|
|
|
|
Greenland ice sheet
|
| |
|
| |
The Greenland ice sheet is a vast body of ice covering 1.71 million km², roughly 80% of the surface of Greenland. It is the second largest ice body in the World, after the Antarctic Ice Sheet. The ice sheet is almost 2,400 kilometers long in a north-south direction, and its greatest width is 1,100 kilometers at a latitude of 77° N, near its northern margin.
Discussion
Ask a question about 'Greenland ice sheet' Start a new discussion about 'Greenland ice sheet' Answer questions from other users |
Encyclopedia
The Greenland ice sheet is a vast body of ice covering 1.71 million km², roughly 80% of the surface of Greenland. It is the second largest ice body in the World, after the Antarctic Ice Sheet. The ice sheet is almost 2,400 kilometers long in a north-south direction, and its greatest width is 1,100 kilometers at a latitude of 77° N, near its northern margin. The mean altitude of the ice is 2,135 meters. The thickness is generally more than 2 km (see picture) and over 3 km at its thickest point. It is not the only ice mass of Greenland - isolated glaciers and small ice caps cover between 76,000 and 100,000 square kilometers around the periphery. Some scientists believe that global warming may be about to push the ice sheet over a threshold where the entire ice sheet will melt in less than a few hundred years. If the entire 2.85 million km³ of ice were to melt, it would lead to a global sea level rise of 7.2 m (23.6 ft). This would inundate most coastal cities in the World and remove several small island countries from the face of Earth, since island nations such as Tuvalu and Maldives have a maximum altitude below or just above this number.
The Greenland Ice Sheet is also sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis. It is also sometimes referred to as an ice cap. Ice sheet, however, is considered the more correct term as ice cap generally refers to less extensive ice masses.
The ice in the current ice sheet is as old as 110,000 years. However,
it is generally thought that the Greenland Ice Sheet formed in the late Pliocene or early Pleistocene by coalescence of ice caps and glaciers. It did not develop at all until the late Pliocene, but apparently developed very rapidly with the first continental glaciation.
The massive weight of the ice has depressed the central area of Greenland; the bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery, confining the sheet along its margins. If the ice were to disappear, Greenland would most probably appear as an archipelago. The ice surface reaches its greatest altitude on two north-south elongated domes, or ridges. The southern dome reaches almost 3,000 metres at latitudes 63° - 65° N; the northern dome reaches about 3,290 metres at about latitude 72° N. The crests of both domes are displaced east of the centre line of Greenland. The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, so that no large ice shelves occur. The ice margin just reaches the sea, however, in a region of irregular topography in the area of Melville Bay southeast of Thule. Large outlet glaciers, which are restricted tongues of the ice sheet, move through bordering valleys around the periphery of Greenland to calve off into the ocean, producing the numerous icebergs that sometimes occur in North Atlantic shipping lanes. The best known of these outlet glaciers is Jakobshavn Isbræ , which, at its terminus, flows at speeds of 20 to 22 metres per day.
On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The lowest mean annual temperatures, about -31°C (-24°F), occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about -20°C (-4°F).
During winter, the ice sheet takes on a strikingly clear blue/green color. During summer, the top layer of ice melts leaving pockets of air in the ice that makes it look white.
The ice sheet as a record of past climates
The ice sheet, consisting of layers of compressed snow from more than a hundred thousand years, contains in its ice today's most valuable record of past climates. In the past decades, scientists have drilled ice cores up to four kilometres deep. Scientists have, using those ice cores, obtained information on (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, desert extent and forest fires. This variety of climatic proxies is greater than in any other natural recorder of climate, such as tree rings or sediment layers.
The melting ice sheet
Positioned in the Arctic, the Greenland ice sheet is especially vulnerable to global warming. Arctic climate is now rapidly warming and much larger Arctic shrinkage changes are projected. The Greenland Ice Sheet has experienced record melting in recent years and is likely to contribute substantially to sea level rise as well as to possible changes in ocean circulation in the future. The area of the sheet that experiences melting has increased about 16% from 1979 (when measurements started) to 2002 (most recent data). The area of melting in 2002 broke all previous records. The number of glacial earthquakes at Helheim and the northwest Greenland glaciers increased substantially between 1993 and 2005. In 2006, estimated monthly changes in the mass of Greenland's ice sheet suggest that it is melting at a rate of about 239 cubic kilometres (57.3 cubic miles) per year. A more recent study, based on reprocessed and improved data between 2003 and 2008, reports an average trend of 195 cubic kilometres (46.7 cubic miles) per year. These measurements came from the US space agency's Grace (Gravity Recovery and Climate Experiment) satellite, launched in 2002, as reported by BBC. Using data from two ground-observing satellites, ICESAT and ASTER, a study published in Geophysical Research Letters (September 2008) shows that nearly 75 percent of the loss of Greenland's ice can be traced back to small coastal glaciers.
