Sunday, November 6, 2011

Global Warming in Antarctica: Thwaites and Pine Island Glaciers accelerating, West Antarctic Ice Sheet losing mass

Scientists have been studying the climate change impacts on ice shelfs and glaciers for some time in Antarctica, and particularly around the Antarctic Peninsula where there is substantial warming occurring increasing ice shelf melt and the speed and discharge of glaciers. The most recent studies predict a faster retreat for the Thwaites Glacier and that warm ocean currents are already speeding the melting of the Pine Island Glacier and Ice Shelf and Getz Ice Shelf. A NASA Icebridge flight detected a major new rift in the Pine Island ice shelf on October 14 - the start of the calving of a massive iceberg. A recent paper in Nature Geoscience discusses the Stability of the West Antarctic ice sheet in a warming world and the likelihood of collapse that would raise sea level by more than three metres over the course of several centuries or less.

Related: The Wilkins ice Bridge collapsed in April 2009 as Polar regions felt the heat of climate change. I reported as far back as 2004 that warming in Antarctica was cause for concern with ocean food chain crashing due to Antarctic warming. More recently in April 2011 I discussed Penguin numbers suffering with krill decline due to Global Warming.

Thwaites Glacier expected to accelerate

The latest study of the Thwaites glacier has identified an underwater ridge critical to the future rate of flow of the glacier. The ridge currently holds back the glacier hindering its speed and discharge. As the glacier detaches from the ridge sometime in the next 20 years, it is expected to accelerate into the Amundsen Sea.

Geophysicist Robin Bell, study co-author, compared the ridge in front of Thwaites to a person standing in a doorway, holding back a crowd. “Knowing the ridge is there lets us understand why the wide ice tongue that used to be in front of the glacier has broken up,” she said. “We can now predict when the last bit of floating ice will lift off the ridge. We expect more ice will come streaming out of the Thwaites Glacier when this happens.”

“The bathymetry is the roadmap for how warm ocean water reaches the edges of the ice sheet,” she added. “Ridges like this one and the one discovered in front of Pine Island Glacier stabilize ice sheets, but can also be a critical part of the destabilizing process.”

Pine Island Glacier showing continuous acceleration

The neighboring Pine Island glacier has been undermined from below by warmer ocean water speeding the melting and discharge of the glacier as a whole. Scientists used a remote controlled submarine in 2009 to study underneath the Pine Island ice shelf and discovered a ridge about half the size of the one anchoring the Thwaites glacier. They estimated the Pine Island glacier detached from this ridge in the 1970s starting the process of ocean water undermining the glacier.

“More warm water from the deep ocean is entering the cavity beneath the ice shelf, and it is warmest where the ice is thickest,” said study’s lead author, Stan Jacobs, an oceanographer at Columbia University’s Lamont-Doherty Earth Observatory.

The Pine Island glacier’s ice shelf is now moving 50 percent faster than it was in the early 1990s. Pine Island Glacier is moving into the sea at the rate of 4 kilometers a year — four times faster than the fastest-moving section of Thwaites.

The research was published in Nature Geoscience in June 2010 as Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat (Abstract). The abstract reports that

"Thinning ice in West Antarctica, resulting from acceleration in the flow of outlet glaciers, is at present contributing about 10% of the observed rise in global sea level1. Pine Island Glacier in particular has shown nearly continuous acceleration and thinning throughout the short observational record. The floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly, driven by changes in ocean heat transport beneath it. As a result, the line that separates grounded and floating ice has retreated inland. These events have been postulated as the cause for the inland thinning and acceleration....The pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice-sheet collapse triggered by climate change.

During the October 2011 NASA Icebridge Project flight on October 14 a huge crack running across the entire width of the Pine Island ice shelf was observed.

"It's part of a natural cycle, but it's still very interesting and impressive to see up close," said IceBridge project scientist Michael Studinger. "It looks like a significant part of the ice shelf is ready to break off." The IceBridge team made a preliminary calculation that the area that could calve in the coming months covers about 310 square miles (800 square kilometers), Studinger said. The team on the DC-8 observed the crack running across the breadth of the ice shelf.

Watch the related video from the NASA Icebridge flight, or view photos at flickr:


West Antarctic Ice Sheet losing mass - stability threatened

According to a recent review published in Nature Geoscience by Ian Joughin and Richard B. Alley - Stability of the West Antarctic ice sheet in a warming world (Abstract and Full paper) - recent observations by satellite show substantial mass loss from the West Antarctic ice sheet (WAIS). Losses range from 100 to 200 Gigatonnes per year, the equivalent to 0.28 to 0.56 mm per year sea-level rise, with the rate growing over the past two decades.

