Antarctica’s ice sheet passes a tipping point

Nothing we can do will stop the West Antarctic ice sheet from collapsing over the next few centuries, causing big sea level rises. [10 June 2014 | Peter Boyer]

AntarcticaUnderIceThe great white land to our south, which holds 90 per cent of the world’s ice, is suddenly back in the global science spotlight. The news isn’t good.

The West Antarctic ice sheet lies atop a rocky archipelago south of South America. New evidence shows that it’s now in an unstable phase and will break up at an accelerating rate, sharply increasing the rate of sea level rise. And we can do nothing to stop it.

But scientists also believe that a much larger body of ice, on the part of Antarctica that lies due south of Australia, is vulnerable to the same destabilisation caused by warming ocean waters.

There’s still a debate as to how quickly Antarctic ice will disappear. Ice sheets mostly change gradually over many hundreds or thousands of years, but in past warming climates there were periods of rapid collapse, with sea level rising not centimetres per century as it is now, but metres.

Potential large-scale ice sheet collapse resulting from undermining by warming oceans is the theme in no less than five research papers which appeared in May, either published or accepted for publication in leading scientific journals.

The West Antarctic story is compelling. A study by a UK-based team of scientists used satellite altimeter data showing changes in the altitude of Antarctic ice to establish that ice loss from West Antarctica from 2011 to 2013 was 31 per cent higher than over the preceding six years.

A separate NASA study of 19 years of satellite data, soon to be published in the journal Geophysical Research Letters, addresses long-standing fears that the West Antarctic ice sheet is inherently unstable because most of its base is below sea level.

The study focused on a few fast-flowing glaciers in the Amundsen Sea region, south of the eastern Pacific Ocean. It found that all of them were diminishing in length and height and that their grounding line was retreating towards the interior of the continent.

Antarctica as it would be without its ice, showing the vulnerability of West Antarctica and part of East Antarctica to warming seawater.

Antarctica as it would be without its ice, showing the vulnerability of West Antarctica and part of East Antarctica to warming seawater.

All coastal glaciers have a grounding line, the point at which their ice no longer moves over rock but begins to float on the sea, where they are weakened by the motion and slightly higher temperature of sea water.

The NASA team, led by California-based glaciologist Eric Rignot, has pinpointed the grounding lines of the West Antarctic glaciers deep under the icecap, using European Space Agency satellite data to discern where they begin moving up and down with the tides.

Between 1992 and 2011 all the glaciers’ grounding lines retreated, some of them by over 30 km, Rignot’s team found, signalling accelerating melting as seawater penetrates deeper beneath the ice.

Rignot was unequivocal: “We’ve passed the point of no return,” he said. “At current melt rates, these glaciers will be history within a few hundred years. The collapse of this sector of West Antarctica appears to be unstoppable.”

Rignot’s assessment was supported by a paper published in the US journal Science. Using under-ice topography data and computer modelling, the paper concluded that the disappearance of Thwaites Glacier within a few centuries will cause wider collapse of the West Antarctic ice sheet.

East Antarctica, which holds the vast bulk of Antarctic land ice, appears at first sight to be an entirely different story. The UK altimetry study, soon to be published by the American Geophysical Union, showed a thickening of some glaciers, and little change in elevation across the whole region.

But one part of East Antarctica attracted special attention. The study found that Totten Glacier, a large, fast-flowing ice stream that reaches the coast 200 km east of Australia’s Casey station, is thinning significantly all the way back to its grounding line.

Two years ago I reported on a multi-year international study of the Antarctic ice sheet, involving Australian glaciologists based in Hobart, at the Antarctic Climate and Ecosystems CRC and the Australian Antarctic Division.

The study is looking at Wilkes and Aurora basins, two large under-ice rock depressions, below sea level, that extend from the coast into the heart of the Antarctic continent. Its aim is to ascertain whether the ice sheet in this part of Antarctica is nearing its own tipping point.

Totten Glacier, 20 km wide at the coast, carries a huge amount of ice. While the loss of the Amundsen Sea glaciers would ultimately contribute about four metres to global sea levels, the ice drained by the Totten Glacier has the potential to raise sea level by as much as seven metres.

The topography of the bedrock beneath Totten Glacier is similar to that of the Amundsen Sea glaciers. Questions arise: could the Totten’s grounding line be retreating into the Aurora basin beyond, and could this signal irreversible loss of ice for the region drained by the glacier?

Tas Van Ommen, who leads the Australian Antarctic Division’s glaciology program, says that more sophisticated modelling of ice and ocean interaction (such as this study of the Wilkes basin published early in May in the journal Nature) will clarify what’s happening to the Totten and other East Antarctic glaciers, but we can’t rule out large, rapid changes.

There’s a similar issue at play in Greenland, the world’s other major ice sheet. A US study has found that deep fiords in the rocky coast under the ice allows seawater to penetrate long distances inland, opening a door to rapid retreat of grounding lines of glaciers and their eventual collapse.

Antarctica currently contributes 0.45 mm a year to sea level rise, but for Van Ommen that’s not the biggest concern. The IPCC, he says, projects several tenths of a metre more of sea level rise if ice sheets were to become unstable, and the evidence says this is now happening.

Bear in mind, as John Hunter (another Tasmanian climate scientist) has calculated, that every 10 cm of sea level rise creates three times the risk of coastal flooding. A modest 50 cm rise equates to a flooding frequency 300 times greater, or “100-year floods” happening several times a year.

POSTSCRIPT: On 4 June the National Academy of Sciences published a paper by a University of Texas (Austin) geophysics team led by Dustin Schroeder describing evidence for warming by volcanic processes under parts of the West Antarctic ice sheet affecting a western tributary and part of the central tributary of Thwaites Glacier, with possibly significant implications for the glacier’s future. The source of the geothermal heating is near Mt. Sidley, Antarctica’s highest volcano.

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