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Sea level

by LEGOS last modified Feb 04, 2014 10:40 AM
Sea level



An important consequence of climate change is sea level variations. All components of the climate system are involved through a variety of processes:

  1. Ocean expansion or contraction in response to density variations caused by ocean temperature and salinity variations (steric effects).
  2. Water mass exchange with other components of the system: atmosphere, continental waters (soil humidity, snow cover, groundwater, rivers, lakes), mountain and polar ice sheets.
  3. Variations in ocean circulation in response to low-frequency variations from atmospheric forcing.


The first two effects determine global ocean volume and mass variations. (note that salinity changes have negligible contribution in terms of global mean sea level). The third effect has no impact on global average but is responsible for local/regional mean sea level variations. Steric and ocean mass contributions over the last few decades can be estimated through observations of ocean temperature variations, land surface waters and ice mass variations. For decades, tide gauges were the only technique available for directly observing sea level variations. However, tide gauges only measure sea level relative to the coast (vertical Earth crust movements must be taken into account) and in situ measurement coverage is limited to continental coastlines and islands.

Since the early 1990s, "absolute" sea level variations can be measured to a precision of a few tenths of mm/yr, with global coverage of the ocean domain with the ERS, ENVISAT, Topex-Poseidon and Jason-1 altimetry missions. The Topex-Poseidon altimetry measurements indicate that global mean sea level rose by 3.3+0.4 mm/yr between 1993 and 2006 (see "Recents results" page). Satellite altimetry also provides mapping of regional sea level variations. The regional sea level trend map, based on Topex-Poseidon data, is presented on the "Recent results" page.

Recent ocean temperature data indicate that thermal expansion is responsible for ~50% of sea level rise observed over the 1993-2003 period (which does not hold true for the 1950-2000 period when thermal expansion was only responsible for 25% of sea level rise). The melting of mountain glaciers over the recent years, ice mass loss in Greenland and Antarctica, and water exchanges between the continental reservoirs, contribute to the remaining half of the 3 mm/year sea level rise.

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