Antarctic Ice Sheet: How Fast is it Melting?

All you need to know about the glaciers and ice caps of East and West Antarctica: the causes and rate of ice-melt, and the threat to sea levels posed by Southern Ocean warming and rising temperatures on the Antarctic ice sheet.
Scientists Review Ice Melt at Thwaites Glacier
Scientists on West Antarctica’s massive Thwaites Glacier examine a crevasse. Photo: © Dr. Seth Campbell GHC Project

The continent of Antarctica is half the size of Africa but it’s so cold, so barren, and so desolate, that almost nothing survives on it, except for a few species of penguins and seals. This is because 98 percent of its surface is covered by the Antarctic ice sheet, a giant mass of ice with an average thickness of 1,900 m (6,200 ft), and a volume of 30 million cubic kilometers (7.2 million cubic miles). The ice sheet covers nearly 8.6 million square kilometers (5.4 million sq miles), and flows to the ocean through scores of glaciers and ice streams. Some glaciers terminate at the shoreline, while some continue moving into the ocean where they form floating ice shelves.

This vast deep-frozen ‘white’ continent is the heart of the cryosphere, accounting for 90 percent of all the ice on Earth. By comparison, the Greenland ice sheet in the Arctic north has a total volume of 2.9 million cubic kilometers (695,000 million cubic miles). 1

What Happens If Antarctica Melts?

Not surprisingly, as global warming intensifies, scientists are becoming concerned about the sea level rise which might result from any melting of the Antarctic ice sheet, and the consequences for coastal communities around the world. For example, in its Special Report on the Ocean and Cryosphere (2019), the IPCC states there are two main controls on how much sea levels will rise this century: (a) future greenhouse gas emissions, and (b) how warming affects the Antarctic ice sheet.

Because, the fact is, if all the ice in Antarctica were to melt, it would cause sea levels to rise by 58.3 meters (191 ft). 2 And of course, if Antarctica melted, Greenland’s ice would also melt, adding a further 7 meters (24 ft). This 65-meter (214 ft) rise in sea level would submerge every coastal city in the world and place entire countries under water. Although the entire Antarctic is not going to melt for centuries, even millennia, a dangerous fraction of it might melt this century.

Is Antarctica Getting Warmer?

The short answer is Yes. There is recent evidence from several points around the continent that temperatures are rising. However, there is a great deal of natural variability in the climate of Antarctica, which results in periodic bouts of warming and cooling. So, it’s difficult to identify firm, long-term trends in either climate or ice-melt. In addition, Antarctica has three differing climate regions: The Peninsula which is the smallest and warmest region; West Antarctica which is prone to warming because of natural climate fluctuations; and East Antarctica – by far the largest and coldest zone – which shows almost no sign of warming except on its coastal fringes.

Remember also, that “warming” in Antarctica means “slightly less cold.” Yes, there are occasional anomalies, caused by unusual combinations of weather patterns, when temperatures in a particular area (like the Peninsula) can suddenly jump, but in general Antarctic temperatures rarely exceed freezing point except during the short summer.

The Antarctic Peninsula: Warmest Region

Mean temperatures on the Peninsula range from + 1°C to minus 15°C, although temperatures can rise to 15°C in summer. Precipitation varies between 400 mm (16 inches) and 600 mm (25 inches) a year. Overall, this area is experiencing warmer air temperatures in summer although – since it only makes up about 1 percent of the continent – it is far from typical. What’s more, weather patterns here can be confusing. For example, after nearly half a century of warming, a slight cooling trend is now apparent.

Between 1951 and 2000, temperature recordings at one weather station on the Peninsula rose by about 2.8°C (5°F), compared to a little more than 1°F (0.5°C) globally. That’s five times the mean rate of global warming as reported by the Intergovernmental Panel on Climate Change (IPCC). See also: What is Earth’s Temperature?

But since the 1990s, temperatures at the peninsula have started to decline. 3 The cooling is pretty minor — less than 1°C (2°F) since the 1990s — and it doesn’t nullify the global warming that’s happening because of the rise of greenhouse gases in the atmosphere, the researchers say. This was confirmed in 2002 by the spectacular collapse of the Larsen B ice shelf on the north-eastern coast of the Peninsula, an event attributed to the warm air absorbed by the peninsula over the preceding summers.

