The evidence of climate change

The evidence of climate change

Fig. 1: Antarctica showing the Larsen and Wilkins Ice Shelves, Pine Island Glacier, and Thwaites Glacier — an ice shelf roughly the size of UK (176 x103 km2). The front of the glacier—its terminus—is nearly 120 km wide and >1000m below sea level.

Image: John Sontag NASA (Larsen C rift)

Dangerous tipping points: Antarctica

Summary

“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.
  • 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 updated research

The Antarctic has registered a temperature of more than 20°C for the first time on record. The Guardian, 9 February, 2020

Glacier glossary:

  • 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.
Fig. 2: Antarctica without ice. The West Antarctic Ice Sheet (WAIS) sits largely on islands and underwater trenches. This makes it particularly vulnerable to undercutting and collapse. Click the image for the details about the Thwaites Glacier research now underway.

A disaster waiting to happen: the WAIS

Glaciologist John Mercer voiced concerns in 1968 that the West Antarctic Ice Shelf 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 the ocean. 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 planetbroke 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).

In fact the scientific scramble to understand events that failed to be predicted by the climate models had 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

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 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, February, 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).

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).
Fig. 6: Temperatures over Antarctica, 07 February, 2020. The vertical bar on the right shows the anomaly, Temperatures exceeded 20C above normal in some places.

Explainers

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.
Fig. 7 : The Albedo Effect. Clean ice reflects about 90% of the sunlight that strikes it. Dark ocean water only reflects about 6%. This aerial photo shows a small portion of A-68, the iceberg that broke of Larsen C Ice Shelf in 2017 (Photo: NASA/ Nathan Kurtz)
Fig. 7 : The Albedo Effect. Clean ice reflects about 90% of the sunlight that strikes it. Dark ocean water only reflects about 6%. This aerial photo shows a small portion of A-68, the iceberg that broke of Larsen C Ice Shelf in 2017 (Photo: NASA/ Nathan Kurtz)
  • 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