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episode]] Sea level rise is an increase in Sea Level . Multiple complex factors may influence this change. Sea level has risen about 130 Metre s (400 feet) 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.1 Since 1900 the level has risen at 1 to 2 mm/yr; since 1992 Satellite altimetry from TOPEX/Poseidon indicates a rate of rise about 3 mm/yr.2. 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} . Church and White (2006) find an acceleration of 1.3 ± 0.5 mm yr–1 per century over the period 1870 to 2000 {Link without Title} . Sea level rise can be a Product of Global Warming through two main processes: expansion of sea water as the oceans warm, and melting of ice over land. Global warming is predicted to cause significant rises in sea level over the course of the twenty-first century. OVERVIEW OF SEA LEVEL RISE 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 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. 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 inch) 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 8 mm every year. 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. 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 above4 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.http://sealevel.colorado.edu 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 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 70 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 were 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 (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 Reef s 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. Estimates See IPCC TAR, figure 11.4 for a graph of sea level changes over the past 140,000 years.5
FUTURE SEA LEVEL RISE In 2001, IPCC's The Third Assessment Report IPCC predicted that by 2100 , Global Warming will lead to a sea level rise of 9 to 88 cm. At that time no significant acceleration in the rate of sea level rise during the 20th century had been detected.http://www.grida.no/climate/ipcc_tar/wg1/013.htm Subsequently, Church and White found acceleration of 0.013 ± 0.006 mm/yr&2. 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 defenses, and would need more if sea level rose, though they also face issues such as sinking land.Church, J.A. and J.M. Gregory (Co-ordinating Lead Authors), 2001 ''Climate Change 2001: The Scientific Basis, Ch. 11. Changes in Sea Level'', Integovernmental Panel on Climate Change, http://www.grida.no/climate/ipcc_tar/wg1/408.htm Accessed on Dec. 19, 2005 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.7 However, models disagree as to the likely pattern of sea level change.8 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 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. We have obtained estimates 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 rise11 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. 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 also is Arendt et al,12 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.''13 estimate a net contribution from Greenland to be at least 0.13 mm/yr in the , Rignot ''et al.''15 estimated a contribution of 0.04±0.01 mm/yr to sea level rise from southeast Greenland. Rignot and Kanagaratnam16 produced a comprehensive study and map of the Outlet Glacier s 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 reportedhttp://news.independent.co.uk/world/environment/article301493.ece 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 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 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 °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 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 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. [http://www.gsfc.nasa.gov/topstory/20020606greenland.html In 2002, Rignot and Thomas17 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.18 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.
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