“Greenland ice sheet”的意思、由来-开放百科全书

The Greenland ice sheet ({{lang-da|Grønlands indlandsis}}, {{lang-kl|Sermersuaq}}) is a vast body of ice covering {{convert|1710000|km²|sqmi|-3}}, 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 {{convert|2400|km|mi}} long in a north-south direction, and its greatest width is {{convert|1100|km|mi}} at a latitude of 77°N, near its northern margin. The mean altitude of the ice is {{convert|2135|m}}.^[1] The thickness is generally more than {{convert|2|km|mi|1|abbr=on}} and over {{convert|3|km|mi|1|abbr=on}} at its thickest point. In addition to the large ice sheet, isolated glaciers and small ice caps cover between {{convert|76000|and|100000|km²|sqmi|-3}} around the periphery. If the entire {{convert|2850000|km3|cumi|-3}} of ice were to melt, it would lead to a global sea level rise of {{convert|7.2|m|ft|0|abbr=on}}^[2], over 14,000 years at current rates of melting. The Greenland Ice Sheet is sometimes referred to under the term inland ice, or its Danish equivalent, indlandsis. It is also sometimes referred to as an ice cap.

General

The presence of ice-rafted sediments in deep-sea cores recovered from northeast Greenland, in the Fram Strait, and south of Greenland indicated the more or less continuous presence of either an ice sheet or ice sheets covering significant parts of Greenland for the last 18 million years. From about 11 million years ago to 10 million years ago, the Greenland Ice Sheet was greatly reduced in size. The Greenland Ice Sheet formed in the middle Miocene by coalescence of ice caps and glaciers. There was an intensification of glaciation during the Late Pliocene.^[3] Ice sheet formation occurred in connection to uplift of the West Greenland and East Greenland uplands. The Western and Eastern Greenland mountains constitute passive continental margins that were uplifted in two phases, 10 and 5 million years ago, in the Miocene epoch.{{efn-ua|The timing of the uplift of Greenland is known from the study of planation surfaces formed near sea-level. Greenland has two large planation surfaces: the older Upper Planation Surface and the younger Lower Planation Surface. The Upper Planation Surface has been uplifted 2000 to 3000 masl. since formation and the Lower Planation Surface has been uplifted 500 to 1000 masl.^[4]}} Computer modelling shows that the uplift would have enabled glaciation by producing increased orographic precipitation and cooling the surface temperatures.^[4] The oldest known ice in the current ice sheet is as old as 1,000,000 years old.^[5]

The 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 suddenly disappeared, Greenland would most probably appear as an archipelago, at least until isostasy lifted the land surface above sea level once again. The ice surface reaches its greatest altitude on two north-south elongated domes, or ridges. The southern dome reaches almost {{convert|3000|m|ft|-3}} at latitudes 63°–65°N; the northern dome reaches about {{convert|3290|m|ft|-2}} at about latitude 72°N (the fourth highest "summit" of Greenland). 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 Glacier ({{lang-kl|Sermeq Kujalleq}}), which, at its terminus, flows at speeds of {{convert|20|to|22|m|ft|0|disp=or}} per day.

On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The lowest mean annual temperatures, about {{convert|-31|°C|°F|0}}, occur on the north-central part of the north dome, and temperatures at the crest of the south dome are about {{convert|-20|°C|°F}}. {{Citation needed|date=April 2017}}

The ice sheet as a record of past climates

The ice sheet, consisting of layers of compressed snow from more than 100,000 years, contains in its ice today's most valuable record of past climates. In the past decades, scientists have drilled ice cores up to {{convert|4|km|mi|1}} 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