This cooling trend was confirmed by another study which found that temperatures at the South Shetland Islands and the north-east region of the Antarctic Peninsula, had shifted from a warming trend of 0.32°C per decade (1979–1997) to a decline of -0.47°C per decade during 1999–2014″ 4

Scientists now say that the period of warming (1951-2000) across the Peninsula occurred because of a combination of greenhouse gas emissions, depletion of the ozone layer, and a series of El Niño-like conditions in the Pacific, all of which favored warming westerly winds. The cooling period that followed, occurred as the ozone hole began to heal and the Pacific Ocean produced La Niña-like conditions, both of which favored more easterly winds – winds that tend to blow sea ice towards the peninsula, thus blocking the transfer of heat from the ocean to the atmosphere.

This doesn’t mean that greenhouse gases have stopped having an effect, either on the peninsula or on the continent as a whole. It simply means that – for the moment – natural variability is overshadowing the signs of long-term climate change.

Recently, in February 2020, Brazilian scientists recorded a record summer temperature of 20.75°C at Marambio base on Seymour Island, just off the Antarctic Peninsula. This is the first occasion that temperatures have exceeded 20°C, and is almost a full degree higher than the previous record of 19.8°C, which occurred in the South Orkney Islands, in 1982. A few days earlier in February 2020, a record measurement of 18.3°C was taken at the Argentine research station at Esperanza, on the northern tip of the Peninsula. 5

West Antarctic Mainland: Ocean Warming from Westerly Winds

Except for an occasional mountain peak, West Antarctica rises no higher than 2,500 meters (8,200 ft) above sea level. Mean temperatures range from minus 12°C to minus 35°C. Even in summer, temperatures rarely rise higher than 5°C. Precipitation varies from 100 mm (4 inches) to 400 mm (16 inches) per year. Overall, air temperatures have risen, though not as rapidly as the Peninsula.

Even so, the region does have its surprises. For example, in 2012, scientists revealed that the average temperature at the 1,550-meter high Byrd Station in the heart of the West Antarctic Ice Sheet, rose by 2.4 degrees Celsius from 1958 to 2010. 6

But it’s not rising temperatures that are the main worry: it’s warmer ocean waters. This ocean warming is causing ice-melt in the region’s Amundsen Sea embayment, where the huge Thwaites and Pine Island glaciers appear to be melting from the bottom up, as warm ocean water seeps beneath the ice.

What’s causing the ocean warming? A new study provides part of the answer: wind. In the 1920s, the winds over the Amundsen Sea mainly blew toward the west, keeping the warm ocean water at bay. Since then, climate change has caused a long-term change in the winds, so that today the wind oscillates between blowing eastward and westward. When the wind blows toward the east, the deep layer of warm ocean water creeps forward to warm the ice. 7

Are Volcanoes Helping to Warm West Antarctica?

The West Antarctic Ice Sheet (WAIS) sits on top of a major volcanic rift system – the West Antarctic Rift System (WARS) – which is thought to contain more than 138 volcanoes.

In 2008, British Antarctic Survey researchers revealed that a volcanic eruption had occurred 2,200 years ago under the West Antarctic ice sheet, close to Pine Island Glacier. It was the biggest volcanic event in the region for more than 10,000 years. 8

Then in 2018, scientists discovered geochemical evidence (seawater helium isotope ratios) proving the existence of a substantial underground volcanic heat source, upstream of the rapidly-melting Pine Island Ice Shelf, producing roughly half the heat of the active Grimsvotn volcano on Iceland. 9

Professor Karen Heywood, the team’s chief scientist, explained the significance of the find. “The discovery of volcanoes beneath the Antarctic ice sheet means that there is an additional source of heat to melt the ice, lubricate its passage toward the sea, and add to the melting from warm ocean waters,” she said.

Even if the volcanic heat source isn’t a direct cause of the ice shelf or glacier melting, the decreasing pressure exerted on the mantle by the glacier as it melts, allows more heat to escape from underground, which would then increase the melt rate.

Other scientists focus on the broader context. Dr Rob Larter, a marine geophysicist at the British Antarctic Survey commented: “The West Antarctic ice sheet has existed for tens of thousands of years and it has happily coexisted with volcanic activity happening within and underneath it,” he said.

