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.
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.
Between 1993 and 2003, comparisons between the different sources of sea level measurements showed that more than half of the increase was due to the thermal expansion of ocean waters. But since 2003, the warming of the ocean seems to pause, its contribution to sea level rise becomes smaller. This pause occurring in the ocean warming has slowed the rise in global ocean and is partly behind the reverse trend observed on the curve (below) in late 2007.
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.
The reference mean sea level since January 1993 (left) is calculated after removing the annual and semi-annual signals. A 2-month filter is applied to the blue points, while a 6-month filter is used on the red curve. By applying the postglacial rebound correction (-0.3 mm/year), the rise in mean sea level has thus been estimated as
mm/year (mean slope of the plotted data). Analysing the uncertainty of each altimetry correction made for calculating the GMSL, as well as a comparison with tide gauges gives an error in the GMSL slope of approximately 0.6 mm/year with a 90% confidence interval. (Credits CLS/Cnes/Legos)
Download the data (NetCDF).
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. 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).
Whether solid or liquid, sea ice occupies a volume which is an integral part of the ocean volume. When it melts, it therefore does not directly play a part in the sea level rise but the melting accelerates the ocean warming, so by steric effect, it contributes to increasing its volume.
Glaciers are very sensitive to global warming. Observations indicate that since the 1970s most world's glaciers are retreating and thinning, with noticeable accelerationsince the early 1990s. 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 potential contribution to sea level change. 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.