Antarctic Ice: The Cold Truth

This week Science Magazine's on-line SciencExpress reports that Antarctica has been losing large amounts of ice mass over the past three years, contributing to sea level rise at a rate of 0.4 ± 0.2 mm/year. This comes on the heels of a paper published by Science two weeks ago that reported that Greenland was also losing big chunks of ice and contributing to sea level rise at a rate of 0.57 mm/yr.

If this sounds like one of those repeating news stories -- Coup in Haiti, Osama Sends a Tape, etc. -- it is. And so is the response. Natural variability is sufficiently large on yearly and multidecadal time scales that it is simply impossible to conclude that anything other than natural variability is at play in either of these two stories.

The SciencExpress paper by Isabella Velicogna and John Wahr reports on 34 months of data recorded by a new NASA satellite that measures the pull of gravity. Variations in the gravitational field are related to variations in the local mass beneath the satellite. If the mass changes, the satellite observes a different degree of gravitational pull.

Velicogna and Wahl attempted to use the gravity variations observed over Antarctica to determine whether Antarctica was gaining or losing mass. But, their analysis is complicated because variations in gravity can be caused by many things. These include variations in atmospheric pressure (the atmosphere has a certain mass); gravity signals arising from outside of Antarctica; and mass changes from a process known as post-glacial rebound -- slow, ongoing changes to the earth's crust as it adjusts to the removal of its huge ice load from the last ice age. Each of these effects needs to be correctly accounted for before estimating snow and ice changes. After this process, Velicogna and Wahr derived the time history of the variations in ice mass covering Antarctica (from April 2002 through August 2005) shown in Figure 1.

Figure 1. The variations in ice mass (in units of volume) covering Antarctica from April 2002 through August 2005. The blue points are the values uncorrected for post glacial rebound, and the red points represent the ice mass corrected for all external influences. The dashed black line is the trend through the corrected (red) values. (Source: Velicogna and Wahl, 2006).

Additionally, the researchers calculated the ice mass changes for the two major ice sheets across Antarctica -- the Western Antarctic Ice Sheet (WAIS) and the Eastern Antarctic Ice Sheet (EAIS) -- which together cover the vast majority of the continent. Figure 2 shows that the there is no trend in the EAIS (which is about 3 times as large as the WAIS) and that virtually all of the mass loss is coming from the WAIS.

Figure 2. The ice mass variations over the West Antarctic Ice Sheet (red) and the East Antarctic Ice Sheet (green). (Source: Velicogna and Wahl, 2006).

This differs from the results published by Davis et al. in Science magazine just last summer, which used a different satellite and over a longer time period -- May1982 through May 2003. While Davis et al. did find that the smaller WAIS was losing mass, they also found that the much larger EAIS was gaining mass at a rate that exceeded the loss over the WAIS. In total, Davis et al. found that Antarctica was gaining mass (from increased snow accumulation) and contributing to a decline in sea level of about 0.09 mm/yr. The differences between these two results likely lie somewhere in the collection of factors that include different time periods, different spatial coverages, and in analysis uncertainties.

However, one thing is clear. The beginning of the Velicogna and Wahl analysis occurs during an unusually high point in the longer record of Davis et al. (Figure 3). This means that the apparent decline in the record of Velicogna and Wahl may simply be a short term correction to an anomalously high mass gain during a period of long-term mass growth. But who is to know for sure? It is impossible to tell anything about a trend in a system as vast as Antarctica with less than three years worth of data.

Figure 3. The ice mass changes (in terms of elevations change) observed over the East Antarctic Ice Sheet by Davis et al. from May 1992-May 2003. Notice that in mid-2002 (the start of the Velicogna and Wahl analysis) ice mass was at the highest level in the record.

Records of the extent of sea ice surrounding Antarctica are available from satellite observations starting back in the late 1980s. Figure 4 (from the National Snow and Ice Data Center) shows that there has been a slight increase in sea ice during the past two decades. Floating sea ice is a different system than the one being measured by either of the two studies mentioned above. Nevertheless it gives some indication as to what is going on in the environs of the extreme Southern Hemisphere. And it certainly doesn't look like ice is disappearing (notice, however, that there is a lot of variation on the yearly to multi-yearly scale).

Figure 4. Sea ice trends around the coast of Antarctica (Source: http://nsidc.org/data/smmr_ssmi_ancillary/regions/total_antarctic.html)

So, all the Velicogan and Wahl results really demonstrate is that there are short term variations in the amount of ice and snow covering the Antarctic continent. Other data indicate that over the course of the past several decades at least, that the ocean-land system of Antarctica has been experiencing a growth in the amount of snow and ice there (Figure 5).

Figure 5. In some parts of Antarctica, such as East Antarctica, the ice sheet is thickening (+ symbols), whereas in others, primarily in West Antarctica it is thinning (– symbols). (Source: Vaughn, 2005).

There is nothing inherently noteworthy about the results of this three year study of Antarctic ice trends. This is not to disparage the scientific work of Velicogna and Wahl. It is to suggest that their paper serves more as a initial investigation into some of the applications of observations of gravitational variations, rather than bearing any relevance to the issue of global climate change and its implications.

Davis, C. H., et al., 2005. Snowfall-driven growth in East Antarctic ice sheet mitigates recent sea-level rise. Science, 308, 1898-1901.

