Ice sheets

"Continental glacier" redirects here. For the glacier located in Wyoming, see Continental Glacier.
One of Earth's two ice sheets: The Antarctic ice sheet covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers.[1]

In glaciology, an ice sheet, also known as a continental glacier,[2] is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi).[3] The only current ice sheets are the Antarctic ice sheet and the Greenland ice sheet. Ice sheets are bigger than ice shelves or alpine glaciers. Masses of ice covering less than 50,000 km2 are termed an ice cap. An ice cap will typically feed a series of glaciers around its periphery.

Although the surface is cold, the base of an ice sheet is generally warmer due to geothermal heat. In places, melting occurs and the melt-water lubricates the ice sheet so that it flows more rapidly. This process produces fast-flowing channels in the ice sheet — these are ice streams.

In previous geologic time spans (glacial periods) there were other ice sheets: during the Last Glacial Period at Last Glacial Maximum, the Laurentide Ice Sheet covered much of North America, the Weichselian ice sheet covered Northern Europe and the Patagonian Ice Sheet covered southern South America.

Definition

An ice sheet is "an ice body originating on land that covers an area of continental size, generally defined as covering >50,000 km2 , and that has formed over thousands of years through accumulation and compaction of snow".[4]: 2234 

Common properties

Carbon stores and fluxes in present-day ice sheets (2019), and the predicted impact on carbon dioxide (where data exists).
Estimated carbon fluxes are measured in Tg C a−1 (megatonnes of carbon per year) and estimated sizes of carbon stores are measured in Pg C (thousands of megatonnes of carbon). DOC = dissolved organic carbon, POC = particulate organic carbon.[5]
Further information: Ice-sheet dynamics

Ice sheets have the following properties: "An ice sheet flows outward from a high central ice plateau with a small average surface slope. The margins usually slope more steeply, and most ice is discharged through fast-flowing ice streams or outlet glaciers, often into the sea or into ice shelves floating on the sea."[4]: 2234 

Ice movement is dominated by the motion of glaciers, whose activity is determined by a number of processes.[6] Their motion is the result of cyclic surges interspersed with longer periods of inactivity, on both hourly and centennial time scales.

Until recently, ice sheets were viewed as inert components of the carbon cycle and were largely disregarded in global models. Research in the past decade has transformed this view, demonstrating the existence of uniquely adapted microbial communities, high rates of biogeochemical/physical weathering in ice sheets and storage and cycling of organic carbon in excess of 100 billion tonnes, as well as nutrients (see diagram).[5]

Earth's current two ice sheets

Antarctic ice sheet

This section is an excerpt from Antarctic ice sheet.[edit]
The Antarctic ice sheet is a continental glacier covering 98% of the Antarctic continent, with an area of 14 million square kilometres (5.4 million square miles) and an average thickness of over 2 kilometres (1.2 mi).[7] It is the largest of Earth's two current ice sheets, containing 26.5 million cubic kilometres (6,400,000 cubic miles) of ice, which is equivalent to 61% of all fresh water on Earth.[8][9]
This section is an excerpt from West Antarctic Ice Sheet.[edit]
The West Antarctic Ice Sheet (WAIS) is the segment of the continental ice sheet that covers West Antarctica, the portion of Antarctica on the side of the Transantarctic Mountains that lies in the Western Hemisphere. It is classified as a marine-based ice sheet, meaning that its bed lies well below sea level and its edges flow into floating ice shelves. The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.[10]
This section is an excerpt from East Antarctic Ice Sheet.[edit]

The East Antarctic Ice Sheet (EAIS) lies between 45° west and 168° east longitudinally. It was first formed around 34 million years ago,[11] and it is the largest ice sheet on the entire planet, with far greater volume than the Greenland ice sheet or the West Antarctic Ice Sheet (WAIS), from which it is separated by the Transantarctic Mountains. The ice sheet is around 2.2 km (1.4 mi) thick on average and is 4,897 m (16,066 ft) at its thickest point.[12] It is also home to the geographic South Pole, South Magnetic Pole and the Amundsen–Scott South Pole Station.

