Sea-level rise is an increase in sea level. Multiple complex factors may influence this change.
Sea-level has risen about 130 meters (400 ft) since the peak of the last ice age about 18,000 years ago. Most of the rise occurred before 6,000 years ago. From 3,000 years ago to the start of the 19th century sea level was almost constant, rising at 0.1 to 0.2 mm/yr. Since 1900 the level has risen at 1 to 2 mm/yr; since 1993 satellite altimetry from TOPEX/Poseidon indicates a rate of rise of 3.1 ± 0.7 mm yr–1 . Church and White (2006) found a sea-level rise from January 1870 to December 2004 of 195 mm, a 20th century rate of sea-level rise of 1.7 ± 0.3 mm per yr and a significant acceleration of sea-level rise of 0.013 ± 0.006 mm per year. If this acceleration remains constant, then the 1990 to 2100 rise would range from 280 to 340 mm,. Sea-level rise can be a product of global warming through two main processes: thermal expansion of sea water and widespread melting of land ice. Global warming is predicted to cause significant rises in sea level over the course of the twenty-first century.
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
“Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
|Short-term (periodic) causes|| Time scale |
(P = period)
|Periodic sea level changes|
|Diurnal and semidiurnal astronomical tides||12–24 h P||0.2–10+ m|
|Rotational variations (Chandler wobble)||14 month P|
|Meteorological and oceanographic fluctuations|
|Atmospheric pressure||Hours to months||–0.7 to 1.3 m|
|Winds (storm surges)||1–5 days||Up to 5 m|
|Evaporation and precipitation (may also follow long-term pattern)||Days to weeks|
|Ocean surface topography (changes in water density and currents)||Days to weeks||Up to 1 m|
|El Niño/southern oscillation||6 mo every 5–10 yr||Up to 0.6 m|
|Seasonal water balance among oceans (Atlantic, Pacific, Indian)|
|Seasonal variations in slope of water surface|
|River runoff/floods||2 months||1 m|
|Seasonal water density changes (temperature and salinity)||6 months||0.2 m|
|Seiches (standing waves)||Minutes to hours||Up to 2 m|
|Tsunamis (generate catastrophic long-period waves)||Hours||Up to 10 m|
|Abrupt change in land level||Minutes||Up to 10 m|
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the volume of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice. Over much longer geological timescales, changes in the shape of the oceanic basins and in land/sea distribution will affect sea level.
Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.
Ice shelves float on the surface of the sea and, if they melt, to first order they do not change sea level. Likewise, the melting of the northern polar ice cap which is composed of floating pack ice would not significantly contribute to rising sea levels. Because they are fresh, however, their melting would cause a very small increase in sea levels, so small that it is generally neglected. It can however be argued that if ice shelves melt it is a precursor to the melting of ice sheets on Greenland and Antarctica.
The current rise in sea level observed from tide gauges, of about 1.8 mm/yr, is within the estimate range from the combination of factors above but active research continues in this field. The terrestrial storage term, thought to be highly uncertain, is no longer positive, and shown to be quite large.
Since 1992 the TOPEX/Poseidon and Jason 1 satellite programs have provided measurements of sea level change. Current data are available. The data show a mean sea level increase of 2.9±0.4 mm/yr. However, because significant short-term variability in sea level can occur, this recent increase does not necessarily indicate a long-term acceleration in sea level changes.
At times during Earth's long history, continental drift has arranged the land masses into very different configurations from those of today. When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level. However, over most of geologic time, long-term sea level has been higher than today (see graph above). Only at the Permian-Triassic boundary ~250 million years ago was long-term sea level lower than today. Long term changes in sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.
During the glacial/interglacial cycles over the past few million years, sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.
The Mediterranean Basin's gradual growth as the Neotethys basin, begun in the Jurassic, did not suddenly affect ocean levels. While the Mediterranean was forming during the past 100 million years, the average ocean level was generally 200 meters above current levels. However, the largest known example of marine flooding was when the Atlantic breached the Strait of Gibraltar at the end of the Messinian Salinity Crisis about 5.2 million years ago. This restored Mediterranean sea levels at the sudden end of the period when that basin had dried up, apparently due to geologic forces in the area of the Strait.
