Antarctica's Retreating Ice
By Christine Evans
Climate change is having impacts around the globe, but these changes have been particularly pronounced in the polar regions. The rate and extent of change at the poles could have profound effects on the rest of the planet, particularly in relation to global sea level rise. In Antarctica, recent research has pointed to surprising results.
There are two ice sheets remaining in the world today – one covers Greenland and the other Antarctica. Together they contain some 99 percent of the freshwater ice on Earth. The larger of the two, the Antarctic Ice Sheet is the size of the Continental U.S. and Mexico combined and ranges from one to three miles thick.
The Antarctic Ice Sheet is often referred to as two separate ice sheets – the West Antarctic and the East Antarctic – due to the differences in their age and behavior. The East Antarctic Ice Sheet is much older, larger and overall more stable. The West Antarctic Ice Sheet is much more dynamic and is often is separated further into the Southern Antarctic Peninsula and the West Antarctic Ice Sheet.
A critical feature of the Antarctic Ice Sheets are the ice shelves, which are an extension of the land based ice sheet that moves ice into the sea in many places. These thick slabs of ice that are connected to the coast on one side and float over the ocean on the other help to stabilize the ice sheets and slow their advance into the sea. Ice shelves are especially important in areas where the ice is resting on land that is below sea level.
West Antarctic Changes
Much of the West Antarctic Ice Sheet lies below sea level, which makes it particularly prone to instability and melting. A study published in May of 2014 examined satellite radar observations of five different glaciers on the West Antarctic Ice Sheet between 1992 and 2011. The glaciers studied drain into the Amundsen Sea, and scientists found that all are retreating. This glacial retreat is taking place along a seabed that slopes inward, away from the sea, allowing warming ocean water to advance under the ice and melt it from below. The researchers found no significant barriers that would keep the glaciers from advancing further, calling the ice loss from these five glaciers “unstoppable.” According to the study’s lead author, Eric Rignot, “These [glacial] systems, whether Greenland or Antarctica, are changing on faster timescales than we expected. We are kind of rediscovering that every day.”
Glacial evidence shows that this portion of the West Antarctic Ice Sheet has changed in the past, melting and reforming, and thus has been referred to by researchers as “the weak underbelly of the West Antarctic Ice Sheet.” Yet, while the five glaciers in this study are changing more rapidly than expected and appear to be unstoppable, in human timescales the melting is likely to take place over centuries. While overall this region contains enough ice to cause a four-foot rise in global sea levels, this will happen very slowly. For instance, melting from one of the glaciers, the Thwaites, is expected to cause no more than a 2.5 centimeter (0.98 inch) sea level rise by the end of the century.
Southern Antarctic Peninsula
Identified as the “handle” of West Antarctica that extends towards South America, the Southern Antarctic Peninsula also has sections of bedrock that sit below sea level. Like the Amundsen Sea region, the seabed slopes inland under the ice. Such formations are considered to be potentially unstable. A study published this past May in Science found that “a major portion of the region has, since 2009, destabilized” and “ice mass loss…has rapidly accelerated.” The study used measurements of the ice elevation from radar on the European Space Agency’s Cryosat-2 satellite mapped to NASA’s twin Gravity Recovery and Climate Experiment (GRACE) probes.
Lead author Bert Wouters, a glaciologist at the University of Bristol, U.K., noted, “There was nothing happening, and then, all of a sudden in the past five years, all of these glaciers started to send ice into the ocean.” The trigger for the melt is warming ocean water.
The Larsen Ice Shelf, on the east side of the peninsula, was one of the early “messengers of change.” Over a decade ago, in 2002, the Larsen B Ice Shelf, once larger than Rhode Island and at least 10,000 years old, partially collapsed over a period of 35 days. 10,000 years in 35 days – this timescale was a whole new way of thinking for geologists. NASA recently released a study saying that the rest of Larsen B – roughly 625 square miles and 1,640 feet at its thickest point – will likely disappear before 2020. Just 18 years for total collapse of this state-sized ice shelf.
The Wilkins Ice Shelf on the west side of the Antarctic Peninsula is also disappearing, having undergone a number of collapses, first in 1998, then in 2008 and 2009. Ted Scambos, a glaciologist at the University of Colorado, was the first to notice the changes in 2008 while monitoring satellite images of the ice shelf. A 25-by-1.5 mile iceberg broke away from the Shelf, and it appeared all 6,180 square miles – an area roughly the size of Northern Ireland – was in danger of collapsing. “Wilkins is the largest ice shelf on the Antarctic Peninsula yet to be threatened. I didn’t expect to see things happen this quickly. The ice shelf is hanging by a thread,” said Scambos.
East Antarctic Ice Sheet
East Antarctica differs from West Antarctica in many ways, including its ice sheet. Most of the much larger East Antarctic Ice Sheet sits on bedrock far above current sea levels, making it more stable. East Antarctica is less studied owing to its remoteness, freezing temperatures, high altitude, and strong winds. But in recent years satellites and aircraft have provided data on more of this region.
A study recently published in Nature Geoscience found that Totten Glacier is vulnerable to melting and is thinning more quickly than anywhere else in East Antarctica. A further look using radar found an inland trough, or channel, 1,300 to over 1,600 feet down that could allow an influx of warm water to melt the glacier from below. Scientists believe this process, like that seen in West Antarctica, may explain why the glacier has been losing mass.
Totten Glacier acts as a barrier for an inland section of ice that is three-fourths the size of Texas and which contains enough water to raise sea levels by more than 11 feet over a timescale of centuries, were it all to melt. In this study, however, scientists were not able to take measurements of ocean water temperatures around the glacier. According to Eric Rignot, “What we need now is a confirmation of the findings of the paper from oceanographic data, because it is one thing to find potential pathways for warm water to intrude the cavity, it is another to show that this is actually happening.” Other studies are underway to try to determine whether this is what is causing Totten Glacier to thin.
What will happen in Antarctica is one of the most important yet uncertain aspects of climate change. Determining how Antarctica’s ice sheets will behave over the next century, as well predicting sea level rise, is incredibly complex and has many uncertainties.
In the 2013 Intergovernmental Panel on Climate Change (IPCC) report, the estimates of sea level rise are 28 to 98 centimeters (roughly one to three feet) by the end of the century. This projection is over twice the 2007 IPCC estimate, largely due to researchers’ improved understanding of ice sheets. Though there is much about changing sea level that is still not well understood, rising seas will threaten coastal cities around the world.
And while there is still uncertainty, it is clear that changes are taking place more rapidly than scientists previously anticipated. Ongoing research is needed to fully understand the dynamics of ice loss in Antarctica and how these changes will connect to all of us.
Christine Evans is a graduate student in Columbia’s Sustainability Management program.
Special thanks to Margie Turrin of the Lamont-Doherty Earth Observatory for all her help in creating this blog post.