Two collapsing West Antarctic glaciers could add another half a metre to sea levels this century?
24 January 2010
New research suggest that just two collapsing West Antarctic glaciers could add another half a metre to sea levels this century
The Victorian and Queensland governments decisions to stick to an "upper boundary" sea-level rise estimate of 0.8 metres by 2100 (and NSW at 0.9 metre) for planning purposes needs urgent revision, with new modelling showing two West Antarctic glaciers are past their tipping points.
The 0.8 metre estimate for sea-level rises to 2100 is already obsolete:
- The Copenhagen climate science congress of March 2009 estimated a sea-level rise of 0.75–1.9 metres by 2100
- The federal Department of Climate Change’s November 2009 climate update reports estimates of a 0.5–2 metre rise by 2100
- A study published in the Proceedings of the National Academy of Sciences in December found that global average sea levels are likely to rise by between 75cm and190cm by the end of the century.
So how far could we reasonably expect sea levels to rise by 2100? As the world’s oceans warm, they expand and sea-levels rise, but how quickly the loss of polar ice sheets will add to the rise is difficult to estimate, principally because ice-sheet and sea-ice dynamics are not sufficiently well understood, and they are subject to non-linear (rapid and unexpected) changes, such as is occurring with sea-ice in the Arctic. The question is not so longer whether the Greenland and West Antarctic ice sheets are losing mass (they are!) but if and when they pass tipping points for large, irreversible ice mass loss, and how fast that will occur.
Recent research by Blancon et. al published in Nature in 2009 examining the paleoclimate record shows sea level rises of 3 metres in 50 years due to the rapid melting of ice sheets 120,000 years ago. Mike Kearney, of the University of Maryland, said it’s "within the realm of possibility" that global warming will trigger a sudden collapse of the West Antarctic ice sheet, which could lead to a rapid increase in sea levels like that predicted by the study.
Given the catastrophic failure to date of global climate policy-making (Copenhagen outcome =4-degree rise by 2100), big sea level rises are on the way for the sort of temperature increases now on the table. NASA climate science chief James Hansen wrote in New Scientist that:
Oxygen isotopes in the deep-ocean fossil plankton known as foraminifera reveal that the Earth was last 2°C to 3°C warmer around 3 million years ago, with carbon dioxide levels of perhaps 350 to 450 parts per million. It was a dramatically different planet then, with no Arctic sea-ice in the warm seasons and sea level about 25 meters higher, give or take 10 meters.
Even more compelling, Professor Eelco Rohling of University of Southampton says:
Even if we would curb all CO2 emissions today, and stabilise at the modern level (387 parts per million by volume), then our natural relationship suggests that sea level would continue to rise to about 25 metres above the present.
Then on 13 January this year, New Scientist published this story showing calculations that the Pine Island glacier in the West Antarctic has likely passed its tipping point, with researchers estimating that this one glacier alone could add a quarter of a metre to sea levels by 2100:
Major Antarctic glacier is ‘past its tipping point’
A major Antarctic glacier has passed its tipping point, according to a new modelling study. After losing increasing amounts of ice over the past decades, it is poised to collapse in a catastrophe that could raise global sea levels by 24 centimetres.
Pine Island glacier (PIG) is one of many at the fringes of the West Antarctic ice sheet. In 2004, satellite observations showed that it had started to thin, and that ice was flowing into the Amundsen Sea 25 per cent faster than it had 30 years before.
Now, the first study to model changes in an ice sheet in three dimensions shows that PIG has probably passed a critical "tipping point" and is irreversibly on track to lose 50 per cent of its ice in as little as 100 years, significantly raising global sea levels.
The team that carried out the study admits their model can represent only a simplified version of the physics that govern changes in glaciers, but say that if anything, the model is optimistic and PIG will disappear faster than it projects.
Richard Katz of the University of Oxford and colleagues developed the model to explore whether the retreat of the "grounding line" – the undersea junction at which a floating ice shelf becomes an ice sheet grounded on the sea bed – could cause ice sheets to collapse.
The model suggests that within 100 years, PIG’s grounding line could have retreated over 200 kilometres. "Before the retreating grounding line comes to a rest at some unknown point on the inner slope, PIG will have lost 50 per cent of its ice, contributing 24 centimetres to global sea levels," says Richard Hindmarsh of the British Antarctic Survey, who did not participate in the study.
