Antarctic Tipping Points - the fate of the Amundsen Sea glaciers

This is a guest post by Richard J. Blaustein, a freelance science and environmental journalist based in New York. You can follow him on twitter @richblaustein.

This image shows a still frame captured from a three-dimensional, virtual flight through a new rift in the Pine Island Glacier in October 2011 as part of NASA's Operation IceBridge campaign. These were the first-ever detailed, airborne measurements of a major iceberg calving event while it was in progress.

Climate change tipping points are scenarios in which climate change impacts sensitive parts of the Earth system and starts a self-accelerating dynamic - often referred to as a forcing - that leads to a rapid transformation of that part of the Earth. Whether tipping points are reversible is a complicated point, but the most serious tipping points have global effects and are not reversible in human time. Right now, there is a lot of concern about tipping points in the cryosphere, the frozen part of the Earth, e.g. large parts of Antarctica and Greenland. If these areas tip over and ice melt accelerates, the sea level would rise and be dangerous for coastal communities worldwide.

Currently, mountain glaciers and Greenland ice are melting. However Antarctica, which is huge, carries with it enormous potential for raising sea level. This May, two scientific papers discussing the breakup of West Antarctica's Amundsen Sea Embayment attracted much media attention. The papers found that irreversible and dramatic loss of glacier ice has been set in motion. While the most dramatic change will probably not occur this century, it will be serious. The loss of glaciers in this area will raise sea levels by one meter and could initiate the breakup of other West Antarctic glaciers, causing another 2-3 meter sea level rise. One of the papers is a computer model-based study, and the other is based on direct observations, including information from recent flights over Antarctica.

Ted Scambos, Antarctic glaciologist at the National Snow and Ice Data Center and not a co-author on the papers, says the findings have made it clear that the Amundsen Sea Embayment "has probably embarked upon a very slow runaway acceleration of ice flow and unloading ice from the West Antarctic ice sheet." While glaciologists have been predicting West Antarctic runaway breakup since the late 1970s, most famously with John Mercer's 1978 Nature article, Scambos explains that these 2014 papers are important because they offer a unique "combination of observation and modelling that show that right now we cannot see any way to avoid it." He adds that this research "is enough of a step forward that it was not improper for the amount of press that it got."

The Amundsen Sea Embayment is a glacial system, formed from two large glaciers and four small ones (named the Haynes, Pope, Smith, and Kohler) from which scientists have observed significant ice loss, breakup, and discharge into the sea. For example, from 1992 to 2011 the Smith and Kohler glaciers retreated 35 kilometers. However, the large Thwaites and Pine Island are the most important Amundsen Sea Embayment glaciers, holding the glacier system together and connecting it with other areas of Antarctica. The Thwaites' retreat has been accelerating since 2009.

How this dramatic change occurs is complicated, and it involves the different currents that course through the ocean depths. Warmer water, known as the circumpolar deep water, circulates around Antarctica. Usually, these warmer currents did not make contact with the Antarctic coast, but since the 1970s, stronger winds have pushed Antarctic cold surface water away from the coast, drawing warmer circumpolar deep water inward. The strengthening of the Antarctic winds is believed to have been triggered by temperature disparities caused by climate change, although some scientists have also hypothesized that the ozone hole is playing a role in their intensification. Regardless, the circumpolar deep water is now making contact with the below-water portions of glaciers of the Amundsen Sea Embayment. This contact results in the melting of the above ice shelf and the retreat of the critical glacial grounding line - the furthermost point at which the glacier is fixed to the ground. As the grounding lines buttress the upstream glacier mass, their retreat therefore destabilizes the entire glacial sector.

University of Washington glaciologist Ian Joughin led the model-based analysis published in Science. Based on different melt rates for the Thwaites Glacier, his study predicts the rapid collapse of the Thwaites Glacier in two to nine centuries. The study also accounts for different snowfall rates and degrees of sturdiness of the glacial margin areas, factors which are important for the overall strength of the glacier. Snowfall is important because an increase in snowfall (which global warming could produce because it adds moisture to the atmosphere) may lead ice to accrue in Antarctica, offsetting glacial ice loss. However, Joughin's paper concludes that increased snow fall would not offset glacial retreat and discharge. Weakening of the Thwaites' margins, on the other hand, would be significant. At the highest melt rate, it was predicted that the Thwaites glacier would rapidly retreat in 212 years with weakened margins in contrast to 292 years without the weak margin factor.

Joughin emphasizes that the Thwaites Glacier holds in balance other West Antarctic areas. He explains, "You can't just remove Thwaites or Pine Island glaciers from that part of Antarctica because you leave a huge gap; the rest of the ice will flow in and the bed is all connected."

The other publication, an AGU Geophysical Research Letters observation-based article, led by NASA glaciologist Eric Rignot, who is also with University of California-Irvine where he heads the Rignot Research Group, pulls together data for the Amundsen Sea Embayment for the past 40 years and documents dramatic glacial loss. A second paper, led by Rignot Research Group geophysicist Jeremie Mouginot, appearing online in May gives a new topography describing the natural features and elevations of this Antarctic area. The topographical study was important for ascertaining whether there are ridges or other geological formations that would thwart the glaciers' retreat.

Rignot and colleagues combined data from IceBridge (NASA's six year polar survey project), surveys conducted in 2004 by the British Antarctic Survey and the University of Texas, Austin, and radar satellite observations which indicate the position of the grouding lines by detecting fluctuations in the ice shelf - in order to determine grounding line locations, as well as gaining a new sense of the terrain of the Amundsen Sea Embayment.

Not only did they find significant grounding line retreats in the Amundsen Sea Embayment but, equally important, they located no significant geographic formations along the glaciers' paths. As Rignot explains, this means that "We see glaciers retreating in deep valleys with no obstacles to stop their retreat."

Scambos, Joughin and Rignot do not rule out a more rapid collapse than recent modelling estimates. Rignot says that "the jury is still out" on the time frame for the breakup of glaciers in the Amundsen Sea Embayment, adding that "I think by 2100, there will be dramatic changes in that sector."

According to Scambos, the Amundsen Sea Embayment glacial collapse has probably begun, but it's not too late to make a difference. "We can slow this down by slowing down the things that are affecting climate," Scambos says. "It is within our power to have less impact on the future, that's where the morality comes in... I say we owe the future of humanity a better tended planet that gives them a reasonable chance of making their way in the world."

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