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Effects & Impacts: Antarctic melting

Larsen C ice shelf fracture image: John Sontag NASA

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Antarctic melting

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Summary

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Home > Climate wiki > Effects > Antarctic melting

Summary

Video 1: What’s happening in Antarctica and what it means for sea levels around Aotearoa.

Video 3: Prof. Matt King, Director of the Australian Centre for Excellence in Antarctic Science outlines the changes and surprises emerging from East Antarctica. This is also a good summary of the most recent research around Antarctica in general, including the massive reduction in the Antarctic Circumpolar Current (December 2023).

Video 2: Prof.Tim Naish from Wellington University explains what Antarctica may be melting faster than anyone realizes, and the implications for humanity are potentially disastrous (October 2022).

Video 4: New model (November 2023) produces High Resolution Simulation of Melting Antarctic Ice: NCI Australia

“A disaster waiting to happen”: West Antarctic Ice Sheet (WAIS)

“The irreversible loss of the WAIS likely lies between 1.5°C and 2°C of global average warming above pre-industrial levels. With warming already at around 1.1°C and the Paris Agreement aiming to limit warming to 1.5°C or “well-below 2°C”, the margins for avoiding this threshold are fine indeed.”  – Prof. Christina Hulbe, University of Otago

Our ice sheet modelling…suggests that this [West Antarctic] ice sheet lies close to a “tipping point” under projected warming.”   Turney, et al 2020

Using genetic analyses of a type of circum-Antarctic octopus, Pareledone turqueti, Lau et al. showed that the WAIS collapsed completely during the last interglacial period, when global sea levels were 5 to 10 meters higher than today and global average temperatures were only about 1°C warmer.”   Science Editorial, 2023

In 1968, glaciologist John Mercer voiced concerns that the West Antarctic Ice Shelf (WAIS) could abruptly collapse, leading to a rapid rise in sea levels because the WAIS contains enough ice to add 3.3m to global sea levels. In spite of the geological evidence that past climate change has led to abrupt sea level risen (as much as a 4cm/year) the notion that a few degrees of warming could have any impact on the coldest place on Earth was largely dismissed.

Then in 1995, the Larsen A Ice Shelf on the Antarctic Peninsular (the northern tip of the WAIS)—one of the fastest warming areas on the planet—broke apart. In 2002, its neighbour, the Larsen B Ice Shelf disintegrated in spectacular fashion in six weeks, not hundreds of years as previously assumed it might happen:

“It [the ice shelf] was sitting there stable for 10,000 years and then it was just…gone.” – Dr. Jeremy Bassis (Video 5).

Video 5: Larsen B & C ice shelves break off.

“We see things today that five years ago would have seemed completely impossible, extravagant, exaggerated.”  – Eric Rignot, JPL/NASA in The big thaw, National Geographic, June 2008.

In spite of this and the growing evidence that similar dramatic ‘non-linear’ abrupt changes to glaciers were being seen in Greenland, the 2009 IPCC 4th Assessment Report stated that sea levels were not likely to be greatly affected by melting glaciers, either from Antarctica or anywhere else one Earth, in the twenty-first century:

“Current global model studies project that the Antarctic Ice Sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall.” – IPCC 4th Assessment Report

Video 6: Pine Island Glacier, the fastest moving glacier in Antarctica, is being undercut by warm ocean currents. This is causing it’s grounding line to retreat for the same reasons as the much larger Thwaites Glacier (Fig. 3).

Meanwhile, Pine Island Glacier, the fastest melting glacier in Antarctica that drains about 10% of the WAIS, was thinning and accelerating (Video 6). Then in 2017, a section of Larsen C broke off as a single iceberg 5,800 km2—an area the size of the Waimakariri Distict, Christchurch, and Banks Peninsula combined (Video 5).

The scientific scramble to understand these events, which failed to be predicted by the climate models, had in fact been underway since 1986, with similar abrupt collapses being seen in Greenland.

The ‘tipping point’ processes in Greenland are also happening in Antarctica although here warmer oceanic waters play a larger role (Video 7), particularly to the eastern section of the massive Thwaites shelf, projected to collapse before 2030 (Videos 8 and press conference Video 9).

Video 7: Prof. Eric Rignot explains the ‘top down’ and ‘bottom up’ processes melting glaciers and ice sheets.

Video 8: Dr Ella Glibz explains the most recent research (June 2024) indicating sea levels will rise much faster than currently modelled, as Thwaites Glacier becomes increasingly destabilised.
Video 9.
Note: the processes described below and in Video 4 are the same as those described on the Greenland page.
 
Bottom up melting from warmer deep ocean waters: Pushed by westerly winds, which are strengthening with climate change, the warm deep (400-700m) saltier layers of the Antarctic Circumpolar Current are pushing closer to the shoreline. This warm water eats away at the underside of ice shelves (which can be well over 1km deep), thinning them from below. Continued undercutting allows more water to travel further under the ice shelf, eroding it and thinning it until it’s detached from the ‘grounding line’ and the ice begins to float.

The Jakobshavn effect now comes into play. As the thinning glacier becomes more buoyant, instead of being part of a solid ice mass, it floats at the calving front. And this means it’s forced to move up and down with the tides. These forces travel up the length of the glacier, ultimately assisting the leading edge to break at the weakest point. Additionally, because the glacier is thinner at the front the slope is steeper so the glacier speeds up due to gravity, allowing huge volumes of ice to surge downstream and into the sea (Fig. 3).

