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episode]] Sea level rise is an increase in Sea Level . Multiple complex factors may influence such changes. The sea level has risen more than 120 of Global Warming on sea level, which is predicted to cause significant rises in sea level over the course of the twenty-first century. OVERVIEW Local and eustatic sea level , Atmosphere , and Glacier s.]] 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 largely removed. One must adjust perceived changes in LMSL to take into account vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur due to the Isostatic adjustment of the Mantle to the melting of ice sheets at the end of the last Ice Age . Atmospheric pressure (the Inverse Barometer Effect ), ocean currents and local ocean temperature changes can all 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 a variety of factors which can produce short-term (a few minutes to 14 months) changes in sea level. Longer term changes 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 ocean basins and in land/sea distribution will affect sea level. Observational estimates are that the rise in sea level due to rising temperature is about 1 mm/yr over recent decades.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. Glaciers and ice caps Each year about 8 mm (0.3 inches) of water from the entire surface of the oceans goes into the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 80 mm (3 inches) every 10 years. Although approximately the same amount of water returns to the ocean in icebergs and from ice melting at the edges, scientists do not know which is greater -- the ice going in or the ice coming out. The difference between the ice input and output is called the Mass Balance and is important because it causes changes in global sea level. Ice Shelves float on the surface of the sea and, if they melt, to first order they do not change sea level. 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 {Link without Title} but active research continues in this field. The uncertainty in the terrestrial storage term is particularly large. Since 1992 the TOPEX and JASON satellite programs have provided measurements of sea level change. The current data are available at {Link without Title} . 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. Geological influences 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 during at the Permo-Triassic boundary ~250 million years ago was long-term sea level lower than today. 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 melting of the Greenland and Antarctica ice sheets would result in a sea level rise of approximately 80 meters. 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. PAST CHANGES IN SEA LEVEL The sedimentary record For generations, geologists have been trying to explain the obvious cyclicity of Sedimentary deposits observed everywhere we look. The prevailing theories hold that this cyclicity primarily represents the response of depositional processes to the rise and fall of sea level. In the rock record, geologists see times when sea level was astoundingly low alternating with times when sea level was much higher than today, and these anomalies often appear worldwide. For instance, during the depths of the last Ice Age 18,000 years ago when hundreds of thousands of cubic miles of ice was stacked up on the continents as glaciers, sea level was 390 feet (120 m) lower, locations that today support coral reefs were left high and dry, and coastlines were miles farther basinward from the present-day coastline. It was during this time of very low sea level that there was a dry land connection between Asia and Alaska over which humans are believed to have migrated to North America (see Bering Land Bridge ). However, for the past 6,000 years (long before mankind started keeping 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 Reef s left stranded about 3 meters above today's sea level along the southwestern coastline of West Caicos Island in the British West Indies. These once-submerged reefs and nearby paleo-beach deposits are silent testament 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). Abundant similar evidence of geologically recent sea level positions can be found around the world. Estimates See IPCC TAR, figure 11.4 for a graph of sea level changes over the past 140,000 years.
FUTURE SEA LEVEL RISE Tide gauges and satellite altimetry suggest an increase in sea level of 1.5-3 mm/yr over the past 100 years. The IPCC predicts that by 2100 , Global Warming will lead to a sea level rise of 110 to 880 mm (details below). These sea level rises could lead to difficulties for shore-based communities: for example, many major cities such as London and New Orleans already need storm-surge defences, and would need more if sea level rose. TAR chapter 11 . Future sea level rise, like the recent rise, is not expected to be globally uniform. 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 [http://www.grida.no/climate/ipcc_tar/wg1/fig11-13.htm . Intergovernmental Panel on Climate Change results The results from the IPCC Third Assessment Report (TAR) sea level chapter (convening authors John A. Church and Jonathan M. Gregory ) are given below. 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 modelling 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 {Link without Title} . A common perception is that the rate of sea level rise should have accelerated during the latter half of the . 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 {Link without Title} . Uncertainties and criticisms regarding IPCC results
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 Ann. Glac. 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 {Link without Title} ) Of interest is also Arendt et al, (''Science'', 297, p. 382, July 2002) 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.'' (''Science'', Vol 289, Issue 5478, 428-430, , Rignot ''et al.'' (''Geophysical Research Letters'', v31, L10401) estimated a contribution of 0.04±0.01 mm/yr to sea level rise from southeast Greenland. Rignot and Kanagaratnam ( Science, 311, pp. 986 et seq., 2006 ) produced a comprehensive study and map of the Outlet Glacier s and basins of Greenland. They found widespread glacial accleration 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 {Link without Title} 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 degrees 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 millenia, resulting in a global sea level rise of about seven meters {Link without Title} . 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 degrees 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 degrees C. So even a projected 4 degrees 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 could 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 might contribute about 0.5 metres; this number is in the uncertainty of the estimates from Antarctica or Greenland but could be expected to be fast (within the coming century) whereas Greenland would be slow (perhaps 1500 years to fully deglaciate at the fastest likely rate) and Antarctica even slower {Link without Title} . In 2002, Rignot and Thomas (''Science'', v297, 1502-1506, 2002) 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. (''Geophysical Research Letters'', v31, L10401) 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. (Science, v306, 255-258, 2004) 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. (Davis et al., ''Science 2005'') . 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. {Link without Title} 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. 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 and will not necessarily be negative in all situations. To date, sea level changes have not been implicated in any substantial environmental, humanitarian, or economic losses. Previous claims have been made that parts of the island nations of . --> Despite President Gayoom speaking in the past about the impending dangers to his country, the Maldives , research found that the people of the Maldives have in the past survived a higher sea level about 50-60 cm and there is evidence of a significant sea level fall in the last 30 years in that Indian Ocean area. 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 . {Link without Title} Since 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 : {Link without Title}
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. {Link without Title} Also, it should be noted that since satellite results are partially calibrated against tide gauge readings, they are not an entirely independent source. {Link without Title} 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." {Link without Title} Other satellites:
Other sea level analysis:
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