Sea level rise


Sea level rise

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. [Citation | title=Climate Change 2001: The Scientific Basis | contribution-url=http://www.grida.no/climate/ipcc_tar/wg1/425.htm | author=Houghton, J.T. (ed.) | contribution = Long-term mean sea level accelerations] 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 citation | author= Bindoff, NL et al. | contribution = Observations: Oceanic Climate Change and Sea Level | title= Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change | publisher=Cambridge University Press | contribution-url = http://ipcc-wg1.ucar.edu/wg1/Report/AR4WG1_Print_Ch05.pdf] . 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. [Citation | title=Climate Change 2001: The Scientific Basis | contribution-url=http://www.grida.no/climate/ipcc_tar/wg1/409.htm | author=Houghton, J.T. (ed.) | contribution = Chapter 11: Changes in sea level: executive summary.] Global warming is predicted to cause significant rises in sea level over the course of the twenty-first century.

Overview of sea-level change

Local and eustatic sea level

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 and periodic changes

There are many factors which can produce short-term (a few minutes to 14 months) changes in sea level.

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.Citation | title=Climate Change 2001: The Scientific Basis | url=http://www.grida.no/climate/ipcc_tar/wg1/426.htm#fig1110 | accessdate=2005-12-19] 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.

Uncertainties and criticisms regarding IPCC results

* Tide records with a rate of 180 mm/century going back to the 19th century show no measurable acceleration throughout the late 19th and first half of the 20th century. The IPCC attributes about 60 mm/century to melting and other eustatic processes, leaving a residual of 120 mm of 20th century rise to be accounted for. Global ocean temperatures by Levitus et al are in accord with coupled ocean/atmosphere modelling of greenhouse warming, with heat-related change of 30 mm. Melting of polar ice sheets at the upper limit of the IPCC estimates could close the gap, but severe limits are imposed by the observed perturbations in Earth rotation. (Munk 2002)
* By the time of the IPCC TAR, attribution of sea-level changes had a large unexplained gap between direct and indirect estimates of global sea-level rise. Most direct estimates from tide gauges give 1.5–2.0 mm/yr, whereas indirect estimates based on the two processes responsible for global sea-level rise, namely mass and volume change, are significantly below this range. Estimates of the volume increase due to ocean warming give a rate of about 0.5 mm/yr and the rate due to mass increase, primarily from the melting of continental ice, is thought to be even smaller. One study confirmed tide gauge data is correct, and concluded there must be a continental source of 1.4 mm/yr of fresh water. (Miller 2004)
* From (Douglas 2002): "In the last dozen years, published values of 20th century GSL rise have ranged from 1.0 to 2.4 mm/yr. In its Third Assessment Report, the IPCC discusses this lack of consensus at length and is careful not to present a best estimate of 20th century GSL rise. By design, the panel presents a snapshot of published analysis over the previous decade or so and interprets the broad range of estimates as reflecting the uncertainty of our knowledge of GSL rise. We disagree with the IPCC interpretation. In our view, values much below 2 mm/yr are inconsistent with regional observations of sea-level rise and with the continuing physical response of Earth to the most recent episode of deglaciation."
* The strong 1997-1998 El Niño caused regional and global sea level variations, including a temporary global increase of perhaps 20 mm. The IPCC TAR's examination of satellite trends says " the major 1997/98 El Niño-Southern Oscillation (ENSO) event could bias the above estimates of sea-level rise and also indicate the difficulty of separating long-term trends from climatic variability".

Glacier contribution

It is well known that glaciers are subject to surges in their rate of movement with consequent melting when they reach lower altitudes and/or the sea. The contributors to Annals of Glaciology [http://www.igsoc.org/annals/] , Volume 36 [http://www.igsoc.org/annals/36/] (2003) discussed this phenomenon extensively and it appears that slow advance and rapid retreat have persisted "throughout the mid to late Holocene" in nearly all of Alaska's glaciers. Historical reports of surge occurrences in Iceland's glaciers go back several centuries. Thus rapid retreat can have several other causes than CO2 increase in the atmosphere.

