Image: John Sontag NASA (Larsen C rift)
Dangerous tipping points: Antarctica
“Research over the past couple decades revealed the Antarctic plateau, the coldest and one of the most remote places on Earth, had been cooling while global temperatures were increasing…Our study has found that this is no longer the case. The south pole is now one of the fastest warming regions on the planet, warming at an incredible three times faster than the global average rate.” – Dr. Kyle Clem, Victoria University, Wellington.
- The continent of Antarctica is almost twice the size of Australia and contains 30 million cubic km., or 90% of the world’s freshwater that, if it all melted, would add ~60-70m to sea levels.
- Over the past 50 years, the west coast of the Antarctic Peninsula (Fig. 1) has been one of the most rapidly warming parts of the planet, with air temperatures 3°C; five times the average rate of global warming.
- Upper ocean temperatures to the west of the Antarctic Peninsula have increased over 1°C since 1955.
- Ice loss 1994 – 2017 = 9 trillion tonnes; many glaciers and ice shelves along the Peninsular have retreated and some have collapsed completely.
- The East Antarctic Ice Sheet (EAIS) was considered relatively stable (Fig. 1). However, recent research shows that this is not the case and that melting is contributing to sea level rise.
- The Western Antarctic Ice Sheet (WAIS) covers islands and land below sea level (Fig. 2), which makes it particularly vulnerable to collapse, and that would allow the inland ice sheet to flow faster into the ocean.
- The Thwaites Glacier on the WAIS is currently of greatest concern. It’s ~75% the size of New Zealand, and increasingly warm ocean water is melting it from below, undercutting it so that it’s collapsing. Nicknamed the ‘Doomsday Glacier’, it acts like a plug, holding back the ice sheets.
- See Wellington University’s Antarctic Research Centre for up to date research
“The Antarctic has registered a temperature of more than 20°C for the first time on record. – The Guardian, 9 February, 2020
- Ice sheet: continental glaciers that have joined together to cover the surrounding land in an area greater than 50,000 km². There are only three in the world: Greenland and Antarctica which has two: the WAIS and the EAIS (Fig. 1). The existence of these ice sheets are why we are still in an ice age.
- Marine ice sheet: an ice sheet whose base is on ground below sea level. This makes it particularly vulnerable to undercutting by warming waters (Fig. 3). The WAIS is a marine ice sheet (Figs. 1 & 2).
- Outlet glacier: drains inland glaciers/ice sheets through gaps in the surrounding topography. If an outlet glacier reaches the coast (some terminate inland), it can become an:
- Ice shelf: a tidewater (coastal) glacier or ice sheet that flows down to a coastline and onto the ocean surface, where it floats. Thwaites and Pine Island Glaciers are outlet glaciers with ice shelves.
- Grounding line: the point where the bottom or ‘basal’ side of a glacier leaves land and extends out over the ocean.
- If the grounding line is below sea level, the glacier is prone to undercutting by increasingly warmer ocean waters.
- If land behind the grounding line slopes down inland instead of up, warm water can flow further underneath, destabilising the glacier even faster (Fig. 3). Thwaites Glacier, now considered the most unstable (Video 3) is grounded below sea level, and much of the the land behind the grounding line slopes down.
A disaster waiting to happen: the 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
Glaciologist John Mercer voiced concerns in 1968 that the West Antarctic Ice Shelf (WAIS ) could abruptly collapse, leading to a disastrously rapid rise in sea levels because the WAIS contains enough ice to add 3.3m of water 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 make any substantial changes to 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 (Video 1).
“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.
“It [the ice shelf] was sitting there stable for 10,000 years and then it was just…gone.” – Dr. Jeremy Bassis (Video 1).
Video 1: The collapse of the Larsen Ice Shelves
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
Meanwhile, Pine Island Glacier, the fastest melting glacier in Antarctica that drains about 10% of the WAIS, was thinning and accelerating (Video 2). 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 1).
Video 2: 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).
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 3).
Note: the processes described below and in Video 3 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 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 become 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 (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.”
The Zwally effect :top down melting from warm air melts ice into giant meltwater lakes on the surface of ice shelves. 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. The water finds crevasses in the ice, whereupon it drains down moulins that it scours out, into the heart of glacier. Until the late 1990s it was assumed this water would re-freeze. Instead, through hydrofracturing, the weight of the water widens the moulin as it drops, until it reaches the ocean. The ice shelf is effectively turned into Swiss cheese and rapidly breaks up. A good example is the Larsen B Ice Shelf (Video 1).
