West Antarctic Ice Sheet May Not Be Losing Ice As Fast As Once Thought

Oct. 19, 2009

AUSTIN, Texas — New ground measurements made by the West Antarctic GPS Network (WAGN) project, composed of researchers from The University of Texas at Austin, The Ohio State University, and The University of Memphis, suggest the rate of ice loss of the West Antarctic ice sheet has been slightly overestimated.

"Our work suggests that while West Antarctica is still losing significant amounts of ice, the loss appears to be slightly slower than some recent estimates," said Ian Dalziel, lead principal investigator for WAGN. "So the take home message is that Antarctica is contributing to rising sea levels. It is the rate that is unclear."

In 2006, another team of researchers used data from the Gravity Recovery and Climate Experiment (GRACE) satellites to infer a significant loss of ice mass over West Antarctica from 2002 to 2005. The GRACE satellites do not measure changes in ice loss directly but measure changes in gravity, which can be caused both by ice loss and vertical uplift of the bedrock underlying the ice.

Now, for the first time, researchers have directly measured the vertical motion of the bedrock at sites across West Antarctica using the Global Positioning System (GPS). The results should lead to more accurate estimates of ice mass loss.

Antarctica was once buried under a deeper and more extensive layer of ice during a period known as the Last Glacial Maximum. Starting about 20,000 years ago, the ice began slowly thinning and retreating. As the ice mass decreases, the bedrock immediately below the ice rises, an uplift known as postglacial rebound.

Postglacial rebound causes an increase in the gravitational attraction measured by the GRACE satellites and could explain their inferred measurements of recent, rapid ice loss in West Antarctica. The new GPS measurements show West Antarctica is rebounding more slowly than once thought. This means that the correction to the gravity signal from the rock contribution has been overestimated and the rate of ice loss is slower than previously interpreted.

"The published results are very important because they provide precise, ground-truth GPS observations of the actual rebound of the continent due to the loss of ice mass detected by the GRACE satellite gravity measurements over West Antarctica" said Vladimir Papitashvili, acting director for the Antarctic Earth Sciences Program at the National Science Foundation, which supported the research.

WAGN researchers do not yet know how large the overestimation was. A more definitive correction will be conducted by other researchers who specialize in interpreting GRACE data. Previous estimates of postglacial rebound were made with theoretical models. Assimilation of the direct GPS results into new models will therefore produce significant improvements in estimations of ice mass loss.

The results will appear in "Geodetic Measurements of Vertical Crustal Velocity in West Antarctica and the Implications for Ice Mass Balance" (M. Bevis et al., 2009), published in the electronic journal Geochemistry, Geophysics, Geosystems of the American Geophysical Union and the American Geochemical Society. [View the paper online.]

A team from The University of Texas at Austin's Jackson School of Geosciences (Ian Dalziel, lead principal investigator), The Ohio State University's School of Earth Sciences (Michael Bevis), and The University of Memphis' Center for Earthquake Research and Information (Robert Smalley, Jr.) performed the WAGN project.

The network consists of 18 GPS stations installed on bedrock outcrops across West Antarctica. Precise, millimeter level, three-dimensional locations of the stations, which are bolted into the bedrock, were determined during measurements made from 2001 to 2003 and from 2004 to 2006, the two measurements being at least three years apart. The difference in the positions during the two time periods indicates the motion of the bedrock.

The WAGN data were supplemented with data from the first year of the Polar Earth Observing Network (POLENET) project, a project to establish a more sophisticated, continuously recording network of GPS and seismic stations, including the already established WAGN sites. POLENET will further improve our understanding of  the interaction between the solid earth and ice sheets at both poles. The lead principal investigator of the U.S. Antarctic contribution to POLENET is Terry Wilson of The Ohio State University.

The West Antarctic GPS Network and the U.S. Antarctic contribution to the Polar Earth Observing Network of the International Polar Year were both funded and logistically supported by the Office of Polar Programs of the National Science Foundation.

For more information, contact: J.B. Bird, University of Texas at Austin, 512-750-3512 (cell), 512 471 4550.

9 Comments to "West Antarctic Ice Sheet May Not Be Losing Ice As Fast As Once Thought"

1.  Ben said on Oct. 20, 2009

Well, this fits the other data better. Rapidly declining West Antarctic ice made no sense in the context of the record-high sea ice in 2007.

Now, this begs the question: "What evidence remains that the Antarctic is losing ice?" and its follow-up "Why is it losing ice?" It's not like the Antarctic is warm enough for melting, and ice loss can be caused by temperature decreases which stop precipitation.

2.  Seth Davis said on Oct. 20, 2009

Although the Geochemical Society (www.geochemsoc.org) is headquartered in the United States, our scope and membership is international. The label of "American Geochemical Society" in this story is incorrect.

~Seth Davis, Geochemical Society Business Manager

3.  Tom the Pom said on Oct. 21, 2009

From previous comments it's worth reminding readers of the title of the article. Scientists aren't the parodies some of you reckon they are.

For information, should you choose to use it, here's a summary of some mass balance studies up to circa 2007 and comments on factors that affect InSAR, GRACE and laser/radar altimetry measurements and their interpretation.

4.  Emily Sejetski said on Oct. 21, 2009

I enjoyed reading this article because it keeps me updated with what is going on in the world.

5.  Michael Bohlig said on Oct. 21, 2009

Since I cannot read the online paper (requires a login on the society's Web site), I would like to ask here: Why do the researchers only have two data points from the 18 GPS sites over a six-year period. These data seem to be rather sparse and could potentially be misleading. If the changes in position actually fluctuated non-linearly, what would the implications be for the results?

6.  John W. Bales said on Oct. 22, 2009

The lead sentence contains the claim "the rate of ice loss of the West Antarctic ice sheet has been slightly overestimated." Note the word "slightly."

However, readers who make it to paragraph number eight learn that "researchers do not yet know how large the overestimation was."

How's that for spin?

7.  Henning Fangel said on Oct. 22, 2009

Very interesting observations. When judging glacier increase or decrease, also glacier surging should be taken into account. When any glacier flows and loses height, it also loses energy. Such energy loss converts to heat at the rock base, ice melting and lubrication of the glacier flow, i.e. increased rate of flow and reduced volume of the glacier. When surging stops, the volume of the glacier will again increase.

8.  chris latter said on Oct. 22, 2009

I am a beginning geology student, so much of what this article explains is new for me. However, after reading this news I am more aware of three things: 1) projects like WAGN are intensively collaborative, both nationally (Texas, Memphis, Ohio) and internationally (West Antarctic GPS Network and the Polar Earth Observing Network of the International Polar Year ) as well as prior research; 2) like other kinds of research, the WAGN project is open to interpretation and creates more questions for further research ("may be losing ice" and "suggests . . . slight[ly] overestimated"); and 3) are important, because they help establish arguments that others will use for political and economic ends. (An example is New York Time's Oct. 14, 2009 article "Saudis seek payment for any drop in oil revenues.")

9.  Billy Catringer said on Jan. 17, 2010

Why is there thinning of ice in Antarctica? It has far less to do with temperature than people assume and far more to do with ice under pressure. It flows.

So, we have ice flowing off into the ocean where? On the driest continent known!