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. [Dyurgerov, Mark. 2002. Glacier Mass Balance and Regime: Data of Measurements and Analysis. INSTAAR Occasional Paper No. 55, ed. M. Meier and R. Armstrong. Boulder, CO: Institute of Arctic and Alpine Research, University of Colorado. Distributed by National Snow and Ice Data Center, Boulder, CO. A shorter discussion is at [http://nsidc.org/sotc/sea_level.html] ]

Of interest also is Arendt et al, [cite journal| last=Arendt| coauthors=et al| journal=Science| volume=297| pages=382–386| month= July| year= 2002 | title=Rapid Wastage of Alaska Glaciers and Their Contribution to Rising Sea Level | pmid=12130781 | doi = 10.1126/science.1072497 ] 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.

Greenland contribution

Krabill "et al." [cite journal| last=Krabill| coauthors= et al| journal=Science| volume= 289| issue= 5478| pages= 428–430| month=21 July| year=2000 | pmid=10903198 | doi = 10.1126/science.289.5478.428 | title=Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning] estimate a net contribution from Greenland to be at least 0.13 mm/yr in the 1990s. Joughin "et al." [cite journal| last=Joughin| coauthors=et al| journal=Nature| volume= 432| pages=608–610| month=December| year=2004 | pmid=15577906 | doi = 10.1038/nature03130 | title=Large fluctuations in speed on Greenland's Jakobshavn Isbræ glacier] have measured a doubling of the speed of Jakobshavn Isbræ between 1997 and 2003. This is Greenland's largest-outlet glacier; it drains 6.5% of the ice sheet, and is thought to be responsible for increasing the rate of sea level rise by about 0.06 millimeters per year, or roughly 4% of the 20th century rate of sea level increase. [ [http://www.spaceref.com/news/viewpr.html?pid=15611 Report shows movement of glacier has doubled speed | SpaceRef - Your Space Reference ] ] In 2004, Rignot "et al."cite journal| last=Rignot| coauthors=et al| journal=Geophysical Research Letters| year=2004| volume=31| pages=L10401 | title=Rapid ice discharge from southeast Greenland glaciers| doi = 10.1029/2004GL019474 ] estimated a contribution of 0.04±0.01 mm/yr to sea level rise from southeast Greenland.

Rignot and Kanagaratnam [cite journal| last=Rignot| coauthors=Kanagaratnam| url=http://www.sciencemag.org/cgi/content/abstract/311/5763/986?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=luckman&searchid=1140284328766_4322&FIRSTINDEX=0&journalcode=sci | journal=Science| volume= 311| pages= 986 et seq.| year= 2006 | title = Changes in the Velocity Structure of the Greenland Ice Sheet | doi = 10.1126/science.1121381 | pmid = 16484490 ] 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 [ [http://news.independent.co.uk/world/environment/article301493.ece Melting Greenland glacier may hasten rise in sea level - Environment - Independent.co.uk ] ] 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. [http://www.metoffice.gov.uk/corporate/pressoffice/adcc/BookCh4Jan2006.pdf]

Effects of snowline and permafrost

The snowline altitude is the altitude of the lowest elevation interval in which minimum annual snow cover exceeds 50%. This ranges from about 5500 metres above sea-level at the equator down to sea-level at about 65° N&S latitude, depending on regional temperature amelioration effects. Permafrost then appears at sea-level and extends deeper below sea-level pole-wards. The depth of permafrost and the height of the ice-fields in both Greenland and Antarctica means that they are largely invulnerable to rapid melting. Greenland Summit is at 3200 metres, where the average annual temperature is minus 32 °C. So even a projected 4 °C rise in temperature leaves it well below the melting point of ice. Frozen Ground 28, December 2004, has a very significant map of permafrost affected areas in the Arctic. The continuous permafrost zone includes all of Greenland, the North of Labrador, NW Territories, Alaska north of Fairbanks, and most of NE Siberia north of Mongolia and Kamchatka. Continental ice above permafrost is very unlikely to melt quickly. As most of the Greenland and Antarctic ice sheets lie above the snowline and/or base of the permafrost zone, they cannot melt in a timeframe much less than several millennia; therefore they are unlikely to contribute significantly to sea-level rise in the coming century.