No one is seriously suggesting that volcanoes are responsible for the warming of the Antarctic continent, or even the widespread ice thinning which appears to be happening across the West Antarctic ice sheet. But the discovery of a significant volcanic heat source beneath a major WAIS glacier highlights the need to learn more about subglacial volcanism, and its potential contribution to the future integrity of the WAIS.

East Antarctica: Least Signs of Warming

Nowhere on Planet Earth is colder than East Antarctica. This vast elevated plateau – rising 4,000 meters (13,123 ft) above sea level – makes up two-thirds of the continent, with ice sheets up to 4.7 kilometers (nearly three miles) thick in places and temperatures capable of plunging to minus 97°C (minus 144°F). Temperatures rarely exceed minus 20°C, and during the winter months they average around minus 55°C. Precipitation varies between 50 mm (2 inches) and 100 mm (4 inches) per year. Generally speaking, East Antarctica shows little sign of warming, except on its coastal fringes, though there are exceptions.

For example, when scientists examined surface air temperatures at the Amundsen-Scott station at the South Pole – where winter temperatures regularly drop below minus 73°C (minus 100°F) – they discovered that temperatures had been rising by about 0.55°C (1°F) each decade since the start of the 1990s. That’s roughly three times faster than the global average. This warming was a major change, because over the previous 40 years – ever since the station was first established in 1956 – the South Pole had been cooling down. 10

So why is the South Pole warming? According to the study, a big contributor has been a shift in climate patterns in the tropics – notably changing ocean temperatures in the tropical western Pacific. This is regulated by a natural climate mechanism called the Interdecadal Pacific Oscillation, or IPO. The IPO causes western Pacific temperatures to oscillate between warm and cool phases every few decades. When the ocean is warmer, it causes more warm air to move south towards Antarctica.

A second climate mechanism, known as the Southern Annular Mode (SAM), is also involved. This regulates the westerly winds that flow around the Antarctic continent. The SAM can cause these currents of air to periodically strengthen or weaken. When stronger, they push that warm air over Antarctica, heating up the South Pole. The combination of these two natural climate mechanisms is believed to be the main cause of warming over the South Pole since the 1990s. However, climate change is still believed to affect both the IPO and the SAM, albeit in ways that are not yet fully understood.

All this shows that even the heart of the polar Antarctic is prone to warming. It also illustrates one of the most confusing aspects of Antarctica’s climate – namely, that – unlike the Arctic – different regions can experience different climate trends at the same time. Thus, even as the South Pole was cooling in previous decades, most of West Antarctica and the Antarctic Peninsula was warming. Then, from the 1990s, as the South Pole was warming, the Peninsula was cooling.

The Antarctic Ice Sheets: How Much Ice Are They Losing?

Glaciologists recognize three ice sheets in Antarctica: the Antarctic Peninsula Ice Sheet (APIS), the East Antarctic Ice Sheet (EAIS), and the West Antarctic Ice Sheet (WAIS). The last two – which contain 99 percent of the continent’s ice – are separated by the 2,000-kilometer long Transantarctic Mountain range, which rises up about 4 kilometers (13,100 ft) high and whose peaks are visible only as nunataks, poking up through the ice sheet.

The EAIS is 9 times bigger (by volume) than the West Antarctic sheet (WAIS) and 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 contains enough water to raise sea levels by about 53 meters (173 ft), while the WAIS holds enough to raise seas by about 17 feet. The Peninsula Ice Sheet, by comparison, contains enough water to raise sea levels by about 24 centimeters.

The Antarctic Peninsula Ice Sheet

This is roughly 500 meters (1,640 ft) thick, with outlet glaciers flowing both east and west. The APIS is particularly sensitive to its warming temperatures, as shown by the collapse of numerous ice shelves and increased ice velocities, as well as the retreat and reduction of glaciers and ice caps. Many of its tidewater glaciers are grounded, notably those on the north-western coast, but large ice shelves survive on both sides of the Peninsula south of 70°S (in the west) and 68°S (in the east).

Evidence shows that many glaciers in the Antarctic Peninsula have already shifted to a negative mass balance and are in retreat. 11

How Much Ice is the Antarctic Peninsula Ice Sheet Losing?

