What is an Ice Sheet?
An ice sheet is a huge expanse of dense, glacial ice covering an area of land greater than 50,000 square kilometers (19,300 sq mi). At present, there are only two surviving ice sheets in the world: the Greenland ice sheet in the northern polar region and the much larger Antarctic ice sheet, in the southern polar region. The Greenland ice sheet occupies an area of 1,710,000 square kilometers (660,000 sq mi), compared to 14 million square kilometers (5.4 million sq mi) for Antarctica.
Ice sheets are sometimes called continental glaciers because they are constantly in motion, slowly moving downhill towards the sea under pressure of their own weight. Towards the coast, most of the ice becomes absorbed into slightly faster-moving glaciers and ice streams, that transport it to the shoreline. Here, if it doesn’t melt, it forms floating platforms called ice shelves, that act as a brake on the flow of ice into the ocean. Pieces of ice that break off glaciers or ice shelves, are called icebergs.
Ice sheets play a hugely important role in our climate system, helping to balance the higher temperatures of the tropics. Since the advent of global warming, this cooling role has become even more important.
If Planet Earth were to lose the Greenland ice sheet, it would suffer catastrophic sea level rise and the submergence of most low-lying coastal areas around the world. If the ice sheet in Antarctica were to disappear, human civilization itself would be at risk.
- What is an Ice Sheet?
- How Much Ice Does an Ice Sheet Contain?
- What Happens if the Ice Sheets Melt?
- How Thick Are Ice Sheets?
- What is an Ice Stream?
- What’s the Difference Between an Ice Sheet and an Ice Shelf?
- How Old Are Ice Sheets?
- Why are Ice Sheets Important?
- How are Ice Sheets Formed?
- How Do Ice Sheets Grow? How Do Ice Sheets Shrink?
- How Fast Do Ice Sheets Move?
- Are the Ice Sheets Melting?
How Much Ice Does an Ice Sheet Contain?
The Greenland ice sheet contains 2,850,000 cubic kilometres (684,000 cu mi) of ice; the Antarctic sheet, 26.5 million cubic kilometres (6,400,000 cubic mi) of ice. A cubic kilometer of ice weighs roughly one billion metric tons (tonnes). Thus, the Greenland ice sheet weighs about 2.8 billion tonnes, compared to the Antarctic’s 26 billion tonnes. The weight of the ice depresses the level of the land which will rise if the ice melts. The weight of both the Antarctic and Greenland ice sheets have caused the underlying bedrock beneath under them to sink. Eastern Antarctica is about 2.5 kilometers (1.6 miles) below sea level because of the colossal weight of the ice sheet above it.
What Happens if the Ice Sheets Melt?
Ice sheets are a vital element in the cryosphere because they store 99 percent of Earth’s fresh water. The downside, is that whenever land ice melts, it causes sea levels to rise around the globe.
How high will sea levels rise if the ice sheets melt? Answer: if the Greenland ice sheet melts, it is likely to cause a global rise of about 7.3 m (24 ft). If the Antarctic sheet melts, the rise will be about 58.3 m (190 ft). A combined total of more than 65 m (214 ft). 1 Fortunately, neither is likely to disappear for centuries.
How Thick Are Ice Sheets?
Ice sheets are usually dome-shaped (with one or more domes) and they spread downwards from these dome-like centers. The Greenland ice sheet has an average thickness of about 1,500 meters (5,000 ft) and attains a maximum thickness of 3,000 meters (10,000 ft). Although the average altitude of the ice is 2,100 meters (7,000 ft), most parts of the bedrock underlying the ice sheet are at or slightly below sea level.
In Antarctica, there is a significant difference between the West Antarctic and East Antarctic ice sheets.
The East Antarctic ice sheet (EAIS) has about 9 times the mass of the West Antarctic sheet (WAIS). Plus, it has an average thickness of 2,226 meters (7,303 ft) compared with the WAIS maximum of 1,306 meters (4,284 ft). The EAIS climbs to 4,000 meters (13,123 ft) above sea level at Dome Argus, the highest ice dome on the East Antarctic Plateau, while the maximum altitude of WAIS and the Antarctic Peninsula Ice Sheet is about 2,500 meters (8,200 ft) above sea level. Also, the WAIS sits on bedrock that is significantly lower than EAIS, reaching a low point of 2,496 meters below sea level. 3
What is an Ice Stream?
