So what is really happening in Antarctica?

The recent paper by Zwally et al. in the Journal of Glaciology has been widely reported as evidence that Antarctic is gaining mass, and hence lowering sea level. Is it? Expert Jonathan Bamber weighs in.

Guest post by Jonathan Bamber, University of Bristol

There have been quite few big media stories related to Antarctica recently, including a paper on the irreversible collapse of the marine portion of the West Antarctic Ice Sheet and a NASA-funded study that finds, contrary to numerous previous results, that the Antarctic ice sheet as a whole has been gaining mass between 1992 and 2008. This most recent study received a lot of media attention because it runs counter to what was said in the last IPCC Report. Certain parts of the media hailed this as another sign that the impacts of climate change had somehow been exaggerated a risk that the lead author Jay Zwally was concerned about before the research was published.

So what did Zwally and his colleagues do, what did they find, and why does it contradict a plethora of previous studies that suggest Antarctica has been losing mass over the same time period?

Figure 1. Comparison of various estimates of Antarctic mass balance trends. Vertical dimension of boxes gives the published uncertainty; horizontal gives the time period covered. Figure courtesy of Luke Trusel, Woods Hole Oceanographic Institution.

Zwally and his team measured the changing height of the ice covering Antarctica using two types of instruments — a radar altimeter and a laser altimeter — on two different satellites. The radar provided elevation changes for 1992-2003 and the laser from 2003-2008. They compared the trends in height from the two instruments over a flat part of East Antarctica that covers subglacial Lake Vostok, finding a very good match between the trend for ‘92-’03 and the one for ’03-‘08. So far, so good (though note that they couldn’t compare the two instruments directly because they do not overlap in time.)

Using measurements of elevation change to estimate changes in mass requires knowing the density of the snow. This is the difficult part and one of the key reasons that Zwally’s numbers are so different from previous estimates. The density of snow at the surface of Antarctica is about 1/3 than that of solid ice. Most scientists working with similar data sets agree that over the last ~25 years the surface of the East Antarctic Ice Sheet (EAIS) has been going up very slightly (at about 1-3 cm a year). Previous studies employing altimetry over the EAIS have assumed that the change in elevation is due to a recent increase in snowfall and used a density of snow. They have done this primarily because the interior of the EAIS is moving very slowly and will react very slowly to changes in climate. But there is no evidence for an increase in snowfall from ice cores in East Antarctica and if anything, regional climate models suggest the opposite has been happening over at least the last decade. Zwally and his team argue that instead, mass is actually accumulating in spite of no increase in snowfall. How can this be? The answer is that the ice sheet is still adjusting to the ~doubling snowfall that took place at the end of the last glacial period, between about 18,000 and 12,000 years ago. Because the accumulation rates and temperature in East Antarctica are so low, the ice sheet has a response time to changes in climate of many millennia. So the ice sheet may still be growing even though there has been no increase in accumulation for more than 10 millennia. The difference here is crucial: if the increasing height of 1-3 cm a year is owing to recent increases in snowfall, then we should use the density of snow in the calculation. One cm of snow over the entire East Antarctic Ice Sheet (EAIS) would increase its mass by around 35 Gt (35 billion metric tons). But if the increase in height simply reflects continuing adjustment since the last glacial period, then — Zwally et al. argue — we should be using the density of ice. That would mean an 1 cm increase in height would reflect an increase in mass of ~92 Gt.

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