According to the latest IMBIE study, the APIS lost an average of 20 billion tonnes of ice per year, over the 25-year study. 12 This loss rate increased during the study and especially since the year 2000. The main reason for the ice loss, say scientists, is because the floating ice shelves sitting in front of some glaciers disintegrated, allowing the glacial ice behind to flow faster.

Map of Glaciers in West Antarctica
Map showing the main glaciers that drain into West Antarctica’s Amundsen Sea Embayment. The ground underlying these glaciers slopes deeper below sea level as it extends inland. Thus, as the glaciers retreat, the warmer ocean water follows, adding further to the melting process. For this reason, glaciologists believe that the disappearance of these glaciers is inevitable, albeit over two centuries or so. 13

The West Antarctic Ice Sheet

The West Antarctic Ice Sheet drains into the sea through several large glaciers, most of which flow into either the Ross Ice Shelf, or the Filchner-Ronne Ice Shelf. Two important exceptions are Pine Island Glacier and its southern neighbor, the Thwaites Glacier, which do not flow into a large ice shelf. Instead they generate their own smaller ice shelves in the Amundsen Sea Embayment.

According to a relatively recent study, the continent’s ice sheet as a whole has thinned by as much as 122 meters (396 ft) in places. This is due to warmer ocean water melting the underneath of its ice shelves and also the base of its glaciers.

The fastest change is happening in West Antarctica where glaciers and ice shelves are shedding significantly more ice from ocean melting and iceberg calving than they’re gaining from snowfall. Since 1992, ice thinning has spread across one quarter of West Antarctica and over most of its major ice streams, which are now losing ice five times more rapidly than they were at the start of the 1990s. 14

West Antarctic Bedrock is Below Sea Level

A major influence on the degree and rate of glacier melt by the ocean in West Antarctica, is the fact that much of its underlying bedrock is below sea level. All the glacier floors slope deeper below sea level as they retreat inland. So, as the glacier and its grounding line retreat down the slope of the bedrock, the glacier becomes grounded in deeper and deeper water and the amount of ice exposed to warming becomes greater.

At the same time, the warm ocean water is continually eroding the underside of the ice shelves. In Pine Island Bay, for example, ice shelves at the front of the Pine Island and Thwaites glaciers are being warmed from below by ocean water, which is causing intense melting, glacier acceleration and faster drawdown of the ice sheet. This area is the so-called “Weak Underbelly” of the West Antarctic Ice Sheet, which – scientists believe – may be vulnerable to collapse.

This is because the ice shelf buttresses and supports the glacier. If it weakens or suffers major loss of ice, it will probably result in rapid glacier acceleration, thinning, and grounding line migration, with serious consequences, including the possible disintegration of the entire ice shelf.

Precisely these reactions were observed in 2002 following the collapse of the Larsen B ice shelf on the Antarctic Peninsula. The glaciers behind the Larsen B, which had been held in place by the huge ice shelf, began moving 5-8 times faster towards the ocean, eventually averaging a 300 percent increase. Loss of ice into the ocean skyrocketed from 2–4 billion tonnes per year (1996-2000) to between 22 and 40 billion tonnes per year in 2006. 15

The close connection between erosion of an ice shelf and the acceleration of glaciers behind it, has been confirmed in a recent study. Its findings show that thinning of the floating ice shelves around Antarctica is driving mass loss from the interior of the continent. The thinning is matched almost exactly by an acceleration in the glaciers behind the shelves. 16

What Is Albedo Effect?
What Is Thermohaline Circulation?

How Much Ice is the West Antarctic Ice Sheet Losing?

The West Antarctic Ice Sheet lost an average of 53 billion tonnes of ice per year from 1992-1997. This increased to an average of 159 billion tonnes of ice per year during the period 2012-2017. 17

East Antarctic Glaciers Map
Map showing the velocity of the massive Totten Glacier, as well as four other glaciers in Vincennes Bay, East Antarctica. As of 2018, these four have lowered their surface height by about 9 feet since 2008, before which no ice loss had been recorded. This raises the possibility of systemic changes in the ocean, similar to West Antarctica. Image: NASA Earth Observatory/Joshua Stevens. (Source: Alex Gardner, NASA-JPL)

The East Antarctic Ice Sheet

East Antarctica, by far the greater part of the continent, is the only region whose ice sheet balance (the difference between ice gain from snowfall and ice loss from melting) has shown some growth – at least according to one major set of figures. This is because (except for its coastline) the region sits high above the ocean and is not subject to the same melting forces seen in the rest of Antarctica. However, because snowfall is light to non-existent, ice gains are quite small – no more than five billion tonnes a year. 17

However, concerns remain about what may be happening in East Antarctica. Nobody is suggesting that its interior plateau is going to start melting: this looks rock solid for centuries, if not millennia. But its coastal strip, in contact with warming ocean waters, is a different matter. If ice shelves on the coast experience serious thinning similar to West Antarctica, the huge glaciers behind them could speed up their slide into the sea.