|Long-term causes||Range of effect||Vertical effect|
|Change in volume of ocean basins|
|Plate tectonics and seafloor spreading (plate divergence/convergence) and change in seafloor elevation (mid-ocean volcanism)||Eustatic||0.01 mm/yr|
|Marine sedimentation||Eustatic||< 0.01 mm/yr|
|Change in mass of ocean water|
|Melting or accumulation of continental ice||Eustatic||10 mm/yr|
|• Climate changes during the 20th century|
|•• Antarctica (the results of increasing precipitation)||Eustatic||-0.2 to 0.0 mm/yr|
|•• Greenland (from changes in both precipitation and runoff)||Eustatic||0.0 to 0.1 mm/yr|
|• Long-term adjustment to the end of the last ice age|
|•• Greenland and Antarctica contribution over 20th century||Eustatic||0.0 to 0.5 mm/yr|
|Release of water from earth's interior||Eustatic|
|Release or accumulation of continental hydrologic reservoirs||Eustatic|
|Uplift or subsidence of Earth's surface (Isostasy)|
|Thermal-isostasy (temperature/density changes in earth's interior)||Local effect|
|Glacio-isostasy (loading or unloading of ice)||Local effect||10 mm/yr|
|Hydro-isostasy (loading or unloading of water)||Local effect|
|Volcano-isostasy (magmatic extrusions)||Local effect|
|Sediment-isostasy (deposition and erosion of sediments)||Local effect||< 4 mm/yr|
|Vertical and horizontal motions of crust (in response to fault motions)||Local effect||1-3 mm/yr|
|Sediment compression into denser matrix (particularly significant in and near river deltas)||Local effect|
|Loss of interstitial fluids (withdrawal of groundwater or oil)||Local effect||≤ 55 mm/yr|
|Earthquake-induced vibration||Local effect|
|Departure from geoid|
|Shifts in hydrosphere, aesthenosphere, core-mantle interface||Local effect|
|Shifts in earth's rotation, axis of spin, and precession of equinox||Eustatic|
|External gravitational changes||Eustatic|
|Evaporation and precipitation (if due to a long-term pattern)||Local effect|
However, for the past 6,000 years (a few centuries before the first known written records), the world's sea level has been gradually approaching the level we see today. During the previous interglacial about 120,000 years ago, sea level was for a short time about 6 m higher than today, as evidenced by wave-cut notches along cliffs in the Bahamas. There are also Pleistocene coral reefs left stranded about 3 meters above today's sea level along the southwestern coastline of West Caicos Island in the West Indies. These once-submerged reefs and nearby paleo-beach deposits are silent testimony that sea level spent enough time at that higher level to allow the reefs to grow (exactly where this extra sea water came from—Antarctica or Greenland—has not yet been determined). Similar evidence of geologically recent sea level positions is abundant around the world.
Tide gauges in the United States show considerable variation because some land areas are rising and some are sinking. For example, over the past 100 years, the rate of sea level rise varies from about an increase of per year along the Louisiana Coast (due to land sinking), to a drop of a few inches per decade in parts of Alaska. The rate of sea level rise increased during the 1993-2003 period compared with the longer-term average (1961-2003), although it is unclear whether the faster rate reflects a short-term variation or an increase in the long-term trend.
These sea level rises could lead to difficulties for shore-based communities in the next centuries: for example, many major cities such as London and New Orleans already need storm-surge defenses, and would need more if sea level rose, though they also face issues such as sinking land.
Future sea level rise, like the recent rise, is not expected to be globally uniform (details below). Some regions show a sea-level rise substantially more than the global average (in many cases of more than twice the average), and others a sea level fall. However, models disagree as to the likely pattern of sea level change.
|IPCC change factors 1990-2100||IS92a prediction||SRES prediction|
|Thermal expansion||110 to 430 mm|
|Glaciers|| 10 to 230 mm|
(or 50 to 110 mm)
|Greenland ice||–20 to 90 mm|
|Antarctic ice||–170 to 20 mm|
|Terrestrial storage||–83 to 30 mm|
|Ongoing contributions from ice sheets in response to past climate change||0 to 55 mm|
|Thawing of permafrost||0 to 5 mm|
|Deposition of sediment||not specified|
| Total global-average sea level rise|
(IPCC result, not sum of above)
|110 to 770 mm|| 90 to 880 mm |
(central value of 480 mm)
The sum of these components indicates a rate of eustatic sea level rise (corresponding to a change in ocean volume) from 1910 to 1990 ranging from –0.8 to 2.2 mm/yr, with a central value of 0.7 mm/yr. The upper bound is close to the observational upper bound (2.0 mm/yr), but the central value is less than the observational lower bound (1.0 mm/yr), i.e., the sum of components is biased low compared to the observational estimates. The sum of components indicates an acceleration of only 0.2 (mm/yr)/century, with a range from –1.1 to +0.7 (mm/yr)/century, consistent with observational finding of no acceleration in sea level rise during the 20th century. The estimated rate of sea-level rise from anthropogenic climate change from 1910 to 1990 (from modeling studies of thermal expansion, glaciers and ice sheets) ranges from 0.3 to 0.8 mm/yr. It is very likely that 20th century warming has contributed significantly to the observed sea-level rise, through thermal expansion of sea water and widespread loss of land ice.