Ice streams are relatively fast-moving corridors of ice that operate within an ice sheet, at a speed of about 800 meters per year. In Antarctica, 90 percent of the melting ice and sediment from the ice sheets is drained by ice streams (typically, around 20 km wide and 150 km long), making them critical for overall ice sheet dynamics. 4 One of the best-known ice streams is Antarctica’s Pine Island Glacier.
What’s the Difference Between an Ice Sheet and an Ice Shelf?
As we have seen, an ice sheet is a body of dense ice that covers more than 50,000 square kilometers of land. In contrast, an ice shelf (or ice tongue) floats on water and is found on the fringes of an ice sheet or ice cap (a smaller ice sheet) where it joins the ocean. When an ice sheet glacier reaches the ocean and crosses into the water, it stops being a glacier and becomes a (floating) ice shelf.
The shelf grows larger as glacier ice behind it continues to flow from the land into the sea, or retreats if the leading edge of the ice shelf calves into icebergs. In bays and along sections of protected coastlines, ice shelves may survive for hundreds, if not thousands of years, supported by the rock of islands and peninsulas. 5
Ice shelves are seen only in Antarctica and (to a lesser extent) in Canada and Greenland. The two largest ice shelves in Antarctica are the Ross Ice Shelf and the Ronne-Filchner Ice Shelf, each of which covers an area larger than Spain. Ice shelves in Canada are found only on the fringes of Ellesmere Island – of which the largest is the Ward Hunt Ice Shelf. In Greenland, the ice sheet does not reach the ocean along a broad front anywhere. As a result, there are no large ice shelves on the Greenland coast. Instead, several of the glaciers in northern and northeast Greenland terminate in fiords with long glacier tongues and floating, ice-shelf-like margins. Examples include Petermann Glacier (northwest Greenland) and Nioghalvfjerdsfjorden Glacier (northeast).
How Old Are Ice Sheets?
The present-day polar ice sheets are relatively young in geological terms. The freezing-up of Antarctica didn’t start until about the middle Eocene about 45.5 million years ago and accelerated during the Eocene–Oligocene extinction event about 34 million years ago. 6 After this, it advanced and retreated many times until the Pliocene (5.3 million to 2.5 million years ago), when it finally occupied almost all of the continent. Greenland’s ice sheet took shape in the middle Miocene around 10 million years ago.
The ice sheets attained their greatest extent about 18,000 years ago, during the glacial maximum of the last ice age, when almost one-third of the Earth’s land surface was covered by ice. In North America, the Laurentide Ice Sheet was almost 3 km (2 mi) deep and blanketed the continent from the Canadian Arctic as far south as Missouri.
The Scandinavian ice sheet occupied most of Western Europe from Ireland to Moscow, while the Barents-Kara ice sheet extended across Northern Asia. In South America the Patagonian ice sheet extended from the Andes Mountains southwards. Most of modern Chile was enveloped in ice.
Why are Ice Sheets Important?
There are four reasons why ice sheets are important.
(A) First, the ice sheets in Greenland and Antarctica lock up 99 percent of the planet’s freshwater. If all the ice were to melt, many coastlines, river deltas and other low-lying areas would be inundated, ruining vast areas of highly fertile land and submerging cities such as: Jakarta (Indonesia), Rotterdam (The Netherlands), Guangzhou (China), Amsterdam (The Netherlands), Mumbai (India), Osaka (Japan), Lagos (Nigeria), Houston (Texas), Dhaka (Bangladesh), Venice (Italy), Bangkok (Thailand), New Orleans (Louisiana), and Miami (Florida), among others. See also: The Water Cycle.
(B) Second, by keeping freshwater locked up, ice sheets help to maintain the ocean’s salinity, or salt content. The point is, any major leak of meltwater into the ocean would have a wide-ranging effect on ocean circulation and ecosystems. This is because salinity directly affects seawater density, which powers the network of thermohaline circulation currents that transport cold, nutrient-rich water around the globe. These massive ‘underwater rivers’ help to balance global temperatures and climate, while bolstering the marine food web wherever they upwell.
(C) Third, ice reflects sunlight. Its so-called albedo effect (reflectivity power) can be as high as 9, meaning it reflects 90 percent of the sunshine it receives. By bouncing sunlight back into space, ice sheets reduce the amount of heat reaching the surface of the Earth and therefore cool the planet.
(D) Fourthly, ice sheets, especially the vast Antarctic sheet, are an important area for scientific research, including several branches of climate science such as glaciology (the study of glaciers), oceanography (the study of the physical and biological features of the ocean), marine microbiology (the study of tiny organisms like phytoplankton. zooplankton and krill), meteorology (the study of the atmosphere, including the ozone layer) and paleoclimatology (the study of climates from the geological past, for which direct measurements were not available).
