Present-day sea level rise: do we understand what we measure?
Anny Cazenave(1) , Alix LOMBARD(1) , Steve NEREM(2) , and Kien DoMinh(1)
18 Avenue E. Belin,
31401 Toulouse Cedex 9,
(2) CCAR, University Colorado, Boulder, CO, United States
Since early 1993, sea level variations are accurately measured by Topex/Poseidon altimetry, complemented for the recent years by Envisat and Jason-1. This >12 years long data set indicates that, in terms of global mean, sea level is presently rising at a rate of ~ 3. +/- 0.4 mm/yr. Such a rate is significantly higher than the mean rate recorded by tide gauges for the past five or more decades (on the order of 1.8 +/- 0.3 mm/yr). The higher rate observed during the 1990s may indicate that sea level rise is accelerating due to enhanced land ice melting and/or increased ocean warming. One cannot exclude also that it only reflects decadal variability of the combined thermal and ocean mass change. During the past few years, our group has examined in detail the steric contribution (mainly thermal expansion) to last decade and past 50 years sea level rise. We have computed the thermal expansion contribution using four different global ocean temperature data sets. Results indicate that thermal expansion accounts to about 25% of the observed sea level rise observed over 1950-2000, while for the last decade (1993-2003), it explains about 60% of the rate of rise. For both periods, there is indirect evidence that ocean mass change due to land ice melting accounts to ~ 1. mm/yr. This magnitude of the land ice melt contribution is confirmed by direct estimates (partly based on remote sensing observations) of ice sheets and glaciers mass balance. These results suggest that the larger rate of sea level rise observed during the 1990s, compared to previous decades, is largely explained by the recent increase of the ocean heat content; a result supported by reanalyses of ocean data assimilation. Satellite altimetry permits also the mapping of the geographical variability of sea level change. Comparison with regional variations in thermal expansion shows that most of the non uniform sea level change is explained by thermal effects. However some regional differences are observed, in particular in the north Atlantic and austral ocean, with potential implication on other processes such as the isostatic adjustement related to present-day ice sheet melting, change in the thermohaline circulation, etc.