Ice thinning in glaciers along the Budd coast in East Antarctica is well documented. 18 19 Now, there is clear evidence that two of them – the Totten and Moscow University glaciers – have been losing ice rapidly for the past 15 years. Losses been estimated by one study at 14.8 billion tonnes per year from April 2002 to November 2015. 20

In addition, a new study, led by NASA’s Eric Rignot (see below), suggests that East Antarctica might be experiencing a net loss of ice – a major change in the previous consensus that the region was quietly gaining mass.

How Much Ice is the Antarctic Ice Sheet Losing in Total?

The state of the Antarctic ice sheet has been thoroughly examined by IMBIE (ice sheet mass balance inter-comparison exercise), an international collaboration of polar scientists, supported by the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA). IMBIE’s mission is to assess satellite measurements of ice sheet mass balance in order to furnish improved estimates of the ice sheet contribution to sea level rise.

According to its latest report, which used 24 satellite surveys of Antarctica and involved 80 scientists from 42 international organizations, ocean melting and ice-shelf collapse has resulted in ice losses from the Antarctic continent of around 2.7 trillion tonnes between 1992 and 2017. Nearly all of these losses are caused by ocean warming, unlike Greenland, where half the ice loss is caused by warmer air temperatures.

The study calculated the continent lost 60 billion tonnes per year during the period 1992-1997, rising to 219 billion tonnes per year during the period 2012-2017. With less confidence, the report estimates that East Antarctica made small gains of approximately 5 billion tonnes per year from 1992 to 2017. 17

IMBIE Report Challenged

But the IMBIE report was contradicted the following year by a study based on newer data sources as well as a new model for projecting overall ice accumulation. The 2019 study, led by NASA’s Eric Rignot, looked at ice mass changes over four decades 1979–2017. It found that the rate of ice mass loss across the entire white continent increased in each decade studied.

Losses totalled 40 billion tonnes per year in 1979–1990, 50 billion tonnes per year in 1989–2000, 166 billion tonnes per year in 1999–2009, and 252 billion tonnes per year in 2009–2017. A key finding was that East Antarctica had incurred ice losses throughout the period. However, these new calculations await corroboration by satellite. 11

How Do Ice Losses from Antarctica Affect Global Sea Levels?

The IMBIE Report found that ice losses from Antarctica have tripled since 2012, raising sea levels by 3 mm (0.12 inch). Up to 2012, the report found that Antarctica shed ice at a steady rate of about 76 billion tonnes per year – enough to cause a 0.2 mm per year rise in sea levels. However, between 2012 and 2017 the continent lost 219 billion tonnes of ice per year, resulting in a 0.6 mm per year sea level contribution.

“A three-fold increase now puts Antarctica in the frame as one of the largest contributors to sea-level rise,” said Professor Andrew Shepherd, co-leader of IMBIE. “The last time we looked at the polar ice sheets, Greenland was the dominant contributor. That’s no longer the case.”

ANTARCTIC WATERS

Krill At Risk from Climate Change
Marine Food Web

The Doomsday Scenario

So far, we have seen that air temperatures in Antarctica are clearly warming, and warming more rapidly than previously thought. But these increases are relatively minor compared to those recorded in the northern hemisphere, where widespread heatwaves have contributed to Arctic fires, premature thawing of permafrost and melting of Arctic sea ice throughout the region.

Instead, Antarctica is being attacked around its coastline by warmer ocean waters that are melting the underside of its floating ice shelves. As the ice shelves thin and break away, the massive glaciers behind them accelerate their slide into the sea. This has caused major ice loss in West Antarctica and the Peninsula, and may also be causing a growing loss of ice in East Antarctica.