A small imbalance of forces caused by some perturbation can cause a substantial non-linear response.”Prof. Terry Hughes, ‘The Jakobshavn Effect’

Or as Prof. Jason Box puts it:

There are too many variables that determine exactly when a glacier calves. A single cracking event could conceivably be triggered by a seagull, acting like the straw that broke the camel’s back.”

Fig. 3: ‘Bottom up’ melting: the ice is normally stabilized by sitting on the seafloor. As warm ocean currents eat away at the base, the ice thins, and lifts away from the seafloor, and breaks, losing its ability to act as a brake on the flow of ice from the continent. Click the image for the interactive webpage.

The Zwally effect: top down melting from warm air melts ice into giant meltwater lakes on the surface of ice shelves (see ‘Gatekeepers of Antarctica‘ below). Thanks to their much lower albedo, like the ocean, the dark pools absorb more heat than the surrounding ice, causing more warming and hence further melting in a feedback effect. When this happens on ice shelves, the water finds crevasses in the ice, whereupon it drains down moulins that it scours out like a drill into the heart of glacier. Until the late 1990s it was assumed this water would re-freeze. Instead, through hydrofracturing, the weight and warm temperature of the water widens the moulin as it drops, fracturing the ice at depth.

When this happens on marine glaciers or ice shelves (ie, sitting on the ocean), when the water reaches the base of the ice, the ice shelf is effectively turned into Swiss cheese and rapidly breaks up. A good example is the Larsen B Ice Shelf (Video 4).

Whereas when this happens to glaciers sitting on land the outcome is different. If the glacier is on land that slopes downhill inland , when the meltwater reaches the bottom of the glacier, it lifts the glacier and/or joins with ocean water that has reached this point, adding to the melting and undercutting from below.

The Zwally effect also happens to glaciers sitting on land, but the outcome is different. If the glacier is on land that slopes downhill inland (Fig. 3) when the water reaches the bottom of the glacier it lifts the glacier and/or meets the ocean water that has reached this point. Together, this water adds to the melting and undercutting from below.

Where the glacier is on land that slopes down towards the ocean, the water lubricates the glacier like a water slide, making it flow faster, which in turn opens or widens more crevasses, allowing yet more meltwater lakes to drain and so on in a feedback effect. Upon reaching the ocean, the warm buoyant freshwater scours the floating base of the glacier, shooting hundreds of metres up the submerged terminus (front). In some instances it appears to ‘boil’ at the surface, erupting in a churning jaccuzi-like swirl of mud and ice. This has been filmed in Greenland glaciers.

Click on the image to be taken to the full story
Click on the image to be taken to the full story

East Antarctic Ice Sheet (EAIS)

‘Recently, relatively warm waters that are normally found offshore are coming onto the Antarctic continental shelves, threatening the stability of East Antarctic ice shelves.’Ribeiro et al, 2023
 
‘Totten Glacier hosts the most rapidly thinning ice in East Antarctica. This record of thinning is due to rapid melt along the grounding line of the Totten Ice Shelf (TIS), where warm ocean water from the open ocean flows into ice shelf cavities…’ Nakayama et al, 2023

Until recently, the East Antarctic Ice Sheet (EAIS), which contains enough ice to raise global sea levels ~54m if it all melted, was considered relatively stable. This was in part due to the bulk of the icecap sitting on land rather than the seabed (Fig. 2). However, measurements using satellite records from 1979 to 2017 show that the EIAS had in fact contributed about 30% to rising sea levels during this period, in part because as the climate warmed, stronger polar westerly winds were pushing more of the warmer circumpolar deep water current toward outlet glaciers, undercutting them (Fig. 3 and Video 3). These outlet glaciers with ice shelves behave in the same way as the WAIS outlet glaciers, however they hold back the far larger EAIS. In 2022, this led to the abrupt collapse of the Conger Ice Shelf.

In 2017, other researchers found more than 65,000 meltwater lakes on the EAIS. While most lakes were found on outlet glaciers, thousands were seen up to 50km inland on the ice sheet, and as high as 1500m altitude (Figs. 4 & 5). For inland and high altitude lakes to form, surface temperatures need to be well above freezing, and for sustained periods. For reasons explained in Video 6, these may not play as large a role as the warm deep waters along the WAIS, but they are also being seen on the southern coasts of the EIS, which is also vulnerable to warm deep waters (Fig. 5a).

Fig. 4: ‘Top down’ melting on the EAIS. Click on the image to see the full report (Image: Stokes et al.)

‘About 400,000 years ago…the global temperature was 1 to 2 degrees Celsius greater. Data indicate[s] that the ice sheet margin at the Wilkes Basin (EAIS) retreated to about 700 kilometres inland from the current position, which—assuming current ice volumes—would have contributed about 3 to 4 metres to global sea levels.”  – Blackburn et al, July 2020

Fig. 5: “Location and density of supraglacial lakes (SGLs) in East Antarctica, alongside examples. (a) Location of 65,459 mapped lakes that appeared on imagery from January 2017, each marked by a red cross. (b) Lake density map showing the cumulative area of SGLs within 1 km2 cells using a 50 km search radius. (c,d) Sentinel 2A satellite image (12th Jan 2017) of the high density of lakes on the Jutulstraumen Glacier, Dronning Maud Land. Note that lakes have developed above and beyond the grounding line (thick black line), but there is a clustering of lakes 5–10 km down-ice from the grounding line. (e,f) Sentinel 2A satellite image (27th Jan 2017) of clusters of lakes towards the ice sheet margin in Kemp Land.” (Stokes et al., click on the image to see the full report).

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