By extracting long tubes of ice, known as ‘ice cores’, from the ice sheet, scientists are able to calculate the temperature of the Earth as it was thousands of years ago, as well as atmospheric levels of carbon dioxide and other climate-related metrics from long ago. In conjunction with computerized climate models, this data helps them to simulate how our current climate crisis might evolve.
How are Ice Sheets Formed?
An ice sheet forms like a glacier. Because, in simple terms, an ice sheet is just one enormous glacier which – instead of flowing in a single direction, like a normal glacier – flows outward (from its high point) in many different directions. 7
An ice sheet starts to form as snow accumulates year after year, getting harder as it becomes compressed by successive layers of snowfall. Over time, the snow changes from fluffy powder into a mass of dense ice, from which most of the air is gradually expelled, turning the ice blue in color. This hard, dense snow is known as firn. The hardening process continues for decades, perhaps hundreds of years, during which the ice grows thicker and thicker. Once it gets to about 50 m (roughly 160 ft) thick, the firn coalesces into a single body of glacial ice. From this point onwards the ice sheet begins to move downslope, under the pressure of its own weight.
Ice sheets and large ice caps tend to be slightly dome-shaped, with one or more domes, from which they move ponderously outwards in all directions to the coast. Spreading like a heavy, viscous liquid, the ice covers almost everything in its path. Meltwater accumulates at the base and lubricates its progress.
Depending on the topography, as the ice and meltwater descend further from the center, most of it drains away to the ocean through faster-moving glaciers and ice streams.
If it empties into a bay or other sheltered shoreline, the ice sheet may form a floating ice shelf. Ice shelves tend to act as a brake on the speed of the ice into the sea, but continuing pressure from behind pushes and extends the ice shelf away from the coast. However, if an ice shelf collapses, the glaciers that were feeding it can accelerate.
In areas of the ice sheet where surface melt in summer exceeds snowfall in winter, the front edge of the ice sheet may ‘retreat’ from the ocean and terminate on land. However, in much of Antarctica and Greenland, the ice sheet reaches all the way to the ocean.
How Do Ice Sheets Grow? How Do Ice Sheets Shrink?
If more snow falls on an ice sheet than it loses to the sea, its mass will increase. Conversely, if more of its mass is lost to the sea than arrives in snowfall, then the ice sheet will shrink. Because the air over Antarctica is so cold, it simply can’t hold much water, so snowfall is low – averaging about no more than about 166 mm (6.5 inches) per year. 8
There are four ways that an ice sheet can lose ice: (a) ablation (evaporation/erosion); (b) melting on the surface; (c) melting on contact with the ocean; and (d) calving of icebergs at the shoreline.
How Fast Do Ice Sheets Move?
Where there is no pronounced slope, an ice sheet may move no faster than a few inches per year. In rapidly flowing outlet glaciers and ice streams, the ice can move hundreds of meters or even several kilometers annually.
In 2012, before its recent slowdown, Greenland’s Jakobshavn glacier – widely believed to be the glacier whose iceberg sank the Titanic in 1912 – attained a record speed of more than 17 kilometers (10.6 miles) per year, an average of 46 metres per day. 9
Are the Ice Sheets Melting?
Yes. The Greenland ice sheet is shrinking; the West Antarctic and East Antarctic ice sheets are also shrinking, though not as fast.
In a new study, researchers discovered that almost 96 percent of the Greenland ice sheet experienced melting at some time in 2019, compared with an average of 64 percent between 1981 and 2010. 10
Another recent study conducted by IMBIE – the Ice Sheet Mass Balance Inter-Comparison Exercise, a consortium of 89 polar experts drawn from 50 international organisations – reveals that Greenland is losing ice seven times faster today than during the 1990s. This conclusion is based on a review of all the satellite observations over a 26-year period.11 12
In an average year, the Greenland ice sheet sheds about 250 billion tonnes of ice. The year 2019, however, has seen record temperatures. At Ilulissat, not far from the huge Jakobshavn Glacier, temperatures reached into the high 20s Celsius. The IMBIE study estimated ice loss in 2019 at about 370 billion tonnes. And things are unlikely to improve, given that the Arctic as a whole has seen a 0.75°C temperature rise in just the last 10 years.
In Antarctica, the situation varies between West Antarctica and East Antarctica.