In West Antarctica, concerns about the stability of the ice sheet have focused on Pine Island Bay, part of the Amundsen Sea, where two huge glaciers – Pine Island and Thwaites – drain into the ocean. Pine Island drains 10 percent of the entire West Antarctic Ice Sheet, while Thwaites covers an area of 181,000 square kilometers (70,000 sq mi).

At the start of the 1980s, both these ice streams were in balance – ice gains from snowfall matched the ice losses into the sea. Since then, they have speeded up considerably, causing Pine Island glacier to lose about 800 billion tonnes of ice and Thwaites 600 billion tonnes, because of warming waters in front of them. The Pine Island ice shelf, for example, has increased its speed by 75 percent from 1973 to 2010. 21

Thwaites Before And After - December 2001 (Before)
Thwaites Ice Shelf 2001
Images showing the ongoing break-up of Thwaites ice shelf in the Amundsen Sea embayment off West Antarctica. Both images capture the slow-moving glacier as it flows into the Amundsen Sea, becoming a thick floating ice shelf. The first image above shows the glacier’s floating ice tongue on December 2, 2001. The second image below shows the glacier on December 28, 2019. Unlike its northern neighbor, the Pine Island Glacier, which calves large icebergs almost annually, Thwaites constantly produces a mass of small broken bits. Once their shelves disappear, these two huge ice rivers are likely to accelerate their slide into the sea, with unpredictable consequences for the stability of the West Antarctic Ice Sheet. Images above and below: Courtesy of NASA Earth Observatory (Landsat 7 & 8)
Thwaites December 2019 - Before And After (After)
Thwaites Ice Shelf 2019

There’s an important knock-on effect here. The ice shelf buttresses the grounded glacier behind it, and the part of the glacier that is closer to the ocean prevents the rest of the glacier from flowing into the sea. Take away the ice shelf and the glacier will immediately accelerate into the sea, adding to sea levels in the process.

Scientists estimate that if both glaciers were to melt, sea levels could rise by 1-2 meters (3 ft 3 inches to 6 ft 6 inches), due to the likely knock-on effects across West Antarctica, and possibly even across the eastern half of the continent as well. 22

For this reason, any rapid melting of Thwaites Glacier is seen as one of the most serious climate tipping points in the cryosphere. If Thwaites goes, so does Pine Island Glacier, and the structure of the WAIS could be seriously compromised.

Paul Cutler, program director for Antarctic glaciology at the U.S. National Science Foundation calls Thwaites a “keystone for the other glaciers around it. By itself, Thwaites could raise sea levels about 65 cm (2 ft) as it melts. But if Thwaites goes, the knock-on effect across the western half of Antarctica would lead to 2-3 meters of sea level rise”.

Ongoing concerns about Thwaites’ stability have prompted the formation of a 5-year scientific collaboration, known as the International Thwaites Glacier Collaboration (ITGC), to study and monitor the condition of the glacier. In particular, there are two types of possible instability that may affect Thwaites and other glaciers in the west: ice sheet instability and ice-cliff instability.

Marine Ice Sheet Instability (MISI)

As far back as 1981, it was suggested that the region around Pine Island Bay could be a “weak underbelly” of the West Antarctic Ice Sheet. 23 The reasoning is twofold. First, that the Pine Island and Thwaites glaciers are not protected from the ocean by large floating ice shelves. Second, that the base of both glaciers lies below sea level, increasing the likelihood of marine ice sheet instability.

A slightly different example of marine ice sheet instability was illustrated by a recent report, detailing the discovery of a giant cavity at the bottom of Thwaites Glacier. The cavity measured 300 meters (1,000 ft) in height, covered an area two-thirds the size of Manhattan and could hold around 14 billion tonnes of ice, although most of this had melted over the past three years. Researchers found the cavity using ice-penetrating radar and other equipment from NASA’s Operation IceBridge. 24

The main lesson to be learned concerned the interaction between sea and ice. When the floor of a glacier slopes deeper below sea level as it retreats inland, the ocean will fill any cracks or cavities that appear between the glacier and the underlying bedrock, with warm water, thus accelerating basal ice melt. The bigger the cavity, the bigger the melting effect.

Polar expert Eric Rignot, a co-author of the study, said understanding how warming oceans were melting glaciers was “essential to project its impact on sea level rise in the coming decades”.

