Q: How much can West Antarctica plausibly contribute to sea-level rise by 2100?
A. 3 to 5 feet — contributing to an increasingly likely total sea-level rise of more than 5 feet by 2100, a rise that will be all but impossible to stop if we don’t sharply reverse CO2 emissions trends within a decade or so.
West Antarctica’s collapsing ice shelves are in the news today. This post will survey what we now know about this unstable ice sheet and the threat it poses to humanity — or is that the threat humanity poses to it? — if we continue on our current suicidal emissions path.
Antarctica is disintegrating much faster than almost anybody imagined. In 2001, the IPCC “consensus” said neither Greenland nor Antarctica would lose significant mass by 2100. They both already are. As Penn State climatologist Richard Alley said in March 2006, the ice sheets appear to be shrinking “100 years ahead of schedule.”
A 2007 study found “The current loss of mass from the Amundsen Sea embayment of the West Antarctic ice sheet [WAIS] is equivalent to that from the entire Greenland ice sheet” (see the new survey report Antarctic Climate Change and the Environment draft here). And WAIS’s 2007’s ice loss was 75% higher than 2006’s (see “The Antarctic ice sheet hits the fan“).
On Saturday, Reuters reported on a major new study on Antarctic ice shelves, “Coastal-Change and Glaciological Map of the Larsen Ice Shelf Area, Antarctica: 1940–2005“:
One Antarctic ice shelf has quickly vanished, another is disappearing and glaciers are melting faster than anyone thought due to climate change, U.S. and British government researchers reported on Friday.
They said the Wordie Ice Shelf, which had been disintegrating since the 1960s, is gone and the northern part of the Larsen Ice Shelf no longer exists. More than 3,200 square miles (8,300 square km) have broken off from the Larsen shelf since 1986.
Climate change is to blame, according to the report from the U.S. Geological Survey and the British Antarctic Survey, available at pubs.usgs.gov/imap/2600/B.
“The rapid retreat of glaciers there demonstrates once again the profound effects our planet is already experiencing — more rapidly than previously known — as a consequence of climate change,” U.S. Interior Secretary Ken Salazar said in a statement.
“This continued and often significant glacier retreat is a wakeup call that change is happening … and we need to be prepared,” USGS glaciologist Jane Ferrigno, who led the Antarctica study, said in a statement.
“Antarctica is of special interest because it holds an estimated 91 percent of the Earth’s glacier volume, and change anywhere in the ice sheet poses significant hazards to society,” she said.
In a remarkable example of the accelerating nature of human-caused climate change, the UK Telegraph reports today:
Satellite images have revealed that a 25 mile long strip of ice, which is believed to have pinned the Wilkins Ice Shelf in place since the beginning of recorded history, had broken at its narrowest point.
Without it, ice will be able to flow more freely between Charcot and Latady islands on the western side of the Antarctic, eventually moving into the open seas.
The Wilkins, the size of Jamaica or half the size of Scotland, is the largest of 10 shelves to have shrunk or collapsed in recent years on the Antarctic Peninsula amid rising temperatures in the region.
You can see a video of the demise here.
The ice shelf has lost a total of 694 square miles – or some 14 per cent of its size – over the past year, which shrank the ice bridge to under 546 yards as its narrowest point….
Ice shelves, some of them hundreds of miles thick [wide], float on the water and contract as they melt, so breakages will not directly raise sea levels.
However, research has shown that when ice shelves are removed, the glaciers and landed ice behind them start to move towards the ocean more rapidly, which will add to the amount of water in the seas.
Antarctic temperatures have risen by up to about 3 Celsius (5.4 Fahrenheit) in the past 50 years, the fastest increase in the southern hemisphere [see “Antarctica has warmed significantly over past 50 years, revisited“].
It is really the warming of the West Antarctic Ice Sheet (WAIS) that you should worry about (at least for this century) because it’s going to disintegrate long before the East Antarctic Ice Sheet does — since WAIS appears to be melting from underneath (i.e. the water is warming, too), and since, as I wrote in the “high water” part of my book, the WAIS is inherently less stable:
Perhaps the most important, and worrisome, fact about the WAIS is that it is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. The WAIS rests on bedrock as deep as two kilometers underwater. One 2004 NASA-led study found that most of the glaciers they were studying “flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is under way.” A 2002 study in Science examined the underwater grounding lines–the points where the ice starts floating. Using satellites, the researchers determined that “bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed.” And that melt rate is positively correlated with ocean temperature.
The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more-and increasingly warmer-water underneath it, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS (see, for instance, here).
You can read every thing a laymen could possibly want to know about what the recent study on Antarctic warming does and doesn’t show at RealClimate here. Andy Revkin blogs on the NYT coverage of the study with expert commentary here.
Now a couple of new papers published by Nature in March have been portrayed as suggesting the WAIS as a whole may be stabler than was previously thought. Yet the first paper, “Obliquity-paced Pliocene West Antarctic ice sheet oscillations” (subs. req’), concludes:
Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to 3 °C warmer than today and atmospheric CO2 concentration was as high as 400 p.p.m.v.
We’ll be at 400 ppm by 2020. We’re on track to be more than 5°C warmer by 2100. So the first paper doesn’t seem terribly reassuring.