Polar ice

The sea level will rise above its current level if more polar ice melts. However, compared to the heights of the ice ages, today there are very few continental ice sheets remaining to be melted. It is estimated that Antarctica, if fully melted, would contribute more than 60 metres of sea level rise, and Greenland would contribute more than 7 metres. Small glaciers and ice caps on the margins of Greenland and the Antarctic Peninsula might contribute about 0.5 metres. While the latter figure is much smaller than for Antarctica or Greenland it could occur relatively quickly (within the coming century) whereas melting of Greenland would be slow (perhaps 1500 years to fully deglaciate at the fastest likely rate) and Antarctica even slower. However, this calculation does not account for the possibility that as meltwater flows under and lubricates the larger ice sheets, they could begin to move much more rapidly towards the sea. [cite journal| url = http://www.sciencemag.org/cgi/content/abstract/297/5579/218 | title=Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow | author=Zwally H.J. et al. | journal = Science | volume = 297 | pages = 218–222 | doi = 10.1126/science.1072708 | year= 2002 | pmid = 12052902 ] [cite web | url = http://www.gsfc.nasa.gov/topstory/20020606greenland.html | title=Greenland Ice Sheet flows faster during summer melting | date = 2006-06-02 | publisher = Goddard Space Flight Center (press release) ]

In 2002, Rignot and Thomas [cite journal| last=Rignot| coauthors=Thomas| journal=Science| volume=297| pages=1502–1506| year= 2002 | pmid=12202817 | doi = 10.1126/science.1073888 | title=Mass Balance of Polar Ice Sheets] 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. [cite journal| last=Thomas| coauthors=et al| year= 2004| journal=Science| volume=306| pages= 255–258 | pmid=15388895 | doi = 10.1126/science.1099650 | title=Accelerated Sea-Level Rise from West Antarctica] 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. [cite journal| last=Davis| journal=Science| year=2005| doi=10.1126/science.1110662| pages=1898| pmid=15905362 | month= 24 June| volume=308| issue= 5730| pages= 1898 - 1901| title=Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise| first=Curt H.| coauthors=Yonghong Li, Joseph R. McConnell, Markus M. Frey, Edward Hanna]

Effects of sea level rise

Based on the projected increases stated above, the IPCC TAR WG II report notes that current and future climate change would be expected to have a number of impacts, particularly on coastal systems. [cite web | title=Climate Change 2001: Impacts, Adaptation and Vulnerability | url=http://www.grida.no/climate/ipcc_tar/wg2/292.htm | accessdate=2005-12-19 ] Such impacts may include increased coastal erosion, higher storm-surge flooding, inhibition of primary production processes, more extensive coastal inundation, changes in surface water quality and groundwater characteristics, increased loss of property and coastal habitats, increased flood risk and potential loss of life, loss of nonmonetary cultural resources and values, impacts on agriculture and aquaculture through decline in soil and water quality, and loss of tourism, recreation, and transportation functions.

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. ["Climate Shocks: Risk and Vulnerability in an Unequal World." Human Development report 2007/2008. hdr.undp.org/media/hdr_20072008_summary_english.pdf] 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 convert|30|ft|m 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.

Are islands "drowning"?