Marine Ice Cliff Instability (MICI)

The most dramatic forecasts of sea level rise caused by Antarctic ice melt, are based on a mechanism known as marine ice cliff instability (MICI), the likelihood of which was reinvigorated by a 2016 paper published in Nature, and co-authored by Robert M. DeConto and David Pollard. Ice cliff instability, the paper says, could result in over three feet of sea level rise by the year 2100.

It works like this. Ice sheets are always moving down to the sea. As we have seen, this flow of ice to the ocean is resisted by the buttressing effect of floating ice shelves. According to DeConto and Pollard, if the ice shelves fragment, exposing the taller cliff front of the grounded glacier behind them, the cliff – if taller than 90m/300 ft – could collapse under its own weight, especially if weakened by hydrofracturing (surface melt draining into crevasses) and atmospheric warming. What’s more, if cliff collapse did occur it would expose even higher cliffs in the interior of a thickening ice sheet, leading to an irreversible domino effect. 25

The ice-cliff collapse mechanism was seen at first hand during the abrupt collapse of Antarctica’s Larsen B ice shelf (2002), and is still seen on the Greenland Ice Sheet during the calving of icebergs from the Jakobshavn and Helheim glaciers.

Up until recently, the consensus was that – despite its faster melting – Thwaites Glacier would survive for centuries. Even a worse-case scenario projected its survival for 150 years. 26

Now, a combination of marine ice sheet instability and ice-cliff collapse suggests that Thwaites may be vulnerable to a more rapid collapse which could cause a significant increase in sea-level rise by the end of this century. 27

Doubts About Marine Ice Cliff Instability Domino Effect

The ice cliff instability theory has stimulated much debate and some disagreement. One 2019 study led by Tamsin Edwards demonstrated that more in-depth analysis is needed to validate the MICI hypothesis. 28

Another 2019 study found that in order for a 90-meter ice cliff to collapse abruptly under its own weight, the ice shelf buttressing it would have to disintegrate very rapidly – within a matter of hours — a rate of collapse never witnessed before. If the ice shelf took longer to break up, the study found that the ice cliff wouldn’t suddenly crack under its own weight. Instead, the weight pressure would be distributed throughout the cliff, causing it to deflate plastically or viscously rather than fall down. 29

Antarctic Ice Sheet: Conclusion

Assuming that we don’t suffer a catastrophic planetary event like an asteroid crashing into the South Pole, the Antarctic Ice Sheet is likely to remain substantially intact for centuries. Even so, the Antarctic alone could add a foot to sea level by 2100, says geoscientist Robert DeConto, and possibly more than 3 feet by 2150. Add to this the effects of a melting Greenland ice sheet, thousands of melting alpine glaciers, plus an extra 40 percent or so to allow for thermal expansion. and global sea levels could rise as much as 6 feet by the end of this century.

In fairness, the Intergovernmental Panel on Climate Change is taking a much more conservative approach. Even under a business-as-usual scenario – that is, without any proper climate change mitigation – the IPCC is forecasting a total sea level rise (taking into account all types of ice melt) of no more than 84cm (2 ft 9 inches) by the end of the century. 30

It’s worth remembering, however, that storm surges can be much more deadly. In temperate seas, storm surges can raise sea levels by an additional 3 meters (10 ft), while in the tropics the rise can be as high as 8 meters (26 ft). The point is, extreme weather events in the ocean – such as hurricanes, cyclones or typhoons – are one of the well-documented effects of global warming on the ocean.

