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 1,673 m (5,489 ft) thick and over 3,488 m (11,444 ft) thick at its maximum. It is almost 2,900 kilometres (1,800 mi) long in a north–south direction, with a maximum width of 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern edge. The ice sheet covers 1,710,000 square kilometres (660,000 sq mi), 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 2 °C (3.6 °F) is achieved, melting of Greenland ice alone would still add around 6 cm (2+1⁄2 in) to global sea level rise by the end of the century. If there are no reductions in emissions, melting would add around 13 cm (5 in) by 2100, with a worst-case of about 33 cm (13 in). For comparison, melting has so far contributed 1.4 cm (1⁄2 in) since 1972, while sea level rise from all sources was 15–25 cm (6–10 in) between 1901 and 2018. If all 2,900,000 cubic kilometres (696,000 cu mi) of the ice sheet were to melt, it would increase global sea levels by ~7.4 m (24 ft). Global warming between 1.7 °C (3.1 °F) and 2.3 °C (4.1 °F) would likely make this melting inevitable. However, 1.5 °C (2.7 °F) would still cause ice loss equivalent to 1.4 m (4+1⁄2 ft) 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

Ice sheets 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 120 km (75 mi) 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 76,000 and 100,000 square kilometres (29,000 and 39,000 sq mi) 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

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