The second paper by Pollard and DeConto (the one that got all the attention), “Modelling West Antarctic ice sheet growth and collapse through the past five million years,” (subs. req’), notes, “Recent melt rates under small Antarctic ice shelves are inferred to be increasing dramatically” and concluded:
… the WAIS will begin to collapse when nearby ocean temperatures warm by roughly 5 °C. Global climate and regional ocean modelling is needed to predict when and if future ocean temperatures and melt rates under the major Antarctic ice shelves will increase by these amounts, and if so, for how long.
Are you reassured yet?
I would note that West Antarctica land temperatures have risen up to 3°C over the past 50 years — some 4 times what the planet as a whole has warmed. And both Hadley and MIT say the planet will warm more than 5°C by 2100, with a 10% chance of warming more than 7°C (see M.I.T. doubles its projection of global warming by 2100 to 5.1°C and “Hadley Center warns of “Catastrophic” 5-7°C warming by 2100 on current emissions path. And while the ocean warms less than the nearby land, the new study Antarctic Climate Change and the Environment warns: “UP TO one-third of all Antarctic sea ice is likely to melt by the end of the century.” So we may yet see polar amplifacation near the South Pole (see “What exactly is polar amplification and why does it matter?“).
Dr. Robert Bindschadler of NASA, who has been an active Antarctic field researcher for the past 25 years, commented on the new study (here):
I’m familiar with the Pollard/DeConto work. They previewed it last fall at an annual science workshop I organize on West Antarctic research. Their model lacks the detail to get the fastest dynamic responses, so the 0.5m/century rate for sea level rise should only be viewed as a lower bound (and a poor one, at that).
Their model is better at getting the longer-term quasi-equilibrium response (it just takes their model a little longer to get there), so it ’s very interesting that they demonstrate the sensitivity to the ocean temperature. That thinking is certainly where Antarctic scientists are being led by both data and models.
Moreover, the entire WAIS need not collapse for it to contribute to catastrophic sea level rise this century.
The Antarctic Peninsula alone contains “a total volume of 95,200 km3 (equivalent to 242 mm of sea-level; Pritchard and Vaughan, 2007), roughly half that of all glaciers and ice caps outside of either Greenland or Antarctica” (see Chapter 5 here) — that would be more than 9 inches of sea level rise from a region of WAIS losing its protective ice shelves on both sides at an alarming pace.
But it is westernmost part of WAIS, that borders on the Amundsen Sea, that may be the most worrisome, as AP reported this year:
Glaciers in Antarctica are melting faster and across a much wider area than previously thought, a development that threatens to raise sea levels worldwide and force millions of people to flee low-lying areas, scientists said Wednesday.
Researchers once believed that the melting was limited to the Antarctic Peninsula, a narrow tongue of land pointing toward South America. But satellite data and automated weather stations now indicate it is more widespread.
The melting “also extends all the way down to what is called west Antarctica,” said Colin Summerhayes, executive director of the Britain-based Scientific Committee on Antarctic Research.
“That’s unusual and unexpected,” he told the Associated Press in an interview.
By the end of the century, the accelerated melting could cause sea levels to climb by 3 to 5 feet — levels substantially higher than predicted by a major scientific group just two years ago.
Making matters worse, scientists said, the ice shelves that hold the glaciers back from the sea are also weakening.
The report Wednesday from Geneva was a broad summary of two years of research by scientists from 60 countries. Some of the findings were released in earlier reports….
The biggest of the western glaciers, the Pine Island Glacier, is moving 40% faster than it was in the 1970s, discharging water and ice more rapidly into the ocean, said Summerhayes, a member of International Polar Year’s steering committee.
The Smith Glacier, also in west Antarctica, is moving 83% faster than in 1992, he said.
The glaciers are slipping into the sea faster because the floating ice shelf that would normally stop them — usually 650 to 980 feet thick — is melting. And the glaciers’ discharge is making a significant contribution to increasing sea levels.
So we have the serious potential for 3 to 5 feet of sea-level rise just from WAIS this century — and that is on top of whatever we get from thermal expansion of the ocean and Greenland. And on top of whatever we get from the melting of the inland glaciers, whose contribution was recently increased:
New research published this month in the journal Geophysical Research Letters found that melting glaciers will add at least 7 inches to the world’s sea level — and that’s if carbon dioxide pollution is quickly capped and then reduced.
Far more likely is an increase of at least 15 inches and probably more just from melting glaciers, the journal said.
So it increasingly looks like we are facing a very serious risk of more than 5 feet of total sea-level rise by 2100 on our current emissions path.
But this is almost not news anymore — see Startling new sea level rise research: “Most likely” 0.8 to 2.0 meters by 2100. Indeed, an important Science article from 2007 used empirical data from last century to project that sea levels could be up to 5 feet higher in 2100 and rising 6 inches a decade (see Inundated with Information on Sea Level Rise. Another 2007 study from Nature Geoscience came to the same conclusion (see “Sea levels may rise 5 feet by 2100“). Leading experts in the field have a similar view (see “Amazing AP article on sea level rise” and “Report from AGU meeting: One meter sea level rise by 2100 “very likely” even if warming stops?“). Even a major report signed off on by the Bush administration itself was forced to concede that the IPCC numbers are simply too out of date to be quoted anymore (see US Geological Survey stunner: Sea-level rise in 2100 will likely “substantially exceed” IPCC projections).
Did I mention the time to act is now!