IPCC assessments suggest that deltas and small island states are particularly vulnerable to sea level rise caused by both thermal expansion and ocean volume. Relative sea level rise (mostly caused by subsidence) is currently causing substantial loss of lands in some deltas. [cite book|last=Tidwell|first=Mike|title="The Ravaging Tide: Strange Weather, Future Katrinas, and the Coming Death of America's Coastal Cities|publisher=Free Press|year=2006|isbn=0-7432-9470-X] Sea level changes have not yet been conclusively proven to have directly resulted in environmental, humanitarian, or economic losses to small island states, but the IPCC and other bodies have found this a serious risk scenario in coming decades. [ [http://www.wbgu.de/wbgu_sn2006_en.pdf The Future Oceans - Warming Up, Rising High, Turning Sour ] ]

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. [cite news| last=Levine| first= Mark| month=December | year=2002| title=Tuvalu Toodle-oo| publisher= Outside Magazine| url=http://outside.away.com/outside/features/200212/200212_tuvalu_1.html| accessdate=2005-12-19] 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.cite news| last=Patel| first= Samir S.| date=5 April 2006| title=A Sinking Feeling| publisher=Nature| url=http://www.nature.com/nature/journal/v440/n7085/full/440734a.html| accessdate=2007-11-15] 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. [cite news| last=Hunter| first=J.A.| date=12 August 2002| title=A Note on Relative Sea Level Rise at Funafuti, Tuvalu| url=http://staff.acecrc.org.au/~johunter/tuvalu.pdf] 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. [cite news| last=Field| first= Michael J.| month=December | year=2001| title=Sea Levels Are Rising| publisher=Pacific Magazine| url=http://www.pacificislands.cc/pm122001/pmdefault.php?urlarticleid=0009| accessdate=2005-12-19]

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. [cite web | title=Policy Implications of Sea Level Rise: The Case of the Maldives. | work=Proceedings of the Small Island States Conference on Sea Level Rise. November 14-18, 1989. Male, Republic of Maldives. Edited by Hussein Shihab | url=http://users.rcn.com/jtitus/Maldives/Small_Island_States_3.html| accessdate=2007-01-12 ]

Satellite sea level measurement

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.cite web | title=Satellite Measurements of Sea Level Rise | url=http://membrane.com/sidd/sealevel.html| accessdate=2005-12-19| last=Mukherjee| first= S.| year=2004] Satellites ERS-1 (July 17 1991-March 10 2000), [cite web | title=ERS | url=http://earth.esa.int/ers/ | accessdate=2005-12-19 ] ERS-2 (April 21 1995-), [cite web | title=ESA - Observing the Earth - ERS overview | url=http://www.esa.int/export/esaEO/SEMGWH2VQUD_index_0_m.html | accessdate=2005-12-19 ] 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.

* TOPEX/Poseidon began its series of global measurements of sea surface height in 1992, and the scientific mission was ended in October 2005.
* Jason-1, launched December 7, 2001, has now taken over the mission and is flying the same groundtrack.
* Ocean Surface Topography Mission/Jason-2 was launched on June 20, 2008, and after calibration and validation will replace Jason-1. Jason-1 will move to one side and continue measuring the ocean surface, doubling the amount of data collected. [cite web|url=http://sealevel.jpl.nasa.gov/mission/ostm-fact-sheet.html|title=JPL OSTM/Jason-2 Spacecraft Fact Sheet|publisher=JPL|accessdate=2008-07-10]

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.

