NEW STUDY: GLOBAL SEA-LEVEL RISE SIGNIFICANTLY LOWER THAN THOUGHT
Discussing: Woeppelmann, 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), Woeppelmann 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.
Original journal abstract follows:
Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide
By G. Woeppelmann et al.
The problem of correcting the tide gauge records for the vertical land motion upon which the gauges are settled has only been partially solved. At best, the analyses so far have included model corrections for one of the many processes that can affect the land stability, namely the Glacial-Isostatic Adjustment (GIA). An alternative approach is to measure (rather than to model) the rates of vertical land motion at the tide gauges by means of space geodesy. A dedicated GPS processing strategy is implemented to correct the tide gauges records, and thus to obtain a GPS-corrected set of 'absolute' or geocentric sea-level trends. The results show a reduced dispersion of the estimated sea-level trends after application of the GPS corrections. They reveal that the reference frame implementation is now achieved within the millimetre accuracy on a weekly basis. Regardless of the application, whether local or global, we have shown that GPS data analysis has reached the maturity to provide useful information to separate land motion from oceanic processes recorded by the tide gauges or to correct these latter. For comparison purposes, we computed the global average of sea-level change according to Douglas [Douglas, B.C., 2001. Sea level change in the era of the recording tide gauge. Int. Geophys. Ser., 75, pp. 37-64.] rules, whose estimate is 1.84 ~ 0.35 mm/yr after correction for the GIA effect [Peltier, W.R., 2001. Global glacial isostatic adjustment and modern instrumental records of relative sea level history. Int. Geophys. Ser., 75, pp. 65-95.]. We obtain a value of 1.31 ~ 0.30 mm/yr, a value which appears to resolve the 'sea level enigma' [Munk, W., 2002. Twentieth century sea level: an enigma. Proc. Natl. Acad. Sci. U.S.A., 99(10), pp. 6550-6555].
Global and Planetary Change, Volume 57, Issues 3-4, June 2007, Pages 396-406
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