A recent study published in the Journal of Glaciology that found the Antarctic ice sheet is expanding because accumulated snowfall is outpacing melting glaciers has drawn sharp criticism from many climate scientists. While it does not contradict the science on global warming, it has pried open a long-standing debate about how warming is effecting the largest ice mass on the planet.
The study, led by Jay Zwally, a glaciologist at NASA’s Goddard Space Flight Center, contradicts findings from years of research concluding that there’s a net ice loss in the Antarctic. Such findings have come from NASA and the Intergovernmental Panel on Climate Change.
The authors of the new study argue that rapid snow accumulation on the ice sheet between 1992 and 2008 offset the depletion from melting. The Antarctic ice sheet gathered mass at the rate of 112 billion tons of ice every year from 1992 to 2001, they said, which slowed to 82 billion tons of ice gain per year between 2003 and 2008.
The IPCC in its 2013 report noted that ice loss in the Antarctic most likely accelerated from 30 billion tons a year between 1992 and 2001 to 147 billion tons a year in the period from 2002 to 2011.
“We’re essentially in agreement with other studies that show an increase in ice discharge in the Antarctic Peninsula and the Thwaites and Pine Island region of West Antarctica,” Zwally said in a statement. “Our main disagreement is for East Antarctica and the interior of West Antarctica—there, we see an ice gain that exceeds the losses in the other areas.”
Robin Bell, a senior research scientist at Columbia University, noted that the disagreement essentially boiled down to “a checkbook problem: how much comes in, how much goes out.”
Others were harsher in their critiques of what they saw as a study defying a solid body of evidence, pointing to weaknesses in the method and analysis. “I think the study is just plain wrong, far too inconsistent with other lines of evidence, and not worth the public’s attention,” said Theodore Scambos, a senior research scientist at the National Snow and Ice Data Center.
Scientists do not dispute that accumulated snowfall is compensating for some of the ice sheet loss. But not many are convinced that the addition is so significant as to offset the losses.
“At the end of the last ice age, the air became warmer and carried more moisture across the continent, doubling the amount of snow dropped on the ice sheet,” Zwally explained. Glacial ice forms because of the accumulation of snow over many millennia, with the gradual compression of flaky snow into compact ice hunks.
If sea-level rise isn’t coming from Antarctica, then from where?
A change in the height of the ice sheet offers a clue to the net balance between accumulation through snowfall and the loss through glacial melt. The ice sheet in East Antarctica and the interior of West Antarctica has thickened by an average of 0.7 inch (1.7 centimeters) every year, according to the research.
The heights of ice sheets can be measured remotely from satellites equipped with altimeters. In the paper, the researchers relied on measurements from radar altimeters, which calculate height based on the time taken for a radio signal to bounce from the surface, for the period from 1992 to 2001. For height measurements for 2003 to 2008, they used laser altimeters on NASA’s Ice, Cloud and land Elevation Satellite (ICESat), which rely on reflected light to compute altitudes.
Ian Joughin of the University of Washington, who was not part of the research, pointed out that measurements from the same satellites as those used in the current study yielded different results in studies done in the past.
Of this, Scambos said: “The satellite is not at fault here, not inherently, but the data are being pushed to a place it can’t go. The data receive several corrections, and some are problematic.”
Joughin said he was open to the possibility that the East Antarctic Ice Sheet could still be “re-equilibrating” and gaining mass because of increase in snowfall accumulation after the last glacial period ended. However, he questioned the results based on the magnitude of the signal that Zwally and his colleagues present as evidence that there was a net gain in ice.
“Because the signal over the plateau is very small relative to the measurement uncertainty, it’s not implausible that this could be calibration error, either,” he said.
Bell echoed this unease with the new findings, “It is a complicated signal, and you see very small changes; that is why the community is uncomfortable with this result.” The discomfit could also arise from what the findings mean for estimates about sea-level rise.
The IPCC has estimated that meltwater from Antarctica could raise sea levels by between 2.2 to 5.5 inches by 2100.
But the new findings suggest that Antarctica is currently not contributing to sea-level rise at all but helping to check it by stashing away 0.23 millimeter of fresh water every year.
“But this is also bad news,” Zwally said. “If the 0.27 millimeters per year of sea-level rise attributed to Antarctica in the IPCC report is not really coming from Antarctica, there must be some other contribution to sea-level rise that is not accounted for.”
However, given the trend captured by the new study of accelerating ice loss and a slowing rate of ice accumulation, the authors predict that the trend of ice mass gain could reverse in about 20 to 30 years.
Before plunging into the debate about sea-level rise, other scientists called for more careful scrutiny of findings from this study.
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500
