The debate over Antarctic Ice

Scientists know with plenty of confidence what is happening to the average temperature at the Earth’s surface, but the effects of global warming at specific locations are not always understood as clearly.  One location that continues to be a subject of extensive research and considerable debate is the continent of Antarctica.  You might expect that a large landmass covered with ice would be experiencing uniformly large losses of ice in a warming world, but the situation is more complicated and nuanced than that.  The most recent (2013) report of the Intergovernmental Panel on Climate Change (IPCC), the international body that reports on the state of climate science to world governments and to the public, concluded with high confidence that the Antarctic Ice Sheet on the whole has been losing ice.  The ice loss comes primarily from the Northern Antarctic Peninsula and the Amundsen Sea sector of West Antarctica (see Figure 1, taken from the IPCC report Climate Change 2013: The Physical Science Basis).  As a consequence, the lost ice is contributing to an acceleration in the rise of seas levels globally.  Note, however, that much of the land mass of Antarctica appears to be gaining some ice, and some sections of the continent are not presently warming.  This conclusion represented the majority of the studies that measured ice changes in Antarctica, but a 2015 study based on satellite data drew a very different conclusion.  The debate over the difference in observed results continues three years later, but a pair of significant papers that came out in June might point toward a resolution.

Figure 1.  West Antarctica is the left side of this illustration, with the Northern Antarctic Peninsula sticking up on the top left.

The major disruption to the consensus opinion of the Antarctic Ice Sheets came from a paper by a research team based at the NASA Goddard Space Flight Center (NASA/GSFC) in Greenbelt, MD.  Led by Jay Zwally, who in 2002 had published a significant paper about the disintegration of the Greenland Ice Sheet, the team looked at data that appeared to show that that Antarctic Ice Sheet was in fact gaining more mass than it was losing.  Much of the data for the study came from the Ice, Cloud, and Land Elevation Satellite (ICESat), a NASA satellite that operated between 2003 and 2009 and used the reflection of laser light to measure changes in height on the Earth’s surface, but other satellite data were used as well.  The data indicated that the height of the ice sheets is rising, not falling, and Zwally and his team concluded from this that the Antarctic Ice Sheets were experiencing a net gain in mass.  This conclusion caused a bit of a stir, to say the least.  Many climate change skeptics posted articles like this one about the result, taking it as proof that global warming was not as bad as “alarmists” in the IPCC were suggesting.  Zwally himself objected to his team’s research being used in such a matter.  At any rate, if the conclusions of the paper held up, it would not change the rate of observed sea level rise; it would, as commentator Jonathan Bamber from the University of Bristol pointed out in a guest post for the RealClimate blog, “make closing the sea level budget a whole lot harder (that is, making the sum of the sinks and sources match the observed rate of sea level rise).”  Ultimately, what that meant was that more research was necessary to understand what caused the discrepancies between what the plurailty of research papers cited in the IPCC had concluded and what Zwally and his team determined from their satellite data.

In their June 14 issue this year, Nature (the UK’s leading weekly magazine on news and major findings within the scientific community) presented a series of papers on Antarctica.  Included in this series was “Mass balance of the Antarctic Ice Sheet from 1992 to 2017,” written collaboratively by a very large number of scientists working under the banner of the Ice sheet Mass Balance Inter-comparison Exercise (IMBIE).  The IMBIE team compiled 24 different estimates of ice sheet balance.  Seven used satellite measurements of surface height, like the Zwally study did, while fifteen used measurements of the strength of the Earth’s gravity at different locations.  The remaining two used the input-output method, where measurements of incoming snowfall are compared against the size of icebergs that have broken off.  The consensus conclusion was that Antarctica has lost ice after all, causing a sea level rise of 7.6 ± 3.9 millimeters globally over the last 25 years.  The findings of the Zwally paper were acknowledged in the appendix, but it was clear that the Zwally paper is an outlier compared to the other studies.

But if the Zwally study’s status as an outlier comes from something that makes its conclusions inaccurate, it still needs to be determined what the source of that inaccuracy may be.  The week after the IMBIE paper was published in Nature, Science (a major weekly publication like Nature, put out by the American Association for the Advancement of Science) published an article from a research team lead by Valentina Barletta of the Technical University of Denmark that looked at how bedrock in West Antartica rises as the weight of the ice above it diminishes due to melting.  (The weight of the ice puts enough pressure on the underlying rock to make it compress, and removing some of the ice causes the rock to decompress.  Both present-day ice melting, and past melting of ice as the Earth emerged from the last Ice Age, can contribute to present-day decompression.)  The team put GPS devices at the top of the bedrock at six different stations in West Antarctica, and found that the bedrock was rising at an average rate of 41 mm per year — much higher than had been anticipated.  This finding suggests that ice mass loss in the West Antarctic Ice Sheet to date has been underestimated.  But on a more optimistic note, it also suggests that the sheet is more stable than previous thought, and less likely to shed a vary large amount of ice into the world’s oceans in a quick but catastrophic collapse.

So how do the two papers tie in to each other?  Zwally’s paper used a series of different values for the rate of bedrock rise, obtained from previous studies at different locations in Antarctica.  For West Antarctica as a whole, the rate of rise cited by Zwally was 26 mm per year.  If the Baretta study’s findings are correct, then the Zwally paper underestimated the bedrock rise by 15 mm/year.  Consequently, in order for the ice height measurements to be correct, an additional 15 mm per year of ice would be melting (or not accumulating in the first place) in West Antarctica.  Now there are some caveats here.  The Baretta study only examined a narrow part of Antarctica, not the continent as a whole.  Further studies on bedrock rise in the rest of the continent are likely forthcoming, however, and if they show that the Zwally study used estimates for the bedrock rise that are too low across the whole of Antarctica, then the conclusions of that paper can be reconciled with other research without needing to explain an error in either the ice height measurements from ICESat or any of the other data that resulted in different conclusions.

The study of the ice sheets in Antarctica is ongoing.  While apparent discrepancies in the findings of different research groups using different methods cannot presently be definitively explained, there is reason to hope that this puzzle will be solved in the relatively near future.

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