This assumes that the grounding line does eventually stabilise, after much of PIG is gone. In reality, PIG could disappear entirely, says Hindmarsh. "If Thwaite’s glacier, which sits alongside PIG, also retreats, PIG’s grounding line could retreat even further back to a second crest, causing sea levels to rise by 52 centimetres." The model suggests Thwaite’s glacier has also passed its tipping point.
…. and now comes a new report in Science that an undersea ridge that may have once helped slow the loss of the Pine Island glacier is no longer doing so…
Antarctic Glacier Off Its Leash
An unmanned autonomous submarine has discovered a sea-floor ridge that may have been the last hope for stopping the now-accelerating retreat of the Pine Island Glacier, a crumbling keystone of the West Antarctic Ice Sheet, researchers announced at the fall meeting of the American Geophysical Union.
An unmanned autonomous submarine has discovered a sea-floor ridge that may have been the last hope for stopping the now-accelerating retreat of the Pine Island Glacier, a crumbling keystone of the West Antarctic Ice Sheet. The ridge appears to have once protected the glacier, but no more. The submarine found the glacier floating well off the ridge and warmer, ice-melting water passing over the ridge and farther under the ice. And no survey, underwater or airborne, has found another such glacier-preserving obstacle for the next 250 kilometers landward.
The Pine Island and adjacent Thwaites glaciers are key to the fate of West Antarctic ice, says glaciologist Richard Alley of Pennsylvania State University, University Park, in an e-mail. And West Antarctica is key to how fast and far sea level will rise in a warming world. "To a policymaker, I suspect that the continuing list of [such] ice-sheet surprises is not reassuring," he writes.
At the meeting, glaciologist Adrian Jenkins of the British Antarctic Survey in Cambridge and colleagues described how the instrument-laden Autosub3 cruised for 94 hours along 510 kilometers of track beneath the floating portion of the Pine Island Glacier in January 2009. The sub found a 300-meter-high ridge across the ocean cavity formed by the floating end of the glacier. Deep, warmer water was overtopping the ridge and passing through the gap between floating ice and the ridge top on its way to melting back more of the glacier. That gap has been growing, Jenkins said, perhaps since the 1970s. An aerial photograph from 1973 shows a bump in the ice where the ridge is now known to be, suggesting that the ice was then resting on the ridge and no warmer water could have been getting through.
Although the last physical obstacle to continued melting and retreat of the Pine Island Glacier has been breached, the ice’s fate remains murky, says glaciologist David Holland of New York University in New York City. That’s because glaciologists aren’t sure what got the glacial retreat started in the first place, he notes. It wasn’t the greenhouse simply warming the ocean, researchers agree. Instead, shifting winds around Antarctica in recent decades may have driven warmer waters up to the ice and dislodged it from its perch on the ridge. But what caused the winds to shift? Global warming? The ozone hole? Random variability? Glaciologists—and policymakers—would like to know.
… which makes Fred Pearce’s prediction (which we quoted in Climate Code Red two years ago, page 47) look spot on….
Another vulnerable place on the West Antarctic ice sheet is Pine Island Bay, where two large glaciers, Pine Island and Thwaites, drain about 40 per cent of the ice sheet into the sea. The glaciers are responding to rapid melting of their ice shelves and their rate of ﬂ ow has doubled, whilst the rate of mass loss of ice from their catchment has now tripled. NASA glaciologist Eric Rignot has studied the Pine Island glacier, and his work has led climate writer Fred Pearce to conclude that ‘the glacier is primed for runaway destruction’. Pearce also notes the work of Terry Hughes of the University of Maine, who says that the collapse of the Pine Island and Thwaites glaciers — already the biggest causes of global sea-level rises — could destabilise the whole of the West Antarctic ice sheet. Pearce is also swayed by geologist Richard Alley, who says there is ‘a possibility that the West Antarctic ice sheet could collapse and raise sea levels by 6 yards [5.5 metres]’, this century.
So much for 0.8 metres being a risk-averse foundation for sea-level rise planning and policy-making.
24 January 2010