Many scientists who study the ice ablation in Greenland consider that a two or three °C temperature rise would result in a complete melting of Greenland's ice.^[6] Positioned in the Arctic, the Greenland ice sheet is especially vulnerable to climate change. Arctic climate is believed to be now rapidly warming and much larger Arctic shrinkage changes are projected.^[7] The Greenland Ice Sheet has experienced record melting in recent years since detailed records have been kept and is likely to contribute substantially to sea level rise as well as to possible changes in ocean circulation in the future if this is sustained. The area of the sheet that experiences melting has been argued to have increased by about 16% between 1979 (when measurements started) and 2002 (most recent data). The area of melting in 2002 broke all previous records.^[7] The number of glacial earthquakes at the Helheim Glacier and the northwest Greenland glaciers increased substantially between 1993 and 2005.^[8] In 2006, estimated monthly changes in the mass of Greenland's ice sheet suggest that it is melting at a rate of about {{convert|239|km3|cumi|0|sp=us}} per year. A more recent study, based on reprocessed and improved data between 2003 and 2008, reports an average trend of {{convert|195|km3|cumi|0|sp=us}} per year.^[9] These measurements came from the US space agency's GRACE (Gravity Recovery and Climate Experiment) satellite, launched in 2002, as reported by BBC.^[10] 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.^[11]

If the entire {{convert|2850000|km3|cumi|-3|abbr=on}} of ice were to melt, global sea levels would rise {{convert|7.2|m|ft|0|abbr=on}}^[2], over 14,000 years at current rates of melting. Recently, fears have grown that continued climate change 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 be {{convert|3|C-change|0}} to {{convert|9|C-change|0}} 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 {{convert|7|m|ft|0}}.^[7] 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 the IPCC 2001 report,^[2] such warming would, if kept from rising further after the 21st Century, result in 1 to 5 meter sea level rise over the next millennium due to Greenland ice sheet melting. Some scientists have cautioned that these rates of melting are overly optimistic as they assume a linear, rather than erratic, progression. James E. 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."^[12]

In a 2013 study published in Nature, 133 researchers analyzed a Greenland ice core from the Eemian interglacial. They concluded that GIS (Greenland Ice Sheet) had been 8 degrees C warmer than today. Resulting in a thickness decrease of the northwest Greenland ice sheet by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than at present.^[13]

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, allows 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.^[7] In July 2012, this melt zone extended to 97 percent of the ice cover.^[14] Ice cores show that events such as this occur approximately every 150 years on average. The last time a melt this large happened was in 1889. This particular melt may be part of cyclical behavior; however, Lora Koenig, a Goddard glaciologist suggested that "...if we continue to observe melting events like this in upcoming years, it will be worrisome."^[15]^[16]^[17] Global warming is increasing growth of algae on the ice sheet. This darkens the ice causing it to absorb more sunlight and potentially increasing the rate of melting.^[18]

Meltwater around Greenland may transport nutrients in both dissolved and particulate phases to the ocean.^[19] Measurements of the amount of iron in meltwater from the Greenland ice sheet show that extensive melting of the ice sheet might add an amount of this micronutrient to the Atlantic Ocean equivalent to that added by airborne dust.^[20] However much of the particles and iron derived from glaciers around Greenland may be trapped within the extensive fjords that surround the island^[21] and, unlike the HNLC Southern ocean where iron is an extensive limiting micronutrient,^[22] biological production in the North Atlantic is subject only to very spatially and temporally limited periods of iron limitation.^[23] Nonetheless high productivity is observed in the immediate vicinity of major marine terminating glaciers around Greenland and this is attributed to meltwater inputs driving the upwelling of seawater rich in macronutrients.^[24]

Recent developments

Researchers have considered clouds to enhance Greenland ice sheet melt. A study published in Nature in 2013 found that optically thin liquid-bearing clouds extended this July 2012 extreme melt zone,^[26] while a Nature Communications study in 2016 suggests that clouds in general enhance Greenland ice sheet's meltwater runoff by more than 30% due to decreased meltwater refreezing in the firn layer at night.^[27]

A 2015 study by climate scientists Michael Mann of Penn State and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research suggests that the observed cold blob in the North Atlantic during years of temperature records is a sign that the Atlantic Ocean's Meridional overturning circulation (AMOC) may be weakening. They published their findings, and concluded that the AMOC circulation shows exceptional slowdown in the last century, and that Greenland melt is a possible contributor.^[28]

A study published in 2016, by researchers from the University of South Florida, Canada and the Netherlands, used GRACE satellite data to estimate freshwater flux from Greenland. They concluded that freshwater runoff is accelerating, and could eventually cause a disruption of AMOC in the future, which would affect Europe and North America.^[29]

A 2018 international study found that the fertilizing effect of meltwater around Greenland is highly sensitive to the glacier grounding line depth it is released at. Retreat of Greenland's large marine-terminating glaciers inland will diminish the fertilizing effect of meltwater- even with further large increases in freshwater discharge volume.^[30]

Recent ice loss events

Media

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.^[34]

(The acceleration lasted between two and 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 outlet glaciers.^[35]

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.^[36]^[37] 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.

