Five glaciers that feed continental ice from Antarctica into the Pacific sector of the Southern Ocean – glaciers long seen as the soft underbelly of the West Antarctic Ice Sheet – are undergoing irreversible decline, two new studies indicate.
The glaciers flowing into these waters, Antarctica's Amundsen Sea, carry enough ice to raise sea levels by 1.2 meters (3.9 feet), with effects that cascade to other sections of the ice sheet.
"We have passed the point of no return," says Eric Rignot, a glaciologist at the University of California at Irvine and the lead author of the study, which has been accepted for publication in the journal Geophysical Research Letters.
The glaciers' retreat "will also influence adjacent sectors of the West Antarctic Ice Sheet, which could triple this contribution to sea level," he said during a briefing Monday. This would amount to a global average of three to four meters of sea-level rise.
Estimates of the timing involved for the retreat of one of the two largest glaciers, the Thwaites Glacier, range from 200 to 900 years, according to another study, set to be published Friday in the journal Science. Through the rest of this century, the study anticipates relatively modest additions to sea level from Thwaites, after which the glacier's retreat accelerates.
Regardless of the time span, however, the loss of the glaciers is virtually unstoppable, the researchers say.
If these results hold up, they suggest that even if all human-generated greenhouse gas emissions were to stop today, the climate would still continue to warm some, as the oceans release captured heat, and the loss of these glaciers would continue unabated.
The study that Dr. Rignot and colleagues have produced "is a really important piece of work" involving a "critically important piece of the continent" for society, says Sridhar Anandakrishnan, a glaciologist at Penn State University in State College, Pa., who did not take part in either study.
Rignot's team reached its conclusion after studying 40 years worth of data on the speed of glacier retreat, as well as more recent data on ice thickness and significantly improved maps of the terrain under the ice.
The causes of the glaciers' retreat represent a confluence of global warming and the region's unusual under-ice terrain, the researchers say.
The weight of the continental ice has depressed the crust. What once were river valleys that made their way to the ocean are now valleys buried under ice and held well below sea level. Continental ice flows seaward via the glaciers until the glaciers hit barriers of elevated underwater terrain known as sills. The upper section of a glacier tries to move across the top of the sill, but friction serves as a brake, significantly slowing the flow of ice.
But human-triggered climate change has added a new twist. It has altered atmospheric circulation patterns over the Southern Ocean, intensifying westerly winds that encircle the continent and driving them deeper south than they otherwise would be, Rignot explains. This has affected ocean currents in ways that allow relatively warm deep water to rise up the seaward side of the sills and begin to melt the ice from underneath.
Once the relatively thin tongue of ice over-topping the sill begins to float free, the brake fails, water moves in behind the sill, driving the glacier's grounding line further inland, which also accelerates the ice loss. The retreat can be slowed if another under-ice hill or mountain upstream is available to act as another sill. Otherwise the glacier retreats until it reaches the inland end of the valley where the terrain begins to rise again. There its retreat slows until the glacier stabilizes.
This process is especially worrisome for the Thwaites Glacier, perhaps the most important glacier of the six, explains Dr. Anandakrishnan.
"It's hanging on by its fingernails," he says. Thwaites currently is losing its grip to a ridge only 10 to 20 kilometers wide (6 to 12 miles). "Once it retreats off of that, it's in an over-deepened bowl; it will keep retreating until it sees a reverse slope" and starts to back itself out of the bowl.
As it does, it will thin, slow, and eventually stabilize, Anandakrishnan adds. But the glacier will have to retreat 300 to 400 kilometers before this happens because the bowl is long and nothing rises sufficiently from the under-ice terrain to stop its retreat.
The story is similar with other glaciers in the sector.
Using radar data from satellites and collected between 1992 and 2011, as well as by subsequent aircraft flights, Rignot's team tracked the retreat of the grounding points, know as grounding lines, for five of the sector's six glaciers: Pine Island, Thwaites, Haynes, Smith, and Kohler.
The Pine Island Glacier, which has retreated some 31 kilometers at its center, slipped its sill's moorings between 2005 and 2009. Although its retreat slowed slightly between 2009 and 2013, the retreat kept accelerating farther inland. Thwaites Glacier's grounding line has retreated by 14 kilometers along its core since 1992. The Smith/Kohler system's grounding line has retreated by 35 kilometers, and the team says the anchor points for its ice shelf are vanishing.
And the data for all glaciers show nothing upstream of their 2011 grounding points to inhibit their retreat once the glaciers' grounding lines move off their current anchor points.
For Thwaites Glacier, a team from the University of Washington led by researcher Ian Joughin used measurements of ice thinning rates, retreat rates, and seafloor maps to model the future of the glacier. The model projects fairly modest melting that would raise sea-levels by about 0.25 millimeters a year through the end of the century. The pace of melting increases and the contribution to sea level rises four-fold once the glacier's grounding line reaches the deepest parts of the elongated bowl it fills. That stage could occur within a few centuries, the team estimates.
But the model results also indicate that the process of destabilizing the glacier is happening now and may be inevitable. Moreover, the model's highest melt rates reach levels comparable to measured melt rates. At those rates, the model indicates that the glacier's collapse may be closer to "a few centuries" rather than to 900 years.