A common perception is that the rate of sea-level rise should have accelerated during the latter half of the 20th century, but tide gauge data for the 20th century show no significant acceleration. Estimates obtained are based on AOGCMs for the terms directly related to anthropogenic climate change in the 20th century, i.e., thermal expansion, ice sheets, glaciers and ice caps... The total computed rise indicates an acceleration of only 0.2 (mm/yr)/century, with a range from -1.1 to +0.7 (mm/yr)/century, consistent with observational finding of no acceleration in sea-level rise during the 20th century. The sum of terms not related to recent climate change is -1.1 to +0.9 mm/yr (i.e., excluding thermal expansion, glaciers and ice caps, and changes in the ice sheets due to 20th century climate change). This range is less than the observational lower bound of sea level rise. Hence it is very likely that these terms alone are an insufficient explanation, implying that 20th century climate change has made a contribution to 20th century sea level rise. Recent figures of human, terrestrial impoundment came too late for the 3rd Report, and would revise levels upward for much of the 20th century.
The results from Dyurgerov show a sharp increase in the contribution of mountain and subpolar glaciers to sea level rise since 1996 (0.5 mm/yr) to 1998 (2 mm/yr) with an average of approx. 0.35 mm/yr since 1960.
Of interest also is Arendt et al, who estimate the contribution of Alaskan glaciers of 0.14±0.04 mm/yr between the mid 1950s to the mid 1990s increasing to 0.27 mm/yr in the middle and late 1990s.
Rignot and Kanagaratnam produced a comprehensive study and map of the outlet glaciers and basins of Greenland. They found widespread glacial acceleration below 66 N in 1996 which spread to 70 N by 2005; and that the ice sheet loss rate in that decade increased from 90 to 200 cubic km/yr; this corresponds to an extra 0.25 to 0.55 mm/yr of sea level rise.
In July 2005 it was reported that the Kangerdlugssuaq glacier, on Greenland's east coast, was moving towards the sea three times faster than a decade earlier. Kangerdlugssuaq is around 1000 m thick, 7.2 km (4.5 miles) wide, and drains about 4% of the ice from the Greenland ice sheet. Measurements of Kangerdlugssuaq in 1988 and 1996 showed it moving at between 5 and 6 km/yr (3.1 to 3.7 miles/yr) (in 2005 it was moving at 14 km/yr (8.7 miles/yr).
According to the 2004 Arctic Climate Impact Assessment, climate models project that local warming in Greenland will exceed 3° Celsius during this century. Also, ice sheet models project that such a warming would initiate the long-term melting of the ice sheet, leading to a complete melting of the Greenland ice sheet over several millennia, resulting in a global sea level rise of about seven meters.
In 2002, Rignot and Thomas found that the West Antarctic and Greenland ice sheets were losing mass, while the East Antarctic ice sheet was probably in balance (although they could not determine the sign of the mass balance for The East Antarctic ice sheet). Kwok and Comiso (J. Climate, v15, 487-501, 2002) also discovered that temperature and pressure anomalies around West Antarctica and on the other side of the Antarctic Peninsula correlate with recent Southern Oscillation events.
In 2004 Rignot et al. estimated a contribution of 0.04±0.01 mm/yr to sea level rise from South East Greenland. In the same year, Thomas et al. found evidence of an accelerated contribution to sea level rise from West Antarctica. The data showed that the Amundsen Sea sector of the West Antarctic Ice Sheet was discharging 250 cubic kilometres of ice every year, which was 60% more than precipitation accumulation in the catchment areas. This alone was sufficient to raise sea level at 0.24 mm/yr. Further, thinning rates for the glaciers studied in 2002-2003 had increased over the values measured in the early 1990s. The bedrock underlying the glaciers was found to be hundreds of meters deeper than previously known, indicating exit routes for ice from further inland in the Byrd Subpolar Basin. Thus the West Antarctic ice sheet may not be as stable as has been supposed.
