20 January 2022

Have tipping points already been passed for critical climate systems? (2) West Antarctica and the "doomsday" glacier

 by David Spratt

Second in a series.    Read 1  |  2  |  3  |  4  |  5  |  6  |  7

Thwaites Glacier fractures. Image: NASA
The Thwaites Glacier in West Antarctica has an eastern ice shelf 45 kilometres wide as it flows into the Amundsen Sea. On 13 December 2021, scientists announced that the ice shelf is likely to break apart in the next five years or so, resulting in a speeding up of the glacier’s flow and ice discharge, possibly heralding the collapse of the glacier itself, and triggering similar increases across the Amundsen Sea glaciers.

The researchers explain: “Over the last several years, satellite radar imagery shows many new fractures opening up… which like a growing crack in the windshield of a car [can] suddenly break apart into hundreds of panes of glass. We have mapped [the] pathway the fractures might take through the ice [and conclude] the final collapse of Thwaites Glacier’s last remaining ice shelf may be initiated … within as little as five years” (emphasis added).

"There is going to be dramatic change in the front of the glacier, probably in less than a decade… both published and unpublished studies point in that direction," said glaciologist Prof Ted Scambos. In plain language, Thwaites has passed a tipping point, and no further warming is required for abrupt change to it and surrounding glaciers. 

The fracturing of the ice shelf means more warm water will penetrate under the ice sheet, helping to free the underbelly of the glacier from the grounding line rock underneath, and allowing water to flow into the deep basin under the glacier, causing the glacier’s collapse. This would raise sea levels by 65 centimetres, though the timing of such collapse — the “doomsday” scenario — is highly uncertain. Since neighbouring glaciers flow into the same basin, the demise of Thwaites could eventually lead to the loss of all of the West Antarctic Ice Sheet (WAIS), resulting in more than three metres of sea level rise, and putting at risk the lives and livelihoods of 250 million people.

Richard Alley, a world-leading Penn State University glaciologist, says that the most likely place to generate the worst-case scenario is Thwaites, warning that “even six feet of sea level rise by 2100 is not the worst-case scenario… We just don’t know what the upper boundary is for how fast this can happen. We are dealing with an event that no human has ever witnessed before. We have no analog for this.”

This “doomsday” glacier story should not have been a shock. A 2020 study showed that "the Paris Agreement target temperature of 1.5°C is sufficient to drive runaway retreat of the WAIS”. And another study from 2021 found that the melting of the Antarctic Ice Sheet (AIS) may have already passed a critical tipping point of no return leading to irreversible loss of parts of the ice sheet below sea level.

A key 2014 study that received a lot of attention at the time concluded that a rapidly melting section of WAIS appeared to be in an irreversible state of decline, with nothing to stop the glaciers in this area collapsing. The study presented multiple lines of evidence, incorporating 40 years of observations that indicate the glaciers in the Amundsen Sea sector “have passed the point of no return”, according to lead author Eric Rignot. 

Writing in The Guardian, Rignot emphasised: “We had collected enough observations to conclude that the retreat of ice in the Amundsen sea sector of West Antarctica was unstoppable, with major consequences… its disappearance will likely trigger the collapse of the rest of the West Antarctic ice sheet, which comes with a sea level rise of between three and five metres.”  

This study stunned the research community. Malte Meinshausen, an IPCC lead author, said Rignot’s study was “a game changer, this is just one surprise with global warming of only 0.8 degrees of warming", and a “tipping point that none of us thought would pass so quickly”, showing we are ”committed already to a change in coastlines that is unprecedented for us humans”. So the news from December 2021 should not have been a shock — the same conclusion had been reached seven years earlier!

Tipping points and potential cascade effects

Rignot said that "at the current rate, a large fraction of the (West Antarctic) basin will be gone in 200 years" and a Pollard, DeConto and Alley model from 2015 “accelerates the expected collapse of the West Antarctic Ice Sheet to decadal time scales, and also causes retreat into major East Antarctic subglacial basins", producing five metres in the first 200 years.

