Greenland ice sheet
The Greenland ice sheet is an ice sheet which forms the second-largest body of ice in the world. It is an average of thick and over thick at its maximum. It is almost long in a north–south direction, with a maximum width of at a latitude of 77°N, near its northern edge. The ice sheet covers, around 80% of the surface of Greenland, or about 12% of the area of the Antarctic ice sheet. The term 'Greenland ice sheet' is often shortened to GIS or GrIS in scientific literature.
Greenland has had major glaciers and ice caps for at least 18 million years, but a single ice sheet first covered most of the island some 2.6 million years ago. Since then, it has both grown and contracted significantly. The oldest known ice on Greenland is about 1 million years old. Due to anthropogenic greenhouse gas emissions, the ice sheet is now the warmest it has been in the past 1000 years, and is losing ice at the fastest rate in at least the past 12,000 years.
Every summer, parts of the surface melt and ice cliffs calve into the sea. Normally the ice sheet would be replenished by winter snowfall, but due to global warming the ice sheet is melting two to five times faster than before 1850, and snowfall has not kept up since 1996. If the Paris Agreement goal of staying below is achieved, melting of Greenland ice alone would still add around to global sea level rise by the end of the century. If there are no reductions in emissions, melting would add around by 2100, with a worst-case of about. For comparison, melting has so far contributed since 1972, while sea level rise from all sources was between 1901 and 2018.
If all of the ice sheet were to melt, it would increase global sea levels by ~. Global warming between and would likely make this melting inevitable. However, would still cause ice loss equivalent to of sea level rise, and more ice will be lost if the temperatures exceed that level before declining. If global temperatures continue to rise, the ice sheet will likely disappear within 10,000 years. At very high warming, its future lifetime goes down to around 1,000 years.
Beneath the Greenland ice sheet are mountains and lake basins.
Description
s form through a process of glaciation, when the local climate is sufficiently cold that snow is able to accumulate from year to year. As the annual snow layers pile up, their weight gradually compresses the deeper levels of snow to firn and then to solid glacier ice over hundreds of years. Once the ice sheet formed in Greenland, its size remained similar to its current state. However, there have been 11 periods in Greenland's history when the ice sheet extended up to beyond its current boundaries; with the last one around 1 million years ago.The weight of the ice causes it to slowly "flow", unless it is stopped by a sufficiently large obstacle, such as a mountain. Greenland has many mountains near its coastline, which normally prevent the ice sheet from flowing further into the Arctic Ocean. The 11 previous episodes of glaciation are notable because the ice sheet grew large enough to flow over those mountains. Nowadays, the northwest and southeast margins of the ice sheet are the main areas where there are sufficient gaps in the mountains to enable the ice sheet to flow out to the ocean through outlet glaciers. These glaciers regularly shed ice in what is known as ice calving. Sediment released from calved and melting ice sinks accumulates on the seafloor, and sediment cores from places such as the Fram Strait provide long records of glaciation at Greenland.
Geological history
While there is evidence of large glaciers in Greenland for most of the past 18 million years, these ice bodies were probably similar to various smaller modern examples, such as Maniitsoq and Flade Isblink, which cover around the periphery. Conditions in Greenland were not initially suitable for a single coherent ice sheet to develop, but this began to change around 10 million years ago, during the middle Miocene, when the two passive continental margins which now form the uplands of West and East Greenland experienced uplift, and ultimately formed the upper planation surface at a height of 2000 to 3000 meter above sea level.Later uplift, during the Pliocene, formed a lower planation surface at 500 to 1000 meters above sea level. A third stage of uplift created multiple valleys and fjords below the planation surfaces. This uplift intensified glaciation due to increased orographic precipitation and cooler surface temperatures, allowing ice to accumulate and persist. As recently as 3 million years ago, during the Pliocene warm period, Greenland's ice was limited to the highest peaks in the east and the south. Ice cover gradually expanded since then, until the atmospheric carbon dioxide levels dropped to between 280 and 320 ppm 2.7–2.6 million years ago, by which time temperatures had dropped sufficiently for the disparate ice caps to connect and cover most of the island.
