Altimetry applications in videos: Mean Sea Level

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Altimetry satellites are a crucial tool for observing variations in mean sea level. With greater quantities of data, we are able to confirm trends, and attempt to mitigate their effects.

See the Mean Sea Level video: flash (10 Mo) or mpeg (33 Mo)

Anthropogenic activity, increasing greenhouse gases, global warming, rising sea levels, environmental and human consequences. There is no need for subjunctives or superlatives, or logical links between these terms: they are no longer a potential threat, but a reality.

The Earth's climate has already experienced warming periods and submerged continents in the past, but their cause was astronomical (the relative positions of the Sun and the Earth changed our planet's insulation characteristics), and not anthropogenic. These changes, analysed after being stored in our planet's ice cores and sedimentary layers, took place over thousands of years, not over a century.

Demonstrated by the measurements

Since the early 1990's, the continuity and accuracy of altimetry measurements associated with in situ observations have even made it possible to identify an acceleration in the mean sea level rise on a global scale. Altimeter measurements show that between 1993 and 2007, the global mean sea level rise amounted to about 3 mm/yr whereas previously it had not exceeded 1.8 mm/yr over the twentieth century. These measurements are a global average, but in fact the rising sea level is far from uniform. Regional trends have also been observed: in some ocean regions the sea level has risen by up to 20 millimetres a year.

Causes of the rise

The two main phenomena responsible for the global mean rise are thermohaline expansion (due to variations in ocean temperature and salinity) and the exchange between inland waters and the oceans.

Thermal expansion is the phenomenon which occurs when waters expand through warming, causing an increase in volume.  When applied to the oceans, this translates as a rise in their level.

Inland water supplies include soil moisture content, snow cover, groundwater, rivers, lakes, glaciers and the polar ice sheets. The contribution of the polar ice sheets to the rise in sea level is still low, but melting in Greenland seems to be accelerating and could have a major impact on sea level rise.

Consequences

The records always tend to be debatable. Why bother for a few millimetres? But the human, ecological, and economic consequences are significant. From Bangladesh to the Tuvalu islands in the Pacific Ocean, people are already being forced to escape from the rising waters.

How high sea level will rise in the future is a vital question that is closely linked to climate change. Since the second half of 2006, a singular trend has been observed on the global mean sea level curve. The mean sea level trend seems to be weaker, or even reversed. Interannual variations, bias in sampling measurements, contribution of La Niña, there are several assumptions. Jason-2 will help tackle all these questions, by providing continuity of measurements and observing climate change.

Further information :

• Ocean indicators :
  • image selection, maps on Mean Sea Level as seen by altimeters.
  • Mean Sea Level problematics: questions in discussion
  • Processing Mean Sea Level with altimetry data (corrections),
  • Mean Sea Level as seen by other techniques.
• Applications : Mean sea level rise and the Greenhouse effect,
• Data : MSL
• External websites :Legos, IPCC, University of Colorado.