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Global Positioning System Adjustments to Tide Gauge Estimates of Global Sea Level Trend
Woppelmann, G., Miguez, B.M., Bouin, M.-N. and Altamimi, Z. 2007. Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide. Global and Planetary Change 57: 396-406.

What was done
The authors describe a technique they developed for utilizing Global Positioning System (GPS) data, which they obtained from numerous GPS stations situated in close proximity to various tide gauges around the world, to correct the tide gauge records and thus obtain what they call a "set of 'absolute' or geocentric sea-level trends." Based on a number of criteria that had to be met by both the tide gauge and GPS stations, they ultimately used paired data sets from 28 locations that covered a time span of 5.9 years (1999.0-2005.7) to derive their final mean global result, after which they compared it with what they call the "most quoted" tide-gauge results of Douglas (1991, 1997, 2001), which had been corrected for the most common form of vertical land motion by means of theoretical models of Glacial-Isostatic Adjustment (GIA).

What was learned
Whereas the data of Douglas yielded a mean global sea-level rate-of-rise of 1.84 0.35 mm/year after correction for the GIA effect (Peltier, 2001), Woppelmann et al. obtained a much lower mean value of 1.35 0.34 mm/year when employing their correction for measured GPS vertical velocities. The sizable difference between these two results raises the question of how they compare with results obtained from other ways of estimating global sea level trends. In this regard, the four researchers note that Mitrovica et al. (2006) recently indicated there is a 1 mm/year contribution to sea-level rise from the melting of global land ice reservoirs, as well as a 0.4 mm/year contribution from thermal expansion of the global ocean (Antonov et al., 2005). Together, these two numbers yield a value of 1.40 mm/year for the global ocean's total sea-level mean rate-of-rise, which is much closer to the 1.35 mm/year result of Woppelmann et al. than to the Douglas-Peltier result of 1.84 mm/year.

What it means
The mean global sea-level rate-of-rise calculated by Woppelmann et al. appears to resolve the "sea-level enigma" noted by Munk (2002), who called attention to the sizable discrepancy that existed at the time of his writing between estimates of climate-related contributions to sea-level change and what the observed value was thought to be. Now, there is no longer any discrepancy between these two numbers. What is more, the global ocean's mean rate-of-rise is now seen to be much slower than what was previously believed to be the case.

Antonov, J.I., Levitus, S. and Boyer, T.P. 2005. Themosteric sea level rise: 1955-2003. Geophysical Research Letters 32: 10.1029/2005GL023112.

Douglas, B.C. 1991. Global sea level rise. Journal of Geophysical Research 96: 6981-6992.

Douglas, B.C. 1997. Global sea level rise: a redetermination. Surv. Geophys. Res. 18: 279-292.

Douglas, B.C. 2001. Sea level change in the era of the recording tide gauge. In: Douglas, B., Kearney, M. and Leatherman, S. (Eds.), Sea Level Rise: History and Consequences, Academic Press, San Diego, CA, USA, pp. 37-64.

Mitrovica, J.X., Wahr, J., Matsuyama, I., Paulson, A. and Tamisea, M.E. 2006. Reanalysis of ancient eclipse, astronomic and geodetic data: a possible route to resolving the enigma of global sea-level rise. Earth and Planetary Science Letters 243: 390-399.

Munk, W. 2002. Twentieth century sea level: an enigma. Proceedings of the National Academy of Science, USA 99: 6550-6555.

Peltier, W.R. 2001. Global glacial isostatic adjustment and modern instrumental records of relative se3a level history. In: Douglas, B., Kearney, M. and Leatherman, S. (Eds.), Sea Level Rise: History and Consequences, Academic Press, San Diego, CA, USA, pp. 65-95.

Reviewed 8 August 2007