* 140 to 320 mm: Increased height of sea level within 1997-1998 El Niño Pacific region. [cite web | title=40+ Years of Earth Science * TOPEX/Poseidon * | url=http://www.earth.nasa.gov/history/topex/topex.html | accessdate=2005-12-19 ]
* 140 mm: Range of typical regional sea level variations (±70 mm). [cite web | title=http://ibis.grdl.noaa.gov/SAT/hist/tp_products/dev/Fall_2001_Deviation.gif| url=http://ibis.grdl.noaa.gov/SAT/hist/tp_products/dev/Fall_2001_Deviation.gif | accessdate=2005-12-19 ]
* 100 mm: Accuracy of ERS-1 radar altimeter. [cite web | title=Altimeter design | url=http://earth.esa.int/rootcollection/eeo4.10075/altimeter_design.html | accessdate=2005-12-19 ]
* 43 mm: Accuracy of ocean surface height calculations with "T/P". [cite web | title=Ocean Surface Topography from Space-Technology | url=http://sealevel.jpl.nasa.gov/technology/instrument-altimeter.html | accessdate=2005-12-19 ]
* 30 to 40 mm: Accuracy of TOPEX and POSEIDON-1 radar altimeters, which measure distance to ocean surface.
* 20 to 30 mm: Accuracy of determination of "T/P" satellite orbital height (laser ranging, doppler shifts, GPS).
* 20 mm: Accuracy of Jason-1 POSEIDON-2 radar altimeter. [cite web | title=Aviso/Altimetry - How altimetry works | url=http://www.jason.oceanobs.com/html/alti/principe_uk.html | accessdate=2005-12-19 ]
* 7-14 mm: Global mean sea level surge during 1997-1998 El Niño period. [cite web | title=Ocean Surface Topography from Space-Science | url=http://sealevel.jpl.nasa.gov/science/invest-cazenave.html | accessdate=2005-12-19 ]
* Several mm: Precision of global mean sea level measurement after averaging 10-day coverage. [cite web | title=Ocean Surface Topography from Space-Technology | url=http://sealevel.jpl.nasa.gov/technology/technology.html | accessdate=2005-12-19 ]
* 10 mm: Stability of "T/P" orbit heights over 4 years. [cite web | title=Monitoring the Stability of Satellite Altimeters with Tide Gauges | url=http://ams.allenpress.com/amsonline/?request=get-abstract&issn=1520-0426&volume=015&issue=03&page=0721 | accessdate=2005-12-19 ]
* 2.8 ±0.4 mm: Average annual global sea level rise since 1992 according to "T/P".

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: [cite web | title=Monitoring of the ERS-2 Radar Altimeter range measurement stability | author=P Moore, MD Reynoldsand R J Walmsley | url=http://earth.esa.int/pcs/ers/ra/articles/soc-esafrv.pdf | accessdate=2005-12-19| format=PDF ]
* -4.7 ±1.5 mm/yr for ERS-1
* -5.6 ±1.3 mm/yr for TOPEX
* +9.0 ±2.1 mm/yr for ERS-2

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. [cite web | title=T/P versus Pacific Gauges | url=http://ibis.grdl.noaa.gov/SAT/hist/tp_gauge/index.html | accessdate=2005-12-19 ]

Also, it should be noted that since satellite results are partially calibrated against tide gauge readings, they are not an entirely independent source. [cite web | title=Global mean sea level calibration | url=http://sealevel.colorado.edu/calibration.html | accessdate=2005-12-19]

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." [cite web | title=The dynamics of short-term ocean climate variability | url=http://sealevel.jpl.nasa.gov/science/invest-chelton.html | accessdate=2005-12-19]

Other satellites:
* Geosat Follow-On is a U.S. Navy altimeter mission that was launched on February 10, 1998. On November 29, 2000, the Navy accepted the satellite as operational. During its mission life, the satellite will be retained in the GEOSAT Exact Repeat Mission (ERM) orbit (800 km altitude, 108 deg inclination, 0.001 eccentricity, and, 100 min period). This 17-day Exact Repeat Orbit (ERO) retraces the ERM ground track to +/-1 km. As with the original GEOSAT ERM, the data will be available for ocean science through NOAA/NOS and NOAA/NESDIS. Radar Altimeter - single frequency (13.5 GHz) with 35 mm height precision. Note that the GPS receiver is not functional.
** Geosat Follow-On @ NOAA/LSA [cite web | title= Geosat Follow-On @ NOAA/LSA | url=http://ibis.grdl.noaa.gov/SAT/gfo/ | accessdate=2005-12-19 ]
** NAVY GEOSAT FOLLOW-ON (GFO) ALTIMETRY MISSION [cite web | title= NAVY GEOSAT FOLLOW-ON (GFO) ALTIMETRY MISSION | url=http://gfo.bmpcoe.org/Gfo/default.htm | accessdate=2005-12-19 ]
** NASA WFF Geosat Follow-On [cite web | title= NASA WFF Geosat Follow-On | url=http://gfo.wff.nasa.gov/ | accessdate=2005-12-19 ] Other sea level analysis:
* Sea Level Analysis from ERS Altimetry [cite web | title= Sea Level Analysis from ERS Altimetry | url=http://nng.esoc.esa.de//ers/alti.html | accessdate=2005-12-19 ]
* Ssalto/Duacs multimission altimeter products: [cite web | title= Ssalto/Duacs multimission altimeter products | url=http://www.jason.oceanobs.com/html/donnees/duacs/welcome_uk.html | accessdate=2005-12-19 ] Combined current data from Topex/Poseidon, Geosat Follow On, Jason-1 and Envisat.