References

  1. Bedmap2: improved ice bed, surface and thickness datasets for Antarctica“. Fretwell, P. et al; The Cryosphere, 7, 375–393, 2013. []
  2. IPCC Fifth Assessment Report. (Vaughan et al, 2013). []
  3. Absence of 21st century warming on Antarctic Peninsula consistent with natural variability.” Turner, J., et al. Nature 535, 411–415 (2016). []
  4. Recent regional climate cooling on the Antarctic Peninsula and associated impacts on the cryosphere“. Oliva, M; et al. (February 2017). Science of the Total Environment. 580: 210–223. []
  5. Antarctic temperature rises above 20 degrees Celsius for first time on record.[]
  6. Central West Antarctica among the most rapidly warming regions on Earth.” Bromwich, D., Nicolas, J., Monaghan, A. et al. Nature Geosci 6, 139–145 (2013). []
  7. West Antarctic ice loss influenced by internal climate variability and anthropogenic forcing.” Holland, P.R., Bracegirdle, T.J., Dutrieux, P. et al. Nat. Geosci. 12, 718–724 (2019). []
  8. “A recent volcanic eruption beneath the West Antarctic ice sheet”. Corr, H. F. J.; Vaughan, D. G. (2008). Nature Geoscience. 1 (2): 122–125. []
  9. Evidence of an active volcanic heat source beneath the Pine Island Glacier“. Brice Loose; et al. (2018). Nature Communications. 9: 2431. []
  10. Record warming at the South Pole during the past three decades.” Clem, K.R., Fogt, R.L., Turner, J. et al. Nat. Clim. Chang. (2020). []
  11. Four decades of Antarctic Ice Sheet mass balance from 1979–2017.” Eric Rignot, et al. PNAS January 22, 2019 116 (4) 1095-1103.[][]
  12. Mass balance of the Antarctic Ice Sheet from 1992 to 2017.” Shepherd, A., Ivins, E., Rignot, E. et al. Nature 558, 219–222 (2018). []
  13. NASA []
  14. Trends in Antarctic Ice Sheet Elevation and Mass.” Andrew Shepherd, et al. Geophysical Research Letters. 16 May 2019 []
  15. Collapse of Larsen B Ice Shelf.” NASA. []
  16. Instantaneous Antarctic ice sheet mass loss driven by thinning ice shelves.” G. Hilmar Gudmundsson, et al. Geophysical Research Letters. 20 November 2019. []
  17. Mass balance of the Antarctic Ice Sheet from 1992 to 2017.” Shepherd, A., Ivins, E., Rignot, E. et al. Nature 558, 219–222 (2018). [][][]
  18. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets“. Pritchard, Hamish D; et al. (2009). Nature. 461 (7266): 971–975. []
  19. More Glaciers in East Antarctica are waking Up.[]
  20. Mass Loss of Totten and Moscow University Glaciers, East Antarctica, Using Regionally Optimized GRACE Mascons.” Yara Mohajerani, Isabella Velicogna, Eric Rignot. Geophysical Research Letters. 25 July 2018 []
  21. Source: Cumulative mass balance data from NASA Jet Propulsion Laboratory. New York Times Oct 26, 2017. []
  22. Collapse of the West Antarctic Ice Sheet after local destabilization of the Amundsen Basin.” Johannes Feldmann et al. PNAS November 17, 2015 112 (46) 14191-14196; November 2, 2015. []
  23. “The weak underbelly of the West Antarctic ice sheet.” Terry Hughes. Journal of Glaciology. Access Volume 27, Issue 971981 pp.518-525. []
  24. Heterogeneous retreat and ice melt of Thwaites Glacier, West Antarctica.” P. Milillo, E. Rignot, et al. Science Advances 30 Jan 2019: Vol. 5, no. 1, eaau3433. []
  25. Contribution of Antarctica to past and future sea-level rise.” DeConto, R., Pollard, D. Nature 531, 591–597 (2016). []
  26. Marine ice sheet instability amplifies and skews uncertainty in projections of future sea-level rise.” Alexander A. Robel, Helene Seroussi, Gerard H. Roe. PNAS July 23, 2019 116 (30) 14887-14892; July 8, 2019 []
  27. Note: There is evidence to suggest that, in previous interglacials, the West Antarctic Ice Sheet disappeared entirely, causing sea levels to rise approximately 5m (16ft) higher than at present. See: “West Antarctic Ice Sheet and CO2 Greenhouse effect – threat of disaster.” Mercer, J.H; et al. 1978. Nature, 1978. 271(5643): p. 321-325. []
  28. Revisiting Antarctic ice loss due to marine ice-cliff instability.” Edwards, T.L., Brandon, M.A., Durand, G. et al. Nature 566, 58–64 (2019). []
  29. Marine Ice Cliff Instability Mitigated by Slow Removal of Ice Shelves.” Fiona Clerc, Brent M. Minchew, Mark D. Behn. Geophysical Research Letters. 21 October 2019. []
  30. IPCC. Special Report on the Ocean and Cryosphere in a Changing Climate. 2019. Summary for Policymakers. []
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