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.
Video 3: Prof. Eric Rignot explains the ‘top down’ and ‘bottom up’ processes melting glaciers and ice sheets.
Video 4: Scientists go to great lengths to avoid hyperbole, however many now refer to Thwaites Glacier as the ‘Doomsday Glacier’.
“Our ice sheet modelling…suggests that this [West Antarctic] ice sheet lies close to a “tipping point” under projected warming.” – Turney, et al 2020
East Antarctic Ice Sheet (EAIS)
‘We find that lakes often cluster a few kilometres down-ice from grounding lines and ~60% (>80% by area) develop on ice shelves, including some potentially vulnerable to collapse driven by lake-induced hydro-fracturing. This suggests that parts of the [East Antarctic] ice sheet may be highly sensitive to climate warming.’ – Stokes et al, 2019
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 entire 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). 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 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 3, 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).
New temperature records for Antarctica were recorded in February 2020 (Fig.6).
‘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
Five times the average rate of global warming; due to Arctic or Polar Amplification:
These are terms used to describe why the poles are warming far faster than the rest of the planet. There are several reasons for this:
- The Albedo Effect: Clean ice and snow have a very high albedo, that is, they reflect up to 90% of solar radiation back into space. The ocean is much darker, so it has a very low albedo, reflecting only about 6% of the incoming solar radiation and absorbing the other 94%, warming it much faster than the snow and ice (Fig. 7). This feedback effect then leads to more warming, then more melting, and so on.
- Ozone-depleting substances (this is a new area of research: see here for how this is happening).
- Air pressure differences between the tropics and the poles may also be a factor: warmer (and therefore denser, higher pressure) air tends to travel from the tropics to the cooler (lower pressure less dense air) poles (see 5-min. Video 1 here). However, weather systems are stalling as the jet stream wobbles. While this allows cold arctic air to move further south for longer periods, it also allows warmer tropical air to invade polar latitudes. A very small rise in temperatures for long periods is leading to dramatic melting in Greenland and Arctic sea ice, as well as Antarctica.
- Climate system feedbacks have also changed ocean currents as well as the weather associated with them.
WAIS below sea level:
Firstly, not all of the WAIS is below sea level. And most importantly ice shelves that are anchored to bedrock act as dams or buttresses that hold back ice sheets that are entirely on the land. If they break up, this allows ice sheets to flow into the ocean.
Secondly, glaciers and ice caps that sit below sea level do not technically contribute to sea-level rise when they melt, because they are simply changing from a solid to a liquid while still in the ocean. However, while frozen, this large volume of water is concentrated in relatively small locations. When it melts, it’s distributed across all of the world’s oceans. How much this and other factors contribute to sea levels rising, is covered here.
In 1968, glaciologist John Mercer wrote:
“A disquieting thought is that if the present highly simplified climatic models are even approximately correct, this deglaciation (of the WAIS) may be part of the price that must be paid in order to buy enough time for industrial civilisation to make the changeover from fossil fuels to other sources of energy.” – ‘Antarctic Ice and Sangamon Sea Level’, International Association of Scientific Hydrology Symposium 79, 217–225
“Mercer’s views, first buried in a publication of the International Association of Scientific Hydrology, caught the attention of policy makers when published in 1978 in Nature under the title ‘West Antarctic ice sheet and CO2 greenhouse effect: A threat of disaster’ … By the late 1980s the notion that loss of ice shelves could lead to disintegration of the entire ice sheet fell out of favour.” – Oppenheimer, 2004.
The Jakobshavn Effect:
See also Thomas et al‘s paper: ‘Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbræ, Greenland‘
The Zwally Effect:
The idea was proposed by Jay Zwally when researching the sudden acceleration of the Jakobshavn Isbræ glacier in Greenland in 1998 and 1999.
References and further reading
- National Snow and Ice Data Centre (NSIDC)
- The International Thwaites Glacier Collaboration
- 2021: Eayrs et al: Rapid decline in Antarctic sea ice in recent years hints at future change, Nature Geoscience
- Eayrs; Guest post Deciphering the rise and fall of Antarctic sea ice extent (plain English open access article on Carbon Brief).