New research from Milillo, Rignot et al. shows that the the Pope, Smith and Kohler glaciers in the Amundsen Sea embayment of West Antarctica have experienced enhanced ocean-induced ice-shelf melt, glacier acceleration, ice thinning and grounding-line retreat coincident with high melt rates of ungrounded ice in the past 30 years. The retreat rates are faster than anticipated by numerical models.

Backgrounder: Antarctica is “primed for runaway destruction”   

The West Antarctic Ice Sheet (WAIS), comprising more than two million cubic kilometres of ice, is under pressure from a warming climate, with scientists saying its break-up –– and an eventual global sea-level rise of 3–5 metres –– is not a matter of if, but when. WAIS is now the strongest-warming region on the planet, and glaciers including Pine Island, Thwaites, Smith and Kohler glaciers are discharging ice at an accelerating rate.

Scientific attention is focussed on the fate of ice shelves, floating sheets or platforms of ice that are largely submerged and up to two kilometres in height, that abut the land-based glaciers, and extend over the ocean.  The “grounding line” marks the boundary between grounded ice (glacier) and the floating ice shelf. Generally, an ice shelf will lose volume by calving icebergs from the seaward-facing edge, but it can also be subject to rapid disintegration events, in which cracking can dislodge very large sections of ice. The formation of huge cracks along ice shelves leading to the loss of very large areas of ice have become widespread. One example along the Larsen C ice shelf was 100 kms long, half a kilometre wide and a hundred metres deep.  

Warming Antarctic waters, driven by changed ocean and atmospheric circulation patterns, are melting and thinning the underside of ice shelves, making them more susceptible to disintegration. Ice shelves act as a “plug” that buttresses and slow the rate at which glaciers drain into the ocean, so the loss or diminution of the ice shelf will accelerate the pace of glacier movement and hence the rate of ice mass loss.

Because much of WAIS sits on bedrock that is below sea level (buttressed on two sides by mountains, and held in place on the other two sides by the Ronne and Ross ice shelves), melting of the submerged ice shelf allows warm ocean waters to proceed inland under the ice sheet.  This creates hidden valleys of melting ice, puts pressure on the surface above, and contributes to large-scale rifting (cracking). This process also results in the grounding line being pushed further inland, in effect transforming the lower reaches of the glacier into an ice shelf.

What is happening now with Thwaites should not be a surprise, because such an event was foreseen 50 years ago. In 1968, pioneer glacier researcher John Mercer predicted that the collapse of ice shelves along the Antarctic Peninsula could herald the loss of the ice sheet. Ten years later, Mercer contended that "a major disaster — a rapid deglaciation of West Antarctica — may be in progress … within about 50 years”. He said that warming “above a critical level would remove all ice shelves, and consequently all ice grounded below sea level, resulting in the deglaciation of most of West Antarctica”. Such disintegration, once under way, would “probably be rapid, perhaps catastrophically so”, with most of the ice sheet lost in a century. 

Credited with coining the phrase “the greenhouse effect” in the early 1960s, Mercer’s Antarctic prognosis was widely ignored and disparaged at the time. Retired NASA climate research chief James Hansen says it was not clear at the time whether Mercer or his many critics were correct, but those who labelled Mercer an alarmist were considered more authoritative and better able to get funding. Hansen believes funding constraints can inhibit scientific criticisms of the status quo: “I believe there is pressure on scientists to be conservative”. Hansen is responsible for coining the term “The John Mercer Effect”, meaning to play down your findings for fear of losing access to funding or of being considered alarmist.

Although the much larger East Antarctic Ice Sheet (EAIS) — with potential for a 50-metre sea-level rise if all ice were lost — has generally been considered more stable than WAIS, recent evidence suggests some outlet glaciers there are displaying similar dynamics to those on West Antarctica.

Next post in this series: (3) The Arctic: Abrupt change