Sealevel changes

Global sea levels are currently rising as a result of the melting of the Greenland and Antarctica ice sheets and thermal expansion of the world's ocean waters due to global warming. The Earth is presently in an interglacial stage of an ice age, and sea levels have risen nearly 400 feet (130 m) since the last glacial maximum 20,000 years ago, and about six inches (15.25 cm) in the past 100 years. The rate of sea-level rise seems to be accelerating and may presently be as much as an inch (2.5 cm) every 8-10 years. If all the ice on both ice sheets were to melt, global sea levels would rise by 230 feet (70 m), inundating most of the world's major cities and submerging large parts of the continents under shallow seas. The coastal regions of the world are densely populated and are experiencing rapid population growth. Approximately 100 million people presently live within 3 feet (1 m) of the present day sea level. If sea level were to rise rapidly and significantly, the world would experience an economic and social disaster on a magnitude not yet experienced by the civilized world. Many areas would become permanently (on human timescales) flooded or subject to inundation by storms, beach erosion would be accelerated, and water tables would rise.

The Greenland and Antarctic ice sheets have significant differences that cause them to respond differently to changes in air and water temperatures. The Antarctic ice sheet is about 10 times as large as the Greenland ice sheet, and since it sits on the south Pole, Antarctica dominates its own climate. The surrounding ocean is cold even during summer, and much of Antarctica is a cold desert with low precipitation rates and high evaporation potential. Most meltwater in Antarctica seeps into underlying snow and simply refreezes, with little running off into the sea. Antarctica hosts several large ice shelves fed by glaciers moving at rates of up to a thousand feet (300 m) per year. Most ice loss in Antarctica is accomplished through calving and basal melting of the ice shelves, at rates of about 10-15 inches (25-38 cm) per year.

In contrast, Greenland's climate is influenced by warm North Atlantic currents and its proximity to other land masses. Climate data measured from ice cores taken from the top of the Greenland ice cap show that temperatures have varied significantly in cycles of years to decades. Greenland also experiences significant summer melting and abundant snowfall, and has few ice shelves; its glaciers move quickly at rates of up to miles (several km) per year. These fast-moving glaciers can drain a large amount of ice from Greenland in relatively short periods of time.

The Greenland ice sheet is thinning rapidly along its edges, loosing an average of 15-20 feet (4.5-6 m) in the past decade. In addition tidewater glaciers and the small ice shelves in Greenland are melting on an order of magnitude faster than the Antarctic ice sheets, with rates of melting between 25-65 (7.6-20 m) feet per year. About half of the ice lost from Greenland is through surface melting that runs off into the sea. The other half of ice loss is through calving of outlet glaciers and melting along the tidewater glaciers and ice shelf bases. If just the Greenland ice sheet melts, the water released will contribute another 23 feet (7 m) to sea-level rise, to a level not seen since 125,000 years ago.

These differences between the Greenland and Antarctic ice sheets lead them to play different roles in global sea-level rise. Greenland contributes more to the rapid, short-term fluctuations in sea level, responding to short-term changes in climate. In contrast, most of the world's water available for raising sea level is locked up in the slowly changing Antarctic ice sheet. Antarctica contributes more to the gradual, long-term, sea-level rise.

Data released by the IPCC in 2007 suggest that the current melting of glaciers is largely the result of the recent warming of the planet in the past 100 years through greenhouse warming. Greenhouse gases have been increasing at a rate of more than 0.2 percent per year, and global temperatures are rising accordingly. The most significant contributor to the greenhouse gas buildup is C02, produced mainly by burning fossil fuels. other gases that contribute to greenhouse warming include carbon monoxide, nitrogen oxides, methane (CH4), ozone (03), and chlorofluorocarbons. Methane is produced by gas from grazing animals and termites, whereas nitrogen oxides are increasing because of the increased use of fertilizers and automobiles, and chlorofluorocar-bons are increasing as a result of release from aerosols and refrigerants. Together the greenhouse gases have allowed short-wavelength incoming solar radiation to penetrate the gas in the upper atmosphere but trapped the solar radiation after it is reemitted from the Earth in a longer wavelength. The trapped radiation causes the atmosphere to heat up, leading to greenhouse warming. other factors also influence greenhouse warming and cooling, including the abundance of volcanic ash in the atmosphere and solar luminosity variations, as evidenced by sunspot variations.

Measuring global (also called eustatic) sea-level rise and fall is difficult because many factors influence the relative height of the sea along any coastline. Vertical motions of continents, called epeirogenic movements, may be related to plate tectonics; they rebound from being buried by glaciers or to changes in the amount of heat added to the base of the continent by mantle convection. Continents may rise or sink vertically, causing apparent sea-level change, but these sea-level changes are relatively slow compared with changes induced by global warming and glacial melting. Slow, long-term, sea-level changes can also be induced by changes in the amount of sea-floor volcanism associated with seafloor spreading. At some times in Earth history seafloor spreading was particularly vigorous, and the increased volume of volcanoes and the midocean ridge system caused global sea levels to rise.

steady winds and currents can mass water against a particular coastline, causing a local and temporary sea-level rise. such a phenomena is associated with the El-Nino-Southern Oscillation (ENSO), causing sea levels to rise by 4-8 (10-20 cm) inches in the Australia-Asia region. When the warm water moves east in an ENSO event, sea levels may rise 4-20 inches (10-50 cm) across much of the North and South American coastlines. Other atmospheric phenomena can also change sea level by centimeters to meters locally, on short timescales. Changes in atmospheric pressure, salinity of sea waters, coastal upwelling, onshore winds, and storm surges all cause short-term fluctuations along segments of coastline. Global or local warming of waters can cause them to expand slightly, causing a local sea-level rise. The extraction and use of groundwater and its subsequent release into the sea is likely causing an additional sea-level rise of about 0.05 inches (0.13 cm) per year. Seasonal changes in river discharge can temporarily change sea levels along some coastlines, especially where winter cooling locks up large amounts of snow that melt in spring.

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