- 2021: Gilbert; The fate of Antarctic ice shelves at 1.5C, 2C and 4C of warming
- 2021: Wåhlin et al; Pathways and modification of warm water flowing beneath Thwaites Ice Shelf, West Antarctica, Science Advances 7 | 15, eabd7254
- 2021: Slater et al; Review article: Earth’s ice imbalance, The Cryosphere, 15, pp233–246
- 2020: Jordan et al; New gravity-derived bathymetry for the Thwaites, Crosson, and Dotson ice shelves revealing two ice shelf populations, The Cryosphere 14 pp2869–2882
- 2020: Hogan et al; Revealing the former bed of Thwaites Glacier using sea-floor bathymetry: implications for warm-water routing and bed controls on ice flow and buttressing, The Cryosphere 14 pp2883–2908
- 2020: Lai et al; Vulnerability of Antarctica’s ice shelves to meltwater-driven fracture Nature 584 pp574–578
- Nature article (open access) about the above research: Crevasse analysis reveals vulnerability of ice shelves to global warming
- 2020: Blackburn et al: Ice retreat in Wilkes Basin of East Antarctica during a warm interglacial Nature 583 pp554–559
- 2020: Clem et al; Record warming at the South Pole during the past three decades, Nature Climate Change
- Carbon Brief discussion on this University of Wellington research
- 2020: Thomas et al; Tipping elements and amplified polar warming during the Last Interglacial, Quaternary Science Reviews 233 / 106222
- 2020: Ortega; Unusual Arctic warming explained by overlooked greenhouse gases, Science
- 2020: Smith; The unexpected link between the ozone hole and arctic amplification, The Conversation
- 2020: Turney et al; Early Last Interglacial ocean warming drove substantial ice mass loss from Antarctica, 117 (8) 3996-4006
- 2020: Hulbe; Guest post How close is the West Antarctic ice sheet to a ‘tipping point’? Carbon Brief
- 2020: England et al; Tropical climate responses to projected Arctic and Antarctic sea-ice loss Nature Geoscience 13, 275–281
- 2020: Velicogna et al; Continuity of ice sheet mass loss in Greenland and Antarctica from the GRACE and GRACE Follow-Onmissions, Geophysical Research Letters, 47/8 GL087291
- 2020: The Guardian Antarctic temperature rises above 20C for first time on record
- 2020: Robel et al; A Speed Limit on Ice Shelf Collapse through Hydrofracture, Geophysical Research Letters, 46/21
- 2019 NIWA: New Zealand Fluvial and Pluvial Flood Exposure; prepared for Deep South Challenge
- 2019 NIWA: Coastal Flooding Exposure Under Future Sea-level Rise for New Zealand; prepared for Deep South Challenge
- 2019: Stokes et al; Widespread distribution of supraglacial lakes around the margin of the East Antarctic Ice Sheet, Nature Scientific Reports 9 13823
- 2019: Rignot et al; Four decades of Antarctic Ice Sheet mass balance from 1979–2017,
- 2019 NSIDC: ice shelves
- 2019: Pitcher & Smith; Supraglacial rivers and streams, Annual Review of Earth and Planetary Sciences 47, 421-452
- 2019: Cheng et al; How fast are the oceans warming? Science 363/6423 pp128-129
- 2019 IPCC: Special Report on the Ocean and Cryosphere in a Changing Climate
- 2018 Nature editorial; The scientist who predicted ice-sheet collapse — 50 years ago
- 2018 IPCC: Summary for Policymakers of IPCC Special Report on Global Warming of 1.5°C approved by governments
- 2017: Scambos et al; How much, how fast?: A science review and outlook for research on the instability of Antarctica’s Thwaites Glacier in the 21st century, Global and Planetary Change 153, pp 16-34
- 2011: Schmidt et al: Abrupt Climate Change During the Last Ice Age, Nature Education Knowledge 3(10):11
- 2014: Durand et al; Retreat of Pine Island Glacier controlled by marine ice-sheet instability, Nature Climate Change 4, 117–121
- 2007: IPCC 4th Assessment Report: The Physical Science Basis
- 2004: Openheimer; Global warming and the stability of the West Antarctic Ice Sheet, Nature 393,/6683 325-332.
- 2004: Thomas et al; Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbrae, Greenland, Journal of Glaciology. 50/168: 57–66
- 2002: Zwally et al; Surface Melt-Induced Acceleration of Greenland Ice-Sheet Flow, Science 297/5579 218–222.
- 1986: Hughes; The Jakobshavn Effect. Geophys. Res. Lett.13/1: 46–48.
- 1978: Mercer; West Antarctic Ice Sheet and CO2 Greenhouse Effect: A Threat of Disaster, Nature 271, 321–325.
- 1968: Mercer; ‘Antarctic Ice and Sangamon Sea Level’, Int. Assoc. Sci. Hydrol. Symp. 79, 217–225