ee also

* 8.2 kiloyear event
* Antarctic Cold Reversal
* Climate of Antarctica
* Marine regression
* Older Peron transgression
* Retreat of glaciers since 1850

External links

* [http://www.cmar.csiro.au/sealevel Sea Level Rise:Understanding the past - Improving projections for the future]
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* [http://papers.ssrn.com/sol3/papers.cfm?abstract_id=950329 Providing new homes for climate exiles] Sujatha Byravan and Sudhir Chella Rajan, 2006
* [http://stephenschneider.stanford.edu/Publications/PDF_Papers/MornerEtAl2004.pdf New perspectives for the future of the Maldives] Nils-Axel Mörner, Michael Tooley, Göran Possnert, 2004
*
* [http://www.pol.ac.uk/psmsl/palaeoshoreline_webpage/HTML/HOME.htm Changes in the Earth's shorelines during the past 20 kyr caused by the deglaciation of the Late Pleistocene ice sheets] , from the Permanent Service for Mean Sea Level
* [http://www.pol.ac.uk/psmsl/palaeoshoreline_webpage/HTML/Science.htm Includes picture of sea level for past 20 kyr based on barbados coral record]
* [http://www.agu.org/revgeophys/dougla01/dougla01.html Global sea level change: Determination and interpretation]
* [http://www.radix.net/~bobg/faqs/sea.level.faq.html Sea level rise FAQ] (1997)
* [http://www.gloss-sealevel.org/ The Global Sea Level Observing System (GLOSS)]
* [http://www.bodc.ac.uk/data/information_and_inventories/gloss_handbook/ The GLOSS Station Handbook]
* [http://flood.firetree.net/ Interactive sea level map]
*
* [http://www.thesinkingoftuvalu.com/ The Sinking of Tuvalu]
* [http://www.cresis.ku.edu/research/data/sea_level_rise/index.html Center for the Remote Sensing of Ice Sheets - Maps, Animations, GIS Layers]
* [http://www.interactive-earth.com/visualizations/sea_level_fluctuation.htm Tides and Sea Level Rise Model]

References

Footnotes

General references


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* Warrick, R. A., C. L. Provost, M. F. Meier, J. Oerlemans, and P. L. Woodworth. 1996. Changes in sea level, in Climate Change 1995: The Science of Climate Change. pp. 359-405.
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* Colorado Center for Astrodynamics Research, " [http://sealevel.colorado.edu/ Mean Sea Level] " Accessed December 19, 2005
* Fahnestock, Mark (December 4, 2004), " [http://www.spaceref.com/news/viewpr.html?pid=15611 Report shows movement of glacier has doubled speed] ", University of New Hampshire press release. Accessed December 19, 2005
* Morano, Mark (December 07, 2005), " [http://www.cnsnews.com/ViewCulture.asp?Page=%5CCulture%5Carchive%5C200512%5CCUL20051207a.html Climatologist Rejects 'Global Warming' as Cause for Island Evacuation] ", "CNSNews.com". Accessed December 19, 2005
* Leuliette, E.W., R.S. Nerem, and G.T. Mitchum, 2004: Calibration of TOPEX/Poseidon and Jason Altimeter Data to Construct a Continuous Record of Mean Sea Level Change. Marine Geodesy, 27(1-2), 79- 11 94. 12
* National Snow and Ice Data Center (14 March 2005), " [http://nsidc.org/sotc/sea_level.html Is Global Sea Level Rising?] ". Accessed December 19, 2005
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