If the enhanced meltwater effect is the key, then since meltwater is a seasonal input, velocity would have a seasonal signal and all glaciers would experience this effect. If the force imbalance effect is the key, then 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 1970s–2000. In 2001–2005 the glacier retreated {{convert|7|km|mi|1|abbr=on}} and accelerated from {{convert|20|to|33|m|ft|-1|abbr=on|disp=or}}/day, while thinning up to {{convert|130|m|ft|sp=us}} in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960 to 2002. The glacier velocity was {{convert|13|m|ft|0|abbr=on|disp=or}}/day in the 1990s. In 2004–2005 it accelerated to {{convert|36|m|ft|-1|abbr=on|disp=or}}/day and thinned by up to {{convert|100|m|ft|-2|abbr=on}} in the lower reach of the glacier. On Sermeq Kujalleq the acceleration began at the calving front and spread up-glacier {{convert|20|km|mi|0|abbr=on}} in 1997 and up to {{convert|55|km|mi|0|abbr=on}} inland by 2003.^[38] 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—glaciers that calve into the ocean.^[39] A 2008 study noted that the thinning of the ice sheet is most pronounced for marine-terminating outlet glaciers.^[40]

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.^[39]^[41]^[42]^[43]

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 the study of trends difficult. It has been observed that there is more precipitation where it is warmer, up to 1.5 meters per year on the southeast flank, and less precipitation or none on the 25–80 percent (depending on the time of year) of the island that is cooler.^[44]

Rate of change

Explanation of accelerated glacial coastward movement and iceberg calving fails to consider another causal factor: increased weight of the central highland ice sheet. As the central ice sheet thickens, which it has for at least seven decades, its greater weight causes more horizontal outward force at the bedrock. This in turn appears to have increased glacial calving at the coasts. Visual evidence for increased central highland ice sheet thickness exists in the numerous aircraft that have made forced landings on the icecap since the 1940s. They landed on the surface and later disappeared under the ice.^[45] A notable example is the Lockheed P-38F Lightning World War II fighter plane Glacier Girl that was exhumed from 268 feet of ice in 1992 and restored to flying condition after being buried for over 50 years. It was recovered by members of the Greenland Expedition Society after years of searching and excavation, eventually transported to Kentucky and restored to flying condition.

The IPCC Third Assessment Report (2001) estimated 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.^[2] 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 {{convert|96|km3|cumi|1|abbr=on|disp=or}} per year in volume from its ice sheet. In 2005, this had increased to about {{convert|220|km3|cumi|1|abbr=on|disp=or}} a year due to rapid thinning near its coasts,^[46] while in 2006 it was estimated at {{convert|239|km3|cumi|1|abbr=on}} per year.^[10] It was estimated that in the year 2007 Greenland ice sheet melting was higher than ever, {{convert|592|km3|cumi|1|abbr=on}}. Also snowfall was unusually low, which led to unprecedented negative {{convert|-65|km3|cumi|1|abbr=on}} Surface Mass Balance.^[47] If iceberg calving has happened as an average, Greenland lost 294 Gt of its mass during 2007 (one km³ of ice weighs about 0.9 Gt).

The IPCC Fourth Assessment Report (2007) noted, 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.^[48]

Analysis of gravity data from GRACE satellites indicates that the Greenland ice sheet lost approximately 2900 Gt (0.1% of its total mass) between March 2002 and September 2012. The mean mass loss rate for 2008–2012 was 367 Gt/year.^[49]

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.^[50]

While Arctic temperatures have generally increased, there is some discussion concerning 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,^[51] but this cooling has now been replaced by strong warming in the period 1979–2005.^[52]

See also

Notes

References

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