Rignot, E., and P. Kanagaratnam, 2006. Changes in the velocity structure of the Greenland ice sheet. Science, 31, 986-990.

Velicogna, I., and J. Wahr, 2006. Measurements of time-variable gravity show mass loss in Antarctica. Sciencexpress, March 2, 2006.

Vaughn, D.G., 2005. How does the Antarctic ice sheet affect sea level rise? Science, 308, 1877-1878.

If you are a producer or reporter who is interested in receiving more information about this article or the author, please email your request to interview@tcsdaily.com

I knew that when this article cleared the wires (Anarctic melting!) there would be some level-headed refutation.

I'm glad you didn't say "unbiased", considering that Patrick Michaels has made a career out of a) denying global warming is taking place, and b) once faced with the reality of global warming taking place [and admitting it], denying that anything significant will happen because of it. What's (c) going to be, I wonder? {I hope that wasn't too impolite; Michaels is nothing if not predictable.}

Prior to this study hitting the news, I previously discussed other measurements of the Greenland and Antarctic ice sheets. The Greenland trends are considered in line with global warming predictions when there is a warmer ocean surface; i.e., mass gain at the higher elevations due to increased precipitation caused by increased water vapor in the atmosphere, and increased melting at lower elevations due to higher temperatures. Other Antarctic measurements using satellite altimetry show that the West Antarctic ice sheet is losing significant mass, the East Antarctic ice sheet is gaining a bit, for an overall loss of mass. This is based on nine years of data, not three, but the trends are consistent. Also note that the West Antarctic ice sheet is considered less stable than the East Antarctic ice sheet.

I mean, come on, using a study that catches the downward drop in an otherwise relatively consistent pattern to try to prove an alarmist theory about global warming?

I don't think that they are directly trying to address global warming here. I found a press release on the study, and I'm providing it below. Note the parts I've highlighted.

NASA Mission Detects Significant Antarctic Ice Mass Loss Scientists were able to conduct the first-ever gravity survey of the entire Antarctic ice sheet using data from the joint NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE). This comprehensive study found the ice sheet's mass has decreased significantly from 2002 to 2005.

Isabella Velicogna and John Wahr, both from the University of Colorado, Boulder, conducted the study. They demonstrated for the first time that Antarctica's ice sheet lost a significant amount of mass since the launch of GRACE in 2002. The estimated mass loss was enough to raise global sea level about 1.2 millimeters (0.05 inches) during the survey period; about 13 percent of the overall observed sea level rise for the same period.

The researchers found Antarctica's ice sheet decreased by 152 (plus or minus 80) cubic kilometers of ice annually between April 2002 and August 2005. That is about how much water the United States consumes in three months (a cubic kilometer is one trillion liters; approximately 264 billion gallons of water). This represents a change of about 0.4 millimeters (.016 inches) per year to global sea level rise. Most of the mass loss came from the West Antarctic ice sheet.

"Antarctica is Earth's largest reservoir of fresh water," Velicogna said. "The GRACE mission is unique in its ability to measure mass changes directly for entire ice sheets and can determine how Earth's mass distribution changes over time. Because ice sheets are a large source of uncertainties in projections of sea level change, this represents a very important step toward more accurate prediction, and has important societal and economic impacts. As more GRACE data become available, it will become feasible to search for longer-term changes in the rate of Antarctic mass loss," she said.

Measuring variations in Antarctica's ice sheet mass is difficult because of its size and complexity. GRACE is able to overcome these issues, surveying the entire ice sheet, and tracking the balance between mass changes in the interior and coastal areas.

Previous estimates have used various techniques, each with limitations and uncertainties and an inherent inability to monitor the entire ice sheet mass as a whole. Even studies that synthesized results from several techniques, such as the assessment by the Intergovernmental Panel on Climate Change, suffered from a lack of data in critical regions.

"Combining GRACE data with data from other instruments such as NASA's Ice Cloud and land Elevation Satellite, radar and altimeters that are more effective for studying individual glaciers is expected to substantially improve our understanding of the processes controlling ice sheet mass variations," Velicogna said.

The Antarctic mass loss findings were enabled by the ability of the identical twin GRACE satellites to track minute changes in Earth's gravity field resulting from regional changes in planet mass distribution. Mass movement of ice, air, water and solid earth reflect weather patterns, climate change and even earthquakes. To track these changes, GRACE measures micron- scale variations in the 220 kilometer (137 mile) separation between the two satellites, which fly in formation.

GRACE is managed for NASA by the Jet Propulsion Laboratory (JPL), Pasadena, Calif. The University of Texas Center for Space Research has overall mission responsibility. GeoForschungsZentrum Potsdam (GFZ), Potsdam, Germany, is responsible for German mission elements. Science data processing, distribution, archiving and product verification are managed jointly by JPL, the University of Texas and GFZ. The results will appear in this week's issue of Science.

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Now, I'm sure that policy-sensitive entities will attempt to spin or anti-spin a result like this to suit their biases. But please note that the scientists don't say a whit about global warming, they know that the study is only applicable to the GRACE mission period, and they need more data before they can make longer-term assessments and predictions. Are the scientists involved really trying to prove anything about an "alarmist" theory? Or is Patrick Michaels spinning the story a certain way so that he can soothe his constituency? You decide.

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