1.2.5. Mean Sea Level
Sea level rise and the Greenhouse effect
A large proportion of the world's population lives in coastal areas vulnerable to rising sea levels. Permanent submersion, repeated flooding, faster erosion of cliffs and beaches, increasingly saline estuaries and salt contamination of groundwater are just some of the possible consequences of a rise in sea level in low-lying regions.
Mean Sea Level trends computed from altimetry, from January 1993 to 2005. Regional variations were sometimes more than ten times the average (West Pacific, Indian Ocean), whereas in other regions sea level actually decreased.
(Credits LEGOS/CNRS, France)
As global temperatures rise, mean sea level is rising with them: all the indicators point to an increase in the mean level of the world's oceans. As far as we can tell from a few isolated measurements taken around 1900, this rise has been continuing for at least a century. Today, thanks to the global coverage of altimetry satellites, estimates of the rise in sea level have improved in accuracy. With time series containing fifteen years of data, we can begin to observe trends and attempt to mitigate their effects.
Altimetry provides a powerful tool for determining the extent and causes of sea level change. Since early 1993, Topex/Poseidon has been measuring global sea level variations with great accuracy, complemented and followed by Envisat and Jason-1. This long dataset indicates that, in terms of the global mean, sea level is presently rising at a significantly higher rate than the mean rate recorded by tide gauges for the past five or more decades (of 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 greater land ice melting and/or increased ocean warming. Moreover, we cannot exclude the possibility that it at least partly reflects the decadal variability of the combined change in thermal and ocean mass. Longer datasets are needed to discriminate between the different hypotheses, and to study this question further.
Estimating mean sea level change and its sources is currently a very active research topic. Many teams are participating in fine-tuning the computations and studying the causes.
- Nerem, R.S. and G.T. Mitchum, Sea level change, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001
- Nerem R.S., E; Leuliette and Cazenave A., Present-day sea level change, C.R. Geosciences, in press, 2006.
- A. Cazenave, A. Lombard, R.S. Nerem, and K. DoMinh, Present-day sea level rise: do we understand what we measure?, 15 years of progress in radar altimetry Symposium, Venice, Italy, 2006
- Church, J.A., N.J. White, R. Coleman, K. Lambeck and J.X. Mitrovica, Global and regional sea-level rise, Papers of Note. Bulletin of the American Meteorological Society, 85 (7): 950-952, 2004.
- http://www.aviso.oceanobs.com/msl/ (Aviso/Altimetry, Mean Sea Level in cooperation between LEGOS, CLS and CNES)
- http://sealevel.colorado.edu/ (University of Colorado at Boulder, USA)
- http://copes.ipsl.jussieu.fr/Workshops/SeaLevel/ (Understanding Sea-level Rise and Variability Workshop, June 2006)
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