Late last week, a study published by NASA scientists in the Journal of Glaciology made the surprising claim that the gigantic continent of Antarctica is actually gaining ice, rather than losing it, to the tune of 82 gigatons (or billion metric tons) per year from 2003 to 2008.
The study has drawn massive amounts of media attention — and no wonder. It contradicts numerous prior scientific claims, including a 2012 study in Science by a small army of polar scientists, a study from earlier this year in Earth and Planetary Science Letters (which found 92 gigatons of