If the entire 2.85 million km³ of ice were to melt, global sea levels would rise 7.2 m (23.6 ft.). Recently, fears have grown that continued global warming will make the Greenland Ice Sheet cross a threshold where long-term melting of the ice sheet is inevitable. Climate models project that local warming in Greenland will exceed 3 degrees Celsius during this century. 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 ice sheet (over centuries), resulting in a global sea level rise of about seven meters. Such a rise would inundate almost every major coastal city in the World. How fast the melt would eventually occur is a matter of discussion. According to IPCC, the expected 3 degrees warming at the end of the century would, if kept from rising further, result in about 1 meter sea level rise over the next millennium (see image to the left).
Some scientists have cautioned that these rates of melting as overly optimistic as they assume a linear, rather than erratic, progression. James Hansen has argued that multiple positive feedbacks could lead to nonlinear ice sheet disintegration much faster than claimed by the IPCC. According to a 2007 paper, "we find no evidence of millennial lags between forcing and ice sheet response in paleoclimate data. An ice sheet response time of centuries seems probable, and we cannot rule out large changes on decadal time-scales once wide-scale surface melt is underway."
The melt zone, where summer warmth turns snow and ice into slush and melt ponds of meltwater, has been expanding at an accelerating rate in recent years. When the meltwater seeps down through cracks in the sheet, it accelerates the melting and, in some areas, allow the ice to slide more easily over the bedrock below, speeding its movement to the sea. Besides contributing to global sea level rise, the process adds freshwater to the ocean, which may disturb ocean circulation and thus regional climate.
Researchers monitoring daily satellite images have discovered that a massive 11-square-mile (29-square-kilometer) piece of the Petermann Glacier in northern Greenland broke away between July 10 and July 24, 2008. The last major ice loss to Petermann occurred when the glacier lost 33 square miles (86 square kilometers) of floating ice between 2000 and 2001. Between 2001 and 2005, a massive breakup of Sermeq Kujalleq erased 36 square miles (94 square kilometers) from the ice field and raised the awareness of worldwide of glacial response to global climate change.
Ice sheet acceleration
Two mechanisms have been utilized to explain the change in velocity of the Greenland Ice Sheets outlet glaciers. The first is the enhanced meltwater effect, which relies on additional surface melting, funneled through moulins reaching the glacier base and reducing the friction through a higher basal water pressure. (It should be noted that not all meltwater is retained in the ice sheet and some moulins drain into the ocean, with varying rapidity.) This idea, was observed to be the cause of a brief seasonal acceleration of up to 20 % on Sermeq Kujalleq in 1998 and 1999 at Swiss Camp.
(The acceleration lasted two-three months and was less than 10% in 1996 and 1997 for example. They offered a conclusion that the “coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming”. Examination of recent rapid supra-glacial lake drainage documented short term velocity changes due to such events, but they had little significance to the annual flow of the large glaciers outlet glaciers. The second mechanism is a force imbalance at the calving front due to thinning causing a substantial non-linear response. In this case an imbalance of forces at the calving front propagates up-glacier. Thinning causes the glacier to be more buoyant, reducing frictional back forces, as the glacier becomes more afloat at the calving front. The reduced friction due to greater buoyancy allows for an increase in velocity. This is akin to letting off the emergency brake a bit. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension because of the backforce reduction. For ice streaming sections of large outlet glaciers (in Antarctica as well) there is always water at the base of the glacier that helps lubricate the flow. This water is, however, generally from basal processes, not surface melting.
If the enhanced meltwater effect is the key than since meltwater is a seasonal input, velocity would have a seasonal signal, all glaciers would experience this effect. If the force imbalance effect is the key the velocity will propagate up-glacier, there will be no seasonal cycle, and the acceleration will be focused on calving glaciers.
