Greenland ice sheet
The Greenland ice sheet (, ) is a vast body of ice covering , roughly near 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 long in a north–south direction, and its greatest width is at a latitude of 77°N, near its northern margin. The mean altitude of the ice is . The thickness is generally more than and over at its thickest point. In addition to the large ice sheet, smaller ice caps (such as Maniitsoq and Flade Isblink) as  well as glaciers, cover between around the periphery. If the entire of ice were to melt, it would lead to a global sea level rise of . 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 northwest 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. Ice sheet formed in connection to the 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. Computer modelling shows that the uplift would have enabled  glaciation by producing increased orographic precipitation and cooling the surface temperatures. The oldest known ice in the current ice sheet is as much as 1,000,000 years old. The ice cores also record human impact, such as lead from the Roman Empire. 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 at latitudes 63°–65°N; the northern dome reaches about 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, as well as in Jokel Bay. 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 (), which, at its terminus, flows at speeds of per day. On the ice sheet, temperatures are generally substantially lower than elsewhere in Greenland. The lowest mean annual temperatures, about , occur on the north-central part of the  north dome, and temperatures at the crest of the south dome are about . On 22 December 1991, a temperature of was recorded at an automatic weather station near the topographic summit of the Greenland Ice Sheet, making it the lowest temperature ever recorded in the Northern Hemisphere. The record went unnoticed for more than 28 years and was finally recognized in 2020. Change of the ice sheet. 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 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. Subglacial sediment from ~ beneath the ice stored since 1966 indicates that Greenland was completely ice-free and vegetated at least once within the last million years. This was not expected and may show Greenland to be more fragile and sensitive to climate change than previously thought. General considerations on ice  melting. Many scientists who study the ice losses in Greenland consider that an increase in temperature of two or three degrees Celsius would result in a complete melting of Greenland's ice 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. 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. 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. The number of glacial earthquakes at the Helheim Glacier 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 per year. A more recent study, based on reprocessed and improved data between 2003 and 2008, reports an  average trend of 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 of ice were to melt, global sea levels would rise . 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 to 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 . 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, 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." 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. In  July 2012, this melt zone extended to 97 percent of the ice cover. 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." 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 . Meltwater around Greenland may transport nutrients and organic carbon to the ocean. 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. However much of the particles and iron derived from glaciers around Greenland may be trapped within the extensive fjords that surround the island and, unlike the HNLC Southern ocean where iron is an extensive limiting micronutrient, biological production in the North Atlantic is subject only to very spatially and  temporally limited periods of iron limitation. 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. General considerations on rate of change. Several factors determine the net rate of growth or decline. These are 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. 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. 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 per  year in volume from its ice sheet. In 2005, this had increased to about a year due to rapid thinning near its coasts, while in 2006 it was estimated at per year. It was estimated that in the year 2007 Greenland ice sheet melting was higher than ever, . Also snowfall was unusually low, which led to unprecedented negative Surface Mass Balance. If iceberg calving has happened as an average, Greenland lost 294 Gt of its mass during 2007 (one km 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. 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. A study published in 2020 estimated, by combining 26 individual estimates of mass balance derived by tracking changes in Greenland's ice sheet volume, speed and gravity as part of the Ice Sheet Mass Balance Inter-comparison Exercise, that the Greenland Ice Sheet had lost a total of 3,902 gigatons (Gt) of ice between 1992 and 2018. The rate of ice loss has increased over time from 26 ± 27 Gt/year between 1992 and 1997 to 244 ± 28 Gt/year between 2012 and 2017 with a peak mass loss  rate of 275 ± 28 Gt/year during the period 2007 and 2012. 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 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, but this cooling was replaced by strong warming in the period 1979–2005. Observation and research since 2010. In a 2013 study published in "Nature", 133 researchers analyzed a Greenland ice core from the Eemian interglacial. They concluded that during this geological period, roughly 130,000–115,000 years ago, the GIS (Greenland Ice Sheet) was 8 degrees C warmer than today. This resulted 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. Researchers have considered that clouds may 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, 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. 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. 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. The United States built a secret nuclear powered base, called Camp Century, in the Greenland ice sheet. In 2016, a group of scientists evaluated the environmental impact and estimated that due to changing weather patterns over the next few decades, melt water could release the nuclear waste, 20,000 liters of chemical waste and 24 million liters of untreated sewage into the environment. However, so far neither US or Denmark has taken responsibility for the clean-up. 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. On 13 August 2020, Communications Earth and Environment, a Nature Research journal published a study on "Dynamic ice loss from the Greenland Ice sheet driven by sustained glacier retreat". The situation was described as being past the "point of no return" and attributed to two factors, "increased surface meltwater runoff and ablation of marine-terminating outlet glaciers via calving and submarine melting, termed ice  discharge." On 20 August 2020, scientists reported that the Greenland ice sheet lost a record amount of 532 billion metric tons of ice during 2019, surpassing the old record of 464 billion metric tons in 2012 and returning to high melt rates, and provide explanations for the reduced ice loss in 2017 and 2018. On 31 August 2020, scientists reported that observed ice-sheet losses in Greenland and Antarctica track worst-case scenarios of the IPCC Fifth Assessment Report's sea-level rise projections. Glacial calving 2000–2015. 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. (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 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. 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. 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 and accelerated from /day, while thinning up to in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960 to 2002. The glacier velocity was /day in the 1990s. In 2004–2005 it accelerated to /day and thinned by up to in the lower reach of the glacier. On Sermeq Kujalleq the acceleration began at the calving front and spread up-glacier  in 1997 and up to 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—glaciers that calve into the ocean. A 2008 study 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. 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. Twenty-first century melting events. Based on ice core research, most melting events before the 21st century were localized rather than widespread across the bulk of the Greenland ice sheet. In both 2012 and 2019, however, mass melting events (covering more than 300,000 square miles) occurred  in both June and July, due to cloud cover and high temperatures. In July 2021, a new surge of widespread melting began, covering 340,000 square miles of the ice sheet, and melting more 8 billion tons of ice per day for several days. In August 2021, as high temperatures continued over Greenland, with the melt extent at 337,000 square miles, rain fell for 13 hours at Greenland's Summit Station (at 10,551 feet elevation.) Researchers had no rain gauges to measure the rainfall, because temperatures at the summit have risen above freezing only three times since 1989 and it had  never rained there before.