In 2019, most climatologists assumed that over the previous 40 years West Antarctica had been losing ice, while East Antarctica had been making modest gains. Then in 2019, a new study – using a newly adjusted ice-accumulation climate model – showed that East Antarctica had actually been losing ice continually from 1979 right up to 2017. The study also uncovered increasing losses across Antarctica each decade: from 40 gigatons per year during the period 1979-1990, to 252 gigatons per year during the period 2009-2017. 13
In West Antarctica, concerns about the stability of the ice sheet have centered on the Amundsen Sea Embayment, where the Thwaites, Pine Island, Smith, and other glaciers, drain into the sea. Since the 1980s, for example, the Thwaites glacier alone has suffered a net loss of over 600 billion tons of ice. 14
A number of studies (Alley et al. 2015, Pollard et al. 2015, Feldmann and Levermann 2015) have emphasized the vulnerability of the West Antarctic ice sheet to a rapid collapse which is projected to occur within a matter of decades once it begins, although rapid changes are not expected for centuries.
Further research by DeConto and Pollard suggests that such a collapse might be closer than we think. Pointing to certain weakening mechanisms – such as ice cliff failure and hydro fracture, omitted from most ice-flow models – the study shows a collapse of the Thwaites Glacier could come within a matter of decades. 15 16
A recent study (2020), conducted by the British Antarctic Survey (BAS), which surveyed the deep seafloor channels in front of the Thwaites glacier, provides the first detailed map of very large cavities in the base of the glacier, that provide access for warm water to infiltrate and melt Thwaites’ underbelly. 17 It confirms the suspicions of numerous glaciologists that the Thwaites giant is losing ice at a much more rapid rate than expected.
Ice Sheets Are a Touchstone Issue for Climate Change
The state of the Antarctic ice sheet is a critical issue in climate change, because of the consequences of a possible collapse. If the West Antarctic or East Antarctic ice sheets triggered any serious climate tipping points, hundreds of millions of people could face being displaced because of storm surges and rising seas. Hurricane Katrina flooded 80 percent of New Orleans in 2005, with some areas under 4.6 m (15 ft) of water, but if Antarctica sheds a significant fraction of its ice, New Orleans will disappear completely, along with hundreds of other coastal cities around the world.
Will the ice sheets melt completely? Right now, the answer is No. There seems no likelihood that changes of this magnitude will happen over the next couple of centuries. Then again, when the Wuhan Municipal Health Commission first reported a cluster of cases of pneumonia (31 Dec 2019), how many people believed it signalled the beginning of a worldwide virus pandemic?
- IPCC Fifth Assessment Report (Vaughan et al, 2013)
- “Discovering Antarctica”
- “Ice streams as the arteries of an ice sheet: their mechanics, stability and significance.” Bennett, M.R., 2003. Earth-Science Reviews, 61(3-4): 309-339.
- “Quick Facts on Ice Shelves”
- “Palaeogeography, palaeoclimatology, & palaeoecology.” ISSN 0031-0182, 1992, vol. 93, no1-2, pp. 85–112
- “State of the Cryosphere: Ice Sheets.” NSIDC.
- Vaughan et al, 1999
- “Brief Communication: Further summer speedup of Jakobshavn Isbræ.” Joughin, B, et al; The Cryosphere, 8, 209–214, 2014.
- “Unprecedented atmospheric conditions (1948–2019) drive the 2019 exceptional melting season over the Greenland ice sheet.” Marco Tedesco, Xavier Fettweis. The Cryosphere, 14, 1209–1223, 2020.
- “Mass balance of the Greenland Ice Sheet from 1992 to 2018.” Shepherd, A., Ivins, E., Rignot, E. et al. Nature 579, 233–239 (2020).
- For full article, see citation in: “Climate change: Greenland ice melt ‘is accelerating.’ BBC News.
- “Four decades of Antarctic Ice Sheet mass balance from 1979–2017.” Eric Rignot, et al. PNAS January 22, 2019 116 (4) 1095-103.
- “In Antarctica, Two Crucial Glaciers Accelerate Toward the Sea”. Patel, Jugal K. The New York Times. October 26, 2017
- “Contribution of Antarctica to past and future sea-level rise.” Robert M. DeConto & David Pollard. Nature. Vol 531, p.591. March 2016.
- “Evolving Understanding of Antarctic Ice-Sheet Physics and Ambiguity in Probabilistic Sea-Level Projections.” Robert E. Kopp, et al. Earth’s Future. 13 December 2017.
- “Doomsday Glacier vulnerability seen in new maps” BBC. Sept 8, 2020