The surface of the EAIS is the driest, windiest, and coldest place on Earth. Lack of moisture in the air, high albedo from the snow as well as the surface's consistently high elevation[13] results in the reported cold temperature records of nearly −100 °C (−148 °F).[14][15] It is the only place on Earth cold enough for atmospheric temperature inversion to occur consistently. That is, while the atmosphere is typically warmest near the surface and becomes cooler at greater elevation, atmosphere during the Antarctic winter is cooler at the surface than in its middle layers. Consequently, greenhouse gases actually trap heat in the middle atmosphere and reduce its flow towards the surface while the temperature inversion lasts.[13]

Greenland ice sheet

This section is an excerpt from Greenland ice sheet.[edit]
Greenland ice sheet as seen from space

The Greenland ice sheet is an ice sheet about 1.67 km (1.0 mi) thick on average, and almost 3.5 km (2.2 mi) at its thickest point.[16] It is almost 2,900 kilometres (1,800 mi) long in a north–south direction, with the greatest width of 1,100 kilometres (680 mi) at a latitude of 77°N, near its northern edge.[17] It covers 1,710,000 square kilometres (660,000 sq mi), around 80% of the surface of Greenland, and is the second largest body of ice in the world, after the East Antarctic ice sheet.[16] The acronyms GIS or GrIS are also frequently used in the scientific literature.[18][19][20][21]

While Greenland has had major glaciers and ice caps for at least 18 million years,[22] a single ice sheet first covered most of the island some 2.6 million years ago.[23] Since then, it has both grown, sometimes significantly larger than now,[24][25] and shrunk to less than 10% of its volume on at least one occasion.[26][27][28] Its oldest known ice is about 1 million years old.[29] Due to greenhouse gas emissions by humans, the ice sheet is now the warmest it has been in at least the past 1000 years,[30] and is losing ice at the fastest rate in at least the past 12,000 years.[31]

A narrated tour about Greenland's ice sheet.

Melting due to climate change

Further information: Antarctic ice sheet § Changes due to climate change, and Greenland ice sheet § Recent melting
This section is an excerpt from Effects of climate change § Ice sheets decline.[edit]

The melting of the Greenland and West Antarctic ice sheets will continue to contribute to sea level rise over long time-scales. The Greenland ice sheet loss is mainly driven by melt from the top. Antarctic ice loss is driven by warm ocean water melting the outlet glaciers.[32]: 1215 

Future melt of the West Antarctic ice sheet is potentially abrupt under a high emission scenario, as a consequence of a partial collapse.[33]: 595–596  Part of the ice sheet is grounded on bedrock below sea level. This makes it possibly vulnerable to the self-enhancing process of marine ice sheet instability. Marine ice cliff instability could also contribute to a partial collapse. But there is limited evidence for its importance.[32]: 1269–1270  A partial collapse of the ice sheet would lead to rapid sea level rise and a local decrease in ocean salinity. It would be irreversible for decades and possibly even millennia.[33]: 595–596  The complete loss of the West Antarctic ice sheet would cause over 5 metres (16 ft) of sea level rise.[34]

In contrast to the West Antarctic ice sheet, melt of the Greenland ice sheet is projected to take place more gradually over millennia.[33]: 595–596  Sustained warming between 1 °C (1.8 °F) (low confidence) and 4 °C (7.2 °F) (medium confidence) would lead to a complete loss of the ice sheet. This would contribute 7 m (23 ft) to sea levels globally.[35]: 363  The ice loss could become irreversible due to a further self-enhancing feedback. This is called the elevation-surface mass balance feedback. When ice melts on top of the ice sheet, the elevation drops. Air temperature is higher at lower altitudes, so this promotes further melting.[35]: 362 

In geologic timescales

Antarctic ice sheet during geologic timescales

This section is an excerpt from Antarctic ice sheet § Situation during geologic time scales.[edit]
Polar climatic temperature changes throughout the Cenozoic, showing glaciation of Antarctica toward the end of the Eocene, thawing near the end of the Oligocene and subsequent Miocene re-glaciation.

