 1998). Changes in gravity due to variations in sea level and in continental water table levels, such as the lowering of the High Plains aquifer of the Great Plains, which has dropped by 30 m in some places over the last 40 years (Dugan et al, 1994), should be detectable by making comparisons to earlier gravity surveys.
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