Concerns over the stability of the ice sheet have been raised for the last 40 years. Much of the WAIS sits on bedrock lying well below sea level. At the moment vast ice shelves in the Weddell and Ross seas dam the ice sheet. But if these ice shelves were to substantially melt, large areas of WAIS would be unblocked triggering an acceleration of the ice sheet toward the ocean and a rapid inland migration of the grounding line. The ice siting on inland basins would be undercut and float forming new floating ice shelves further inland, in time precipitating further breakup and collapse. Because of it's essential instability, the rate of collapse of WAIS is unknown.

According to the paper by Ian Joughin and Richard B. Alley:

"Removing the WAIS would leave broad, deep seaways that deepen towards the ice-sheet interior. This bathymetry makes the ice sheet subject to the marine-ice-sheet instability. Portions of the East Antarctic and Greenland ice sheets are also marine, and the discussion here applies to those regions as well, but the issue is quantitatively more important for the WAIS, with its extensive troughs extending to depths of more than 2 km. Despite its marine setting, the present ice sheet exists because various factors promote stability, including buttressing ice shelves and regions where local bathymetric slopes oppose the general trend. Climate forcing, in particular warming that affects ice-shelf viability, could undo this potentially fragile stability. Internal instabilities also have the potential to push the WAIS past a threshold where the marine-based instability may lead to irreversible retreat."

Temperature predications for 2100 approach the thresholds of ice-shelf viability in many simulations, according to the review, but with many uncertainties regarding modelling predictions for high latitudes. "Ice-sheet simulations suggest that loss of the large ice shelves by atmospheric or oceanic forcing would probably presage collapse of the bulk of the marine ice sheet," the report authors say.

However, with CO2 emissions increasing by a record amount in 2010, temperatures by the end of the century are likely to be at the top end of IPCC predictions unless concerted action by governments to reduce emissions is taken. If little action is taken, it makes the collapse of the Greenland and West Antarctic Ice sheets more likely. And once collapse is underway, they will be impossible to stop or reverse this process.

Earlier research by scientists at Penn State University in 2009 published in Nature modeled the ice sheet over the last 5 million years. "We found that the ocean's warming and melting the bottom of the floating ice shelves has been the dominant control on West Antarctic ice variations," said David Pollard, senior scientist, Penn State's College of Earth and Mineral Sciences' Earth and Environmental Systems Institute. Due to it's unstable nature the ice sheet can collapse relatively quickly. "Transitions between glacial, intermediate and collapsed states are relatively rapid, taking one to several thousand years." says the Nature article Abstract.

The research by Pollard and Robert M. DeConto, Professor of Climatology, U. Mass, found that the ice sheet collapsed and rebuilt multiple times matching the cycle of Northern Hemisphere's pattern of glaciation and glacier retreat. Pollard also noted that when atmospheric CO2 hit 400ppm, that ice sheet collapses were much more frequent. We are presently on 388 ppm of atmospheric CO2. "We are a little below 400 parts per million now and heading higher," said Pollard in a media release. "One of the next steps is to determine if human activity will make it warm enough to start the collapse."

Ian Joughin and Richard B. Alley conclude in their review that:

A collapse of the marine ice sheet in West Antarctica would raise sea level by more than three metres over the course of several centuries or less. Such an event seems possible, but improved understanding of the expected atmospheric and oceanographic forcing and the ensuing ice-sheet response is required to quantify its likelihood. Precisely understanding the vulnerability of the West Antarctic ice sheet to a warming climate remains a grand challenge for the ice-sheet and climate-modelling communities.

The melting of the Greenland ice sheet is already well documented with Greenland seting a new melt record in 2010, and Greenland melting in 2011 well above average with near-record mass loss. We may be witnessing the start of the destabilization of the West Antarctic Ice Sheet (WAIS). The collapse of these ice sheets, once started will be impossible to stop and will contribute to substantial sea level rise that will affect coastal areas of Australia and around the world. Sea levels will rise slowly, then accelerate and continue for several centuries. In the distant past sea levels have risen at a speed of up to one metre per 20 years, although we are unlikely to see that rate this century.

Watch David Pollard delivering a talk in May 2011 on "Modeling Cenozoic variations of the Antarctic Ice Sheet" (23:03) at Oregon State University

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