Ice cores and sediment samples
The base of the ice sheet may be warm enough due to geothermal activity to have liquid water beneath it. This liquid water, under pressure from the weight of ice above it, may cause erosion, eventually leaving nothing but bedrock below the ice sheet. However, there are parts of the Greenland ice sheet, near the summit, where the ice sheet slides over a basal layer of ice which had frozen solid to the ground, preserving ancient soil, which can then be recovered by drilling. The oldest such soil was continuously covered by ice for around 2.7 million years, while another deep ice core from the summit has revealed ice that is ~1,000,000 years old.Sediment samples from the Labrador Sea provide evidence that nearly all of the south Greenland ice had melted around 400,000 years ago, during Marine Isotope Stage 11. Other ice core samples from Camp Century in northwestern Greenland show that the ice there melted at least once during the past 1.4 million years, during the Pleistocene, and did not return for at least 280,000 years. These findings suggest that less than 10% of the current ice sheet volume was left during those geologically recent periods, when the temperatures were less than warmer than preindustrial conditions. This contradicts how climate models typically simulate the continuous presence of solid ice under those conditions. Analysis of the ~100,000-year records obtained from long ice cores drilled between 1989 and 1993 into the summit of Greenland's ice sheet had provided evidence for geologically rapid changes in climate, and informed research on tipping points such as in the Atlantic meridional overturning circulation.
Ice cores provide valuable information about the past states of the ice sheet and other kinds of paleoclimate data. Subtle differences in the oxygen isotope composition of the water molecules in ice cores can reveal important information about the water cycle at the time, while air bubbles frozen within the ice core provide a snapshot of the gas and particulate composition of the atmosphere through time. When properly analyzed, ice cores provide a wealth of proxies suitable for reconstructing the past temperature record, precipitation patterns, volcanic eruptions, solar variation, ocean primary production, and even changes in soil vegetation cover and the associated wildfire frequency. The ice cores from Greenland also record human impact, such as lead production during the time of Ancient Greece and the Roman Empire.
Recent melting
From the 1960s to the 1980s an area in the North Atlantic which included southern Greenland was one of the few locations in the world which showed cooling rather than warming. This location was relatively warmer in the 1930s and 1940s than in the decades immediately before or after. More complete data sets have established trends of warming and ice loss starting from 1900 and a trend of strong warming starting around 1979, in line with concurrent observed Arctic sea ice decline. In 1995–1999, central Greenland was already warmer than it was in the 1950s. Between 1991 and 2004, average winter temperature at one location, Swiss Camp, rose almost.Consistent with this warming, the 1970s were the last decade when the Greenland ice sheet grew, gaining about 47 gigatonnes per year. From 1980–1990 there was an average annual mass loss of ~51 Gt/y. The period 1990–2000 showed an average annual loss of 41 Gt/y, with 1996 being the last year the Greenland ice sheet saw net mass gain. As of 2022, the Greenland ice sheet had been losing ice for 26 years in a row, and temperatures there had been the highest in the entire past last millennium – about warmer than the 20th century average.
Several factors determine the net rate of ice sheet growth or decline. These are:
- Accumulation and melting rates of snow in and around the centre
- Melting of ice along the sheet's margins
- Ice calving into the sea from outlet glaciers also along the sheet's edges
Annual ice losses from the Greenland ice sheet accelerated in the 2000s, reaching ~187 Gt/yr in 2000–2010, and an average mass loss during 2010–2018 of 286 Gt per year. Half of the ice sheet's observed net loss happened during those 8 years. When converted to sea level rise equivalent, the Greenland ice sheet contributed about 13.7 mm since 1972.
Between 2012 and 2017, it contributed 0.68 mm per year, compared to 0.07 mm per year between 1992 and 1997. Greenland's net contribution for the 2012–2016 period was equivalent to 37% of sea level rise from land ice sources. These melt rates are comparable to the largest experienced by the ice sheet over the past 12,000 years.
Currently, the Greenland ice sheet loses more mass every year than the Antarctic ice sheet, because of its position in the Arctic, where it is subject to intense regional amplification of warming. Ice losses from the West Antarctic Ice Sheet have been accelerating due to its vulnerable Thwaites and Pine Island Glaciers, and the Antarctic contribution to sea level rise is expected to overtake that of Greenland later this century.