Helheim Glacier, East Greenland had a stable terminus from the 1970’s-2000. In 2001-2005 the glacier retreated 7 km and accelerated from 20 m/day to 33 m/day, while thinning up to 130 meters in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960-2002. The glacier velocity was 13 m/day in the 1990’s. In 2004-2005 it accelerated to 36 m/day and thinned by up to 100 m in the lower reach of the glacier. On Sermeq Kujalleq the acceleration began at the calving front and spread up-glacier 20 km in 1997 and up to 55 km inland by 2003. On Helheim the thinning and velocity propagated up-glacier from the calving front. In each case the major outlet glaciers accelerated by at least 50%, much larger than the impact noted due to summer meltwater increase. On each glacier the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent.
An examination of 32 outlet glaciers in southeast Greenland indicates that the acceleration is significant only for marine terminating outlet glaciers. That is glaciers that calve into the ocean. Further, noted that the thinning of the ice sheet is most pronounced for marine terminating outlet glaciers.
As a result of the above, all concluded that the only plausible sequence of events is that increased thinning of the terminus regions, of marine terminating outlet glaciers, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning. Enhanced meltwater induced acceleration does exist but is of a notably smaller magnitude and duration.
Increased precipitation
Warmer temperatures in the region have brought increased precipitation to Greenland, and part of the lost mass has been offset by increased snowfall. However, there are only a small number of weather stations on the island, and though Satellite data can examine the entire island, it has only been available since the early 1990s, making trending difficult. It has been observed that there is more precipitation where it is warmer, up to 1.5 ma-1 on the SE flank, and where cooler less or nil (25-80% of the island depending on the time of year). Actual figures for precipitation are available in "New precipitation and accumulation maps for Greenland", A. Ohmura and N. Reeh, Journal of Glaciology, 1991.
Data from NASA's Polar program confirms that the average elevation change above 2000m "was not significant".
Rate of change
Several factors determine the net rate of growth or decline. These are
- accumulation of snow in the central parts
- melting of ice along the sheet's margins (runoff) and bottom,
- iceberg calving into the sea from outlet glaciers also along the sheet's edges
IPCC estimates in their third assessment report the accumulation to 520 ± 26 Gigatonnes of ice per year, runoff and bottom melting to 297±32 Gt/yr and 32±3 Gt/yr, respectively, and iceberg production to 235±33 Gt/yr. On balance, the IPCC estimates -44 ± 53 Gt/yr, which means that the ice sheet may currently be melting. The most recent research using data from 1996 to 2005 shows that the ice sheet is thinning even faster than supposed by IPCC. According to the study, in 1996 Greenland was losing about 96 km³ per year in mass from its ice sheet. In 2005, this had increased to about 220 km³ a year due to rapid thinning near its coasts, while in 2006 it was estimated at 239 km³ per year . At this rate of ice loss the Greenland ice sheet would melt in 11,900 years. It was estimated that in the year 2007 Greenland ice sheet melting was higher than ever, 592 km3. Also snowfall was unusually low, which lead to unprecedented negative -65 km3 Surface Mass Balance. If iceberg calving has happened as an average, Greenland lost 294 Gt of its mass during 2007 (one km3 of ice weights about 0.9 Gt).
According to the 2007 report from the IPCC, it is hard to measure the mass balance precisely, but most results indicate accelerating mass loss from Greenland during the 1990s up to 2005. Assessment of the data and techniques suggests a mass balance for the Greenland Ice Sheet ranging between growth of 25 Gt/yr and loss of 60 Gt/yr for 1961 to 2003, loss of 50 to 100 Gt/yr for 1993 to 2003 and loss at even higher rates between 2003 and 2005.
A paper on Greenland's temperature record shows that the warmest year on record was 1941 while the warmest decades were the 1930s and 1940s. The data used was from stations on the south and west coasts, most of which did not operate continuously the entire study period.
While Arctic temperatures have generally increased, there is some discussion over the temperatures over Greenland. First of all, Arctic temperatures are highly variable, making it difficult to discern clear trends at a local level. Also, until recently, an area in the North Atlantic including southern Greenland was one of the only areas in the World showing cooling rather than warming in recent decades, but this cooling has now been replaced by strong warming in the period 1979-2005.
See also
External links
- the Greenland Ice
- GEUS has much scientific material on Greenland.
- Lecture 2: MODERN GLACIERS AND ICE SHEETS.
- "Glacial Earthquakes Point to Rising Temperatures in Greenland"
- "Recent Land Ice Mass Flux from SpaceborneGravimetry"
deletion of reference
|
| |
|
|