In 2005 it was reported that during 1992-2003, East Antarctica thickened at an average rate of about 18 mm/yr while West Antarctica showed an overall thinning of 9 mm/yr. associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12±0.02 mm/yr.
There is an implication that many of these impacts will be detrimental-- especially for the three-quarters of the world's poor who depend on agriculture systems. The report does, however, note that owing to the great diversity of coastal environments; regional and local differences in projected relative sea level and climate changes; and differences in the resilience and adaptive capacity of ecosystems, sectors, and countries, the impacts will be highly variable in time and space.
Statistical data on the human impact of sea level rise is scarce. A study in the April, 2007 issue of Environment and Urbanization reports that 634 million people live in coastal areas within of sea level. The study also reported that about two thirds of the world's cities with over five million people are located in these low-lying coastal areas. The IPCC report of 2007 estimated that accelerated melting of the Himalayan ice caps and the resulting rise in sea levels would likely increase the severity of flooding in the short-term during the rainy season and greatly magnify the impact of tidal storm surges during the cyclone season. A sea-level rise of just 40 cm in the Bay of Bengal would put 11 percent of the country's coastal land underwater, creating 7 to 10 million climate refugees.
Many media reports have focused the island nations of the Pacific, notably the Polynesian island of Tuvalu, which based on more severe flooding events in recent years, was thought to be "sinking" due to sea level rise. A scientific review in 2000 reported that based on University of Hawaii gauge data, Tuvalu had experienced a negligible increase in sea-level of 0.07 mm a year over the past two decades, and that ENSO had been a larger factor in Tuvalu's higher tides in recent years. A subsequent study by John Hunter from the University of Tasmania, however, adjusted for ENSO effects and the movement of the gauge (which was thought to be sinking). Hunter concluded that Tuvalu had been experiencing sea-level rise of about 1.2 mm per year. The recent more frequent flooding in Tuvalu may also be due to an erosional loss of land during and following the actions of 1997 cyclones Gavin, Hina, and Keli.
Reuters has reported other Pacific islands are facing a severe risk including Tegua island in Vanuatu. Claims that Vanuatu data shows no net sea level rise, are not substantiated by tide gauge data. Vanuatu tide gauge data show a net rise of ~50 mm from 1994-2004. Linear regression of this short time series suggests a rate of rise of ~7 mm/y, though there is considerable variability and the exact threat to the islands is difficult to assess using such a short time series.
Numerous options have been proposed that would assist island nations to adapt to rising sea level.
Sea level rise estimates from satellite altimetry are 3.1 +/- 0.4 mm/yr for 1993-2003 (Leuliette et al. (2004)). This exceeds those from tide gauges. It is unclear whether this represents an increase over the last decades; variability; true differences between satellites and tide gauges; or problems with satellite calibration.
Since 1992 the NASA/CNES TOPEX/Poseidon (T/P) and Jason-1 satellite programs have provided measurements of sea level change. The current data are available at http://sealevel.colorado.edu/ and http://sealevel.jpl.nasa.gov/. The data show a mean sea level increase of 2.8±0.4 mm/yr. This includes an apparent increase to 3.7±0.2 mm/yr during the period 1999 through 2004. Satellites ERS-1 (July 17 1991-March 10 2000), ERS-2 (April 21 1995-), and Envisat (March 1 2002-) also have sea surface altimeter components but are of limited use for measuring global mean sea level due to less detailed coverage.
Because significant short-term variability in sea level can occur, extracting the global mean sea level information is complex. Also, the satellite data has a much shorter record than tidal gauges, which have been found to require years of operation to extract trends.
There is a range of distances involved.
There apparently is a problem with the ERS-2 altimeter. Mean sea level changes were compared between satellites for 60°N and 60°S from May 1995 to June 1996:
Ongoing altimeter comparisons are available at: http://www7300.nrlssc.navy.mil/altimetry/intercomp.html
The various readings there are of current sea level variations, not global sea level, so the comparison is only in differences between the values. That data is of variations in centimeters; further processing is done to reach the millimeter-level resolution needed for mean sea level studies.
Comparisons of T/P with Pacific island tide gauge data show that the monthly mean deviations are accurate at the level of 20 mm.
Also, it should be noted that since satellite results are partially calibrated against tide gauge readings, they are not an entirely independent source.
The strong 1997-1998 El Niño event "has imprinted a strong signature on the sea surface height field in the mid-latitude eastern Pacific. This signal will be tracked over the next decade as the eastern boundary manifestation of this El Niño event propagates westward toward the Kuroshio Extension.
Other sea level analysis: