New research confirms that ice sheets in West Antarctica are thinning at a far faster rate than in past millennia. Although scientists are divided as to the cause of the melt, many feel it is directly related to climate change.
The boom must have been deafening last fall as the gigantic chunk of ice finally broke off from the Pine Island Glacier in Antarctica. For almost a year, the creaks and groans from the river of ice had presaged the birth of a new, expansive iceberg. And finally it was there — 34 kilometers long by 20 kilometers wide, an area almost as great as that of New York City.
But as dramatic as the iceberg birth was, it has become a common spectacle in recent years. The gigantic ice shelf that extends into the ocean off of West Antarctica is crumbling — and the glaciers on the continent behind the ice shelf are flowing with increasing speed toward the sea. Concern among scientists is increasing just as quickly. Should the melt-rate continue, or accelerate, many experts fear that the resulting rise in the ocean level could be catastrophic.
Just what is behind the meltdown, however, is not entirely clear. Whereas it is not difficult to pinpoint global warming caused by human activity for increasing temperatures in the Arctic, the southern end of the planet is more difficult. The western side of the continent is thawing out wherever one looks, but on the eastern side, not much is happening.
The cause of the shelf-ice collapse off the coast of West Antarctica is just as disputed as is the question whether such disintegration is at all unusual. Satellite measurements of ice thickness began only at the beginning of the 1980s and little is known about the development of the West Antarctica ice sheet since the last Ice Age some 16,000 years ago.
Now, however, researchers have presented the first data that may provide a clue as to the ice shield’s past. Joanne Johnson and Michael Bentley of the British Antarctic Survey, along with Karsten Gohl from Germany’s Alfred Wegener Institute for Polar and Marine Research, have examined rocks in the search for an isotope of the element beryllium. The isotope is produced when the rocks are free of ice and exposed to high-energy cosmic rays. On the basis of the half-life of the beryllium-10 isotope, scientists can calculate how long the rocks have been free of ice. The same method has been used for years to document the retreat of glaciers.
Johnson and her colleagues analyzed minerals from the Smith, Pope and Pine Island glaciers in four locations. In a paper in the March edition of the prestigious journal Geology, the researchers write that the data, for the first time, places the current massive loss of the West Antarctic ice sheet in the context of past millennia. And they confirm what scientists have long feared: The glaciers are losing their mass at an “unusually rapid” rate.
The paper also states that the thickness of the Pine Island Glacier has shrunk by an average of 3.8 centimeters annually over the past 4,700 years. But the Smith and Pope glaciers have only lost 2.3 centimeters of their thickness annually during the past 14,500 years. Satellite measurements taken between 1992 and 1996, though, show a loss of 1.6 meters in thickness per year on the Pine Island Glacier — a figure that represents 42 times the average melt of the past 4,700 years.
Coincidence or Climate Change?
Nevertheless, the authors of the Geology article admit that they were unable to agree as to whether the ice melt in the West Antarctic has anything to do with human-caused climate change. “We calculated the average values of past millennia,” AWI researcher Gohl explained in an interview with SPIEGEL ONLINE. But the data was not precise enough to show changes over periods of time as short as decades. For that reason, the researchers were unable to definitively rule out the possibility that there had been short thaw periods after the last Ice Age as strong as those currently being experienced. Gohl emphasized that more cores had to be collected in the Antarctic in order to reach a more precise result.
However, the fact that the Antarctic ice melt is happening at the same time as man-made climate change cannot be dismissed out of hand. Even Eric Rignot of NASA’s Jet Propulsion Laboratory, one of the world’s leading researchers in the field of surveying ice via satellite, does not believe it is a coincidence. The sheer speed alone of the glacial recession in the West Antarctic has already led many scientists to postulate that something unusual is happening. “These glaciers could have not thinned at the rate they are thinning now for more than a few decades in the past,” Rignot told SPIEGEL ONLINE via e-mail.
The study by Johnson and her colleagues is a “very solid confirmation” for the hypothesis that the current changes are not due to long-term glacier shrinkage, but to current influences, he feels. According to Rignot, the changes are “clearly related to ongoing climate change.”
Other researchers have also stressed the importance of the study. Klaus Grosfeld from the Alfred Wegener Institute for Polar and Marine Research calls it an “important contribution” to the surveying of the Antarctic ice sheet, while geographer Jonathan Bamber from the University of Bristol called the study “essential” for the calibration of Antarctic computer models.
Why Antarctica’s glaciers are shrinking remains an open question. What is certain is that the surface waters around Antarctica are getting warmer — but it is not clear why that is happening. It could be, for example, that an increase in the upward circulation of the so-called circumpolar deep water, which is up to 1.5 degrees Celsius warmer than the rest of the water, is responsible for the melting of glaciers. Alternatively, the melting could even be due to direct heating of the water as a result of climate change. Snowfalls, the emergence of meltwater pools and volcanic activity could also play important roles.
It is also unclear how much the thaw in Antarctica will contribute in the future to an increase in sea level — but the fact that it will is considered certain. In March 2007, a team of researchers estimated the net loss of ice in Greenland and the Antarctic to be 125 gigatons per year. According to the researchers, melting land ice in the polar regions accounts for around 10 percent of the current rise in sea level, which is about three millimeters per year at present.
But that is far from being the whole story, Rignot says. According to his team’s most recent research, the results of which will soon be published in the scientific journal Geophysical Research Letters, the acceleration of glacier flow in West Antarctica is not only continuing but was larger in 2006-2007 than ever before. “Simple models also predict that this is only the beginning,” Rignot say, “as these glaciers could easily double their speed in years to come.”
According to Rignot, Johnson’s study shows that “the future is now.” SPIEGELnet GmbH