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Causes of rising

Variations in the global mean sea level, as well as local variations in lakes and enclosed seas, are mostly due to climate fluctuations. A rising trend linked to global warming is emerging. Causes of mean sea level rise are more and more known.

Sea level is rising as seen by altimeters since 1992

Temperature and salinity variations

Changes in water temperature impact sea level variations. As water warms, it expands and its volume increases, causing levels to rise. We observe a two-month delay for temperature changes to propagate down. The effects of global warming can thus make themselves felt in the oceans within a few years.

The quantity of salts in the water has also an influence on sea level, since it changes the water density. The more salty the water, the denser it is, and the lower the level.

Water exchanges

Larsen ice shelf breakup in Antarctica, in february 2002 (area : 3250 km², weight 720 billion tons). Modis image of Terra satellite. (Credits Nasa).

Variations in the polar ice caps mass in Greenland and Antarctica also affect sea level. Increased snowfall caused by rising temperatures appeared to be offsetting faster melting of ice along the coasts. But the glaciers acceleration to the coast, observed in the west of the Antarctic continent, lead towards an ice cap mass loss. In Greenland and in the North Pole area, warming is faster than anywhere else. Summer temperature in Greenland are high enough to melt the ice in this area.

Sea ice also suffered the bad effects of global warming. Quantity for its melting is accelerating by a mechanism of amplification (positive feedback) : ice reflects solar radiation by a simple reflection, a decrease of the surface or a change in its texture will reduce albedo and will increase absorbed radiation (further information : Atimetry and Ice or Arctic sea ice extent as observed by Envisat).

 

 

Thickness variations in Icelandic, Alpine and Himalayans glaciers. Losses are similar and strong at low altitude whereas they are limited at high altitude, in accumulation area. (Credits Cnes/Legos).

Most of mountain glaciers has decreased now. The first affected are glaciers at medium or low altitude (~ 3000 m in the Alps) : recent simulations based on the most optimistic scenarios of the IPCC in 2007 (scenario B1 : +1.8 °C in 2100) suggest their disappearance in 2060. Their melting has contributed and significantly contributes to the rise in sea level observed during the 20th century until today.

Changes in quantities of liquid water stored in the Earth, reservoirs and groundwater, are also a cause of rising sea level. Fluctuations in the level of continental waters can also be a direct result of human activities, such as damming of rivers or water abstraction to irrigate crops.

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