The icing of Antarctica began in the Late Palaeocene or middle Eocene between 60[36] and 45.5 million years ago[37] and escalated during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm[38] and had been decreasing from earlier levels in the thousands of ppm. Carbon dioxide decrease, with a tipping point of 600 ppm, was the primary agent forcing Antarctic glaciation.[39] The glaciation was favored by an interval when the Earth's orbit favored cool summers but oxygen isotope ratio cycle marker changes were too large to be explained by Antarctic ice-sheet growth alone indicating an ice age of some size.[40] The opening of the Drake Passage may have played a role as well[41] though models of the changes suggest declining CO2 levels to have been more important.[42]

The Western Antarctic ice sheet declined somewhat during the warm early Pliocene epoch, approximately five to three million years ago; during this time the Ross Sea opened up.[43] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[44]

Greenland ice sheet during geologic timescales

This section is an excerpt from Greenland ice sheet § In geologic timescales.[edit]
Timeline of the ice sheet's formation from 2.9 to 2.6 million years ago[18]

While there is evidence of large glaciers in Greenland for most of the past 18 million years,[22] they were more similar to various smaller modern formations, 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. The conditions in Greenland were not initially suitable to enable the presence of a single cohesive ice sheet, 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 had experienced uplift for the first time, which ultimately formed the Upper Planation Surface at a 2000 to 3000 meter height above mean sea level.[45][46]

Later, during the Pliocene, a Lower Planation Surface, with the 500 to 1000 meter height above sea level, was formed during the second stage of uplift 5 million years ago, and the third stage had created multiple valleys and fjords below the planation surfaces. These increases in height had intensified glaciation due to increased orographic precipitation and cooler surface temperatures, which made it easier for ice to accumulate during colder periods and persist through higher temperature fluctuations.[45][46] While 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,[47] ice cover had gradually expanded since then,[23] until the atmospheric CO2 levels dropped to between 280 and 320 ppm 2.7–2.6 million years ago, which had reduced the temperatures sufficiently for the disparate ice caps build up in the meantime to connect and cover most of the island.[18]

For much of the past 120,000 years, the climate in and around Greenland had been colder than in the last few millennia of recorded history (upper half), allowing the ice sheet to become considerably larger than it is now (lower half).[48]

Often, the base of ice sheet is warm enough due to geothermal activity to have some liquid water beneath it.[49] This liquid water, subject to great pressure from the continued movement of massive layers of ice above it, becomes a tool of intense water erosion, which eventually leaves nothing but bedrock below the ice sheet. However, there are parts of the Greenland ice sheet, near the summit, where the upper layers of the ice sheet slide above the lowest layer of ice which had frozen solid to the ground, preserving ancient soil, which can then be discovered when scientists drill ice cores, up to 4 kilometres (2.5 mi) deep. The oldest such soil had been continuously covered by ice for around 2.7 million years,[28] while another, 3 kilometres (1.9 mi) deep ice core from the summit reveals ice that is around ~1,000,000 years old.[29]

On the other hand, ocean sediment samples from the Labrador Sea provide evidence that nearly all of south Greenland had melted around 400,000 years ago, during the Marine Isotope Stage 11,[26][50] and other ice core samples, taken from Camp Century in northwestern Greenland at a depth of 1.4 km (0.87 mi), demonstrate that the ice there melted at least once during the past 1.4 million years, during the Pleistocene, and that it did not return for at least 280,000 years.[27] Taken together, these findings suggest less than 10% of the current ice sheet's volume was left during those geologically recent periods, when the temperatures were less than 2.5 °C (4.5 °F) warmer than preindustrial, which contradicts how climate models typically simulate continuous presence of solid ice under those conditions.[51][28]

Glaciologist at work
Besides providing crucial information about the past states of the ice sheet and its impact on sea level rise, ice cores are invaluable for other kinds of paleoclimate research as well. The subtle differences in isotope distributions of ice core's water molecules can reveal important information about the water cycle at the time,[52] and air bubbles frozen within the ice core provide a snapshot of the lower atmosphere, detailing the gas and particulate composition it used to have.[53][54]When properly analyzed, ice cores provide a wealth of proxies suitable for reconstructing the past temperature record,[52] precipitation patterns,[55] volcanic eruptions,[56] solar variation,[53] ocean primary production,[54] and even changes in soil vegetation cover and the associated wildfire frequency.[57] The ice cores from Greenland also record human impact, such as lead production during the time of Ancient Greece[58] and the Roman Empire.[59]

See also

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