Antarctic ice sheet

The Antarctic ice sheet is one of the two polar ice caps of Earth. It 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.[2] Separate to the Antarctic sea ice it covers an area of almost 14 million square kilometres (5.4 million square miles) and contains 26.5 million cubic kilometres (6,400,000 cubic miles) of ice.[3] A cubic kilometer of ice weighs approximately 0.92 metric gigatonnes, meaning that the ice sheet weighs about 24,380,000 gigatonnes. It holds approximately 61% of all fresh water on Earth, equivalent to about 58 meters of sea level rise[4] if all the ice were above sea level. In East Antarctica, the ice sheet rests on a major land mass, while in West Antarctica the bed can extend to more than 2,500 m below sea level.

A satellite composite image of Antarctica
Antarctic Skin Temperature Trends between 1981 and 2007, based on thermal infrared observations made by a series of NOAA satellite sensors. Skin temperature trends do not necessarily reflect air temperature trends.[1]
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.

Satellite measurements by NASA indicate a still increasing sheet thickness above the continent, outweighing the losses at the edge.[5] The reasons for this are not fully understood, but suggestions include the climatic effects on ocean and atmospheric circulation of the ozone hole,[6] and/or cooler ocean surface temperatures as the warming deep waters melt the ice shelves.[7]

History

The icing of Antarctica began in the Late Palaeocene or middle Eocene between 60[8] and 45.5 million years ago[9] and escalated during the Eocene–Oligocene extinction event about 34 million years ago. CO2 levels were then about 760 ppm[10] 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.[11] 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.[12] The opening of the Drake Passage may have played a role as well[13] though models of the changes suggest declining CO2 levels to have been more important.[14]

The Western Antarctic ice sheet declined somewhat during the warm early Pliocene epoch, approximately 5 to 3 million years ago; during this time the Ross Sea opened up.[15] But there was no significant decline in the land-based Eastern Antarctic ice sheet.[16]

Late twentieth century

Temperature

According to a 2009 study, Antarctica's average surface temperature trend is positive and significant at >0.05 °C/decade since 1957.[17][18][19][20] West Antarctica has warmed by more than 0.1 °C/decade since 1960. This warming is strongest in winter and spring. Although this is partly offset by fall cooling in East Antarctica, this occurred only during the 1980s and 1990s.[17][18][19]

Floating ice and land ice
See also: Ice shelf and Antarctic sea ice
An image of Antarctica differentiating its landmass (dark grey) from its ice shelves (minimum extent, light grey, and maximum extent, white)
Visualization of NASA's mission Operation IceBridge dataset BEDMAP2, obtained with laser and ice-penetrating radar, collecting surface height, bedrock topography and ice thickness.
The bedrock topography of Antarctica, critical to understand dynamic motion of the continental ice sheets.

Ice enters the sheet through precipitation as snow. This snow is then compacted to form glacial ice that moves under gravity towards the coast. Most of it is carried by fast-moving ice streams. The ice then passes into the ocean, forming floating ice shelves. These shelves then melt or calve to give icebergs that eventually melt.

If the movement of ice to the sea is balanced by snow falling on the land then global sea levels remain unaffected. A warming climate in the southern hemisphere transports more moisture to Antarctica, growing the interior ice sheets, while calving events along the coast increase, allowing interior ice quicker access to the sea.

A 2006 paper derived from satellite data, measuring changes in the gravity of the ice mass, suggested that the total amount of ice in Antarctica had begun decreasing.[21] A 2008 study compared the ice leaving the ice sheet, by measuring the ice velocity and thickness along the coast, to the amount of snow accumulation. It reported that the East Antarctic Ice Sheet was in balance, but the West Antarctic Ice Sheet was losing mass. This was largely due to acceleration of ice streams such as Pine Island Glacier. These results agree closely with the 2006 report.[22][23] An estimate published in November 2012, based on Gravity Recovery and Climate Experiment data as well as on an improved glacial isostatic adjustment model discussed systematic uncertainty in the estimates, and by studying 26 separate regions, estimated an average yearly mass loss of 69 ± 18 Gt/y from 2002 to 2010 (a sea-level rise of 0.16 ± 0.043 mm/y). The mass loss was geographically uneven, mainly occurring along the Amundsen Sea coast, while the West Antarctic Ice Sheet mass was roughly constant and the East Antarctic Ice Sheet gained in mass.[24]

Antarctic sea ice anomalies have roughly followed the pattern of warming, with the greatest declines occurring off the coast of West Antarctica. East Antarctica sea ice has been increasing since 1978, although the increase was not statistically significant. The atmospheric warming is linked to the mass loss in West Antarctica of the 2000s. This mass loss is more likely to be due to increased melting of the ice shelves because of changes in ocean circulation patterns. The patterns may be linked to atmospheric circulation changes that may explain the warming trends in West Antarctica. Melting of the ice shelves in turn allows the ice streams to speed up.[25] The melting and disappearance of the floating ice shelves has only a minor effect on sea level, which is due to salinity differences.[26][27][28] The most important consequence of the increased melting is to increase the speed of the ice streams on land.

Recent observations
Ice mass loss since 2002, as measured by NASA's GRACE and GRACE Follow-On satellite projects, was 152 billion metric tons per year.[29]

A group of scientists with the University of California updated previous results ranging from 1979 to 2017, which improved time series for more accurate results. Their article, published January 2019, covered four decades of information in Antarctica, revealing the total mass loss which increased gradually per decade.

Ice loss (Gigatons)
Period Mean Range
1979-1990 40 ±9
1989-2000 50 ±14
1999-2009 166 ±18
2009-2017 252 ±26

The majority of mass loss was in the Amundsen Sea sector, which experienced loss as high as 159 ±8 Gt/y. Other areas have not experienced significant losses, such as the East Ross ice shelf.

This improved study revealed an acceleration of near 280% over the four decades. The study questions previous hypotheses, such as the belief that the heavy melt began in the 1940s to 1970s, suggesting that more recent anthropogenic actions have caused accelerated melt.[30]

See also

References
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  2. "Ice Sheets". National Science Foundation.
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