|November 03, 2015|
Experts dispute NASA study showing Antarctic ice gain
|A number of experts are disputing the conclusion of a recent NASA study that says more ice is accumulating in Antarctica than is being lost due to climate change. They argue that the study contradicts more than a decade of other scientific measurements --- including previous NASA studies.|
The NASA report issued last week, "Mass Gains of Antarctic Ice Sheet Greater than Losses," argued that snow accumulation in East Antarctica has added enough ice to the continent to outweigh the losses from the continent's thinning glaciers, especially those in West Antarctica. These ice gains, the report noted, would likely not last more 20 to 30 years due to the speed with which ice is melting due to climate change.
But previous NASA studies, including data released last year, have warned that melting in West Antarctica is "unstoppable." Researchers have also said melting ice could add as much as four feet to long-term sea level rise predictions, which warn of a three-foot rise by 2100.
Last week's study --- which challenged a 2013 report by the United Nations Intergovernmental Panel on Climate Change (IPCC) saying Antarctica was losing ice overall --- has triggered heated debate.
"Please don't publicize this study," said Theodore A. Scambos, a senior research scientist at the National Snow & Ice Data Center, a polar research center at the University of Colorado at Boulder.
Other critics said the study contradicts 13 years of satellite measurements of Antarctica's ice by NASA's GRACE mission.
"There is no quality data to support the claims made by the authors of [ice] growth in East Antarctica," said Eric Rignot, principle scientist for the Radar Science and Engineering Section at NASA's Jet Propulsion Laboratory.
But the lead author of the contested report, Jay Zwally, a glaciologist at NASA's Goddard Institute for Space Studies, told Al Jazeera his data are based on improved models that weren't applied to previous measurements. If the same model were applied to other data, it would match his own "more accurate" determination of the state of Antarctica's ice sheets, Zwally said.
Zwally's model focuses on the movement of the bedrock deep under Antarctica's ice. Earth's mantle rises when relieved of the burden of ice sheets and glaciers. The same phenomenon occurred in Antarctica, Zwally said, but it hasn't been accurately included in the old models of bedrock movement. That, he said, may be behind the difference between his measurements and those of the rest of the scientific community.
The old models "didn't take into account the slow growth (in ice) at the center. So we estimated that that unaccounted-for growth is about a centimeter (0.39 inch) or two a year over 10,000 years. That's ... meters on top of the other ice. And instead of the Earth coming up because the ice went away, it's going down because of that," Zwally said.
Benjamin Smith, of the University of Washington's applied physics lab, said he didn't think there was inaccurate data in Zwally's study. The differences in conclusion with other studies, he said, were largely based on the interpretation of that data. It was possible, Smith said, that Zwally's measurements were correct and previous data were wrong.
"There's an interpretation step that needs to go into this. The GRACE measurements and his are both influenced by what rock is doing underneath the ice sheet," Smith said. "You have to understand what's happening with the rock motion to understand what the signal from GRACE means."
Smith said the issue may be laid to rest soon. There are plans underway, he said, to send teams to Antarctica to take measurements of the ice's altitude that way rather than using satellite data.
If Zwally's study is correct, it raises another question: Where did the sea level rise attributed to Antarctic ice melt originate?
In recent decades, the world's oceans have risen an average of 2.8 millimeters (0.11 inch) per year, according to a 2013 report by the IPCC. That rise is attributed to Antarctic and Greenland ice sheets melting, to disappearing glaciers and to thermal expansion --- when heat from climate change causes the ocean to expand, therefore causing a sea level rise.
"The good news is that Antarctica is not currently contributing to sea level rise, but is taking 0.23 mm per year away," Zwally said last week in a press release.
"But this is also bad news," he said. "If the 0.27 mm 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."