Sea level rise
The term sea-level rise generally designates the average long-term global rise of the ocean surface measured from the centre of the earth (or more precisely, from the earth reference ellipsoid), as derived from satellite observations. Relative sea-level rise refers to long-term average sea-level rise relative to the local land level, as derived from coastal tide gauges.
Contributions to sea-level rise
Sea levels are highly variable over periods ranging from seconds to decades. Sea-level rise is the rising trend averaged over longer periods, which is observed at many coastal stations since a few centuries. Global warming due to human emissions of greenhouse gases is thought to be responsible for strengthening this trend over the last several decades at least . Several phenomena contribute to sea-level rise. At global scale, sea-level rise is mainly due to increase of the water mass and water volume of the oceans. This global rise of sea level (often termed Eustatic sea-level rise) has two components:
(1) water volume increase related to decrease of the density (also referred to as steric component), which is mainly due to increasing temperature, and
(2) water mass increase, which is mainly due to glacier melting (and to a lesser degree due to decreasing storage of surface water and groundwater on land).
Other phenomena can substantially influence sea levels at regional scale, inducing either sea-level rise or sea-level fall . Most important are:
(3) vertical earth crust motions - in particular earth crust adjustment to melting of polar ice caps, the so-called isostatic rebound,
(4) land surface subsidence, related in particular to extraction of groundwater and oil/gas mining and compaction of drained soils,
(5) changes in the earth gravitational field, related in particular to melting of polar ice caps,
(6) regional atmospheric pressure anomalies and changes in the strength and distribution of ocean currents, related in particular to ocean-atmosphere interaction, and
(7) changes in seawater salinity.
Due to these phenomena, sea-level rise is not uniform around the globe, but differs from place to place. Relative sea-level rise is the locally observed rise of the average sea level with respect to the land level. It is the sum of the components (1-7).
Observed sea-level rise
Trends in sea-level from world-wide available tide gauge records and from satellite measurements have been analysed by Church and White . The tide gauge data were corrected for vertical land surface motion, by using estimates for glacial isostatic adjustment (assuming that this is the major cause of vertical land surface motion). From these corrected tide gauge data, a linear trend of 1.7 ± 0.2 mm/year sea-level rise was found for the period 1900 to 1990 and a linear trend of 2.8 ± 0.8 mm/year for the period 1990 to 2009. From the satellite data a linear trend of 3.2 ± 0.4 mm/year was derived for the the same perod 1990 to 2009. From this analysis the authors conclude that there is a significant strengthening of sea-level rise during the last decades.
Trend analyses of regularly updated satellite data can be viewed at the NOAA site  for global and regional sea level changes around the world.
Even after correcting for the effect of glacial isostatic adjustment substantial regional differences in sea-level rise occur . Major causes are:
- elastic solid Earth deformation and self-gravitation related to changes in land ice;
- changes in seawater density related to the influence of fresh water input, ocean currents and atmospheric temperature.
Projections of future sea-level rise
Many model studies have been carried out for predicting future sea-level rise . A large spread of future forecasted levels results from uncertainties in future emissions of greenhouse gases, from shortcomings in the present understanding of climate dynamics (including ocean-atmosphere interaction) and from restrictions imposed on model grid scales. Figure 1 shows a compilation of model forecasts up till 2100 presented in the 5th IPCC Assessment Report. The models predict an increase of the rate of sea-level rise. Recent insight in the response of ice cap melting to global warming, which is not yet included in these projections, points to an even stronger increase .
Sea-level rise lags behind global warming. Even if greenhouse gas emissions would stop today, sea levels will continue rising for at least a century .
Impact of sea-level rise
Sea-level rise will impact in particular on low-lying coastal regions, such as river deltas and coral islands. These coastal zones are shaped under the influence of marine bio-geomorphological processes which limit their elevation to the level of high-water. Many of these regions are densely populated and host very large cities, especially in developing countries. In these regions, sea-level rise is generally exacerbated by soil compaction and land subsidence in connection with drainage works and the extraction of groundwater or oil / gas mining. The vulnerability of many of these deltas is further enhanced by coastal erosion, because of sediment retention behind upstream dams, hard coastal structures and/or conversion of mangrove forests to aquacultures , see Human causes of coastal erosion. Considerable investments are required for adaptation to sea-level rise in these vulnerable coastal regions, in particular to reduce flooding risks .
Sea-level rise enhances shoreline retreat (for retreating coasts) or reduces shoreline progradation (for accreting coasts), see Natural causes of coastal erosion. The influence of sea-level rise on the shoreline position can be estimated by means of the Bruun rule . Sea-level rise further threatens coastal wetlands, which may not be capable to keep pace with sea level and be partly lost due to so-called coastal squeeze. This may be the case for mudflats and salt marshes in the Wadden Sea  and for mangrove forests in the tropics and subtropics, see Potential Impacts of Sea Level Rise on Mangroves.
Salt intrusion is another major impact of sea-level rise in low-lying river deltas around the world. This impact is compounded by soil subsidence and by reduced fresh water supply to the coastal zone due to upstream diversion of river water for irrigation and other uses. Salt intrusion threatens crucially important fresh groundwater reservoirs in arid regions, for example in the Nile Delta . Relative sea-level rise also causes loss of fertile agricultural land in the coastal hinterland by increased salt seepage to surface waters , with great economic and social consequences. Salt intrusion may further affect drinking water availability in densely urbanized coastal regions.
Strategies for dealing with the impacts of sea-level rise depend on local conditions. Different strategies are reviewed in the 5th IPCC Assessment report . See also the Coastal Wiki article Climate adaptation policies for the coastal zone.
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