Sea level is a very sensitive index of climate change and variability. As the ocean warms in response to global warming, sea waters expand and, as a result, sea level rises. When mountain glaciers melt in response to increasing air temperature, sea level rises because more freshwater glacial runoff discharges into the oceans. Similarly, ice mass loss from the ice sheets causes sea-level rise. The increase of freshwater flowing into the oceans reduces its salinity, decreasing its density and affecting ocean circulation patterns that in turn affect sea level and its spatial variability.
The global mean level of the oceans is an indicator of climate change. It incorporates the reactions from several different components of the climate system. Precise monitoring of changes in the mean level of the oceans is vitally important for understanding not just the climate but also the socioeconomic consequences of any rise in sea level.
Mean Sea Level is an average over all the oceans of sea surface height, with respect to a reference. However, what are really sought are the variations of this mean sea level along time. For twenty years now, the global mean sea level has been routinely measured over the whole oceanic domain with high-precision satellite altimetry, and such observations show clear evidence of global mean sea level rise ( +3.16 +/-0.5 mm/yr ).
This Mean Sea Level can be global, but basin (e.g. Atlantic) or regional ones can be computed. The rise in the level of the oceans is far from uniform. In fact, while in certain ocean regions the sea level has indeed risen (by up to 20 millimetres a year in places), in others it has fallen an equivalent amount. These regional differences, observed by altimetry satellites, mostly reflect sea level fluctuations over several years.
Past variations can be reconstructed from several indicators, but the very first measurements of sea level were made by monitoring tides in the 18th century. Although we now have a relatively dense network of tide gauges, only 20 stations, mostly along the coasts of Europe and North America, collected data throughout the 20th century. On the basis of their measurements, sea level is estimated to have risen by 10 to 20 centimetres since 1900.
Today, satellite altimetry, autonomous floats (Argo floats since 2003) and gravimetry data (Grace satellite) enable to measure Mean Sea Level variations, or some of their components. Ocean models are also used to understand and quantify those phenomena.
With the satellite altimetry missions, the global mean sea level (GMSL) has been calculated on a continual basis since January 1993. 'Verification' phases, during which the satellites follow each other in close succession (Topex/Poseidon--Jason-1, then Jason-1--Jason-2), help to link up these different missions by precisely determining any bias between them. Envisat, ERS-1 and ERS-2 are also used, after being adjusted on these reference missions, in order to compute Mean Sea Level at high latitudes (higher than 66°N and S), and also to improve spatial resolution by combining all these missions together. In addition, permanent monitoring of quality during the missions and studies of the necessary corrections of altimetry data regularly add to our understanding and knowledge.
There are several possible components which may take their part in the mean rise in sea level: an increase in the temperature of the water, which dilates as it warms, the thawing of mountain glaciers and polar ice caps in Greenland and Antarctica, as well as melting permafrost. Changes to the amount of rainfall and evaporation also play a part, as well as runoff and inland water reserves, mainly owing to human activity such as dam construction and irrigation.
A large proportion of the world’s population lives in coastal areas vulnerable to rising sea level. Permanent submersion, repeated flooding, faster erosion of cliffs and beaches, and increasingly saline estuaries and salt contamination of groundwater are just some of the possible consequences of a big rise in sea level in low-lying regions.