HR: 17:00h
AN: G34A-05 INVITED     [Abstracts]
TI: A Re-Evaluation of Some Geodetic Constraints on Sea Level Rise, and Possible Implications for Munk's Sea Level Enigma
AU: * Wahr, J
EM: wahr@lemond.colorado.edu
AF: University of Colorado, Department of Physics and CIRES, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Mitrovica, J
EM: jxm@physics.utoronto.ca
AF: University of Toronto, Department of Physics, 60 St. George Street, Toronto, ON M5S 1A7 Canada
AU: Matsuyama, I
EM: isamu@cita.utoronto.ca
AF: University of Toronto, Department of Astronomy and Astrophysics, 60 St. George Street, Toronto, ON M5S 3H8 Canada
AU: Paulson, A
EM: archie.paulson@colorado.edu
AF: University of Colorado, Department of Physics and CIRES, Campus Box 390, Boulder, CO 80309-0390 United States
AU: Tamisiea, m
EM: mtamisiea@cfa.harvard.edu
AF: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS42, Cambridge, MA 02138
AB: Tide gauges suggest that sea level has been rising at 1.5-2.0 mm/yr over the last century. Assuming this is a true measure of the global rise and not an artifact caused by the uneven distribution of tide gauges (Cabanes, et al., 2001), it represents some combination of steric (changes in density) and eustatic (changes in mass) effects. Steric effects are probably important, but do not appear to be large enough to explain the entire tide gauge signal. Significant eustatic contributions are required. These would presumably come from the influx of water from the polar ice sheets, mountain glaciers, and liquid water stored on land. Because they involve mass redistribution, eustatic effects could show up in time variable gravity and Earth rotation measurements. In fact, secular trends have been observed in satellite gravity measurements of the Earth's oblateness over the last 25-30 years, in astrometric observations of polar wander over the past century, and in length-of-day estimates inferred from ancient eclipse records extending back over the past few thousand years. A problem with using these observations to detect the effects of water mass variability, is that post-glacial-rebound (PGR) in the solid Earth causes secular trends in these same quantities. In fact, PGR models have been constructed that can simultaneously explain all these geodetic observations, which would then rule out any significant eustatic sea level contributions. This is not an entirely fair assessment, since many of these PGR models are tuned to explain the observations. But, even so, the consistency between the observational results for oblateness and length-of-day suggests that secular mass changes over the last 25-30 years are probably about equal to the changes over the last several thousand years; and this is at odds with what is expected for eustatic variations, as inferred by comparing the tide gauge estimates with geological sea level records from the last several thousand years. Munk (2002) has referred to this confusing situation as the sea level enigma. There are recent indications, though, that the geodetic observations may not be as tightly constraining as previously believed. Most notably, new analyses of satellite gravity data suggest that the secular change in oblateness may be more uncertain than previously recognized, leaving open the possibility that recent secular mass changes could differ from the average over the last several thousand years. Furthermore, recent modifications to the theory of polar wander on a viscoelastic Earth suggest that PGR alone is not likely to cause all of the observed polar wander over the last century, thus leaving the door open for possible eustatic contributions. This talk will summarize these new issues.
DE: 1217 Time variable gravity (7223, 7230)
DE: 1621 Cryospheric change (0776)
DE: 1641 Sea level change (1222, 1225, 4556)
DE: 4556 Sea level: variations and mean (1222, 1225, 1641)
SC: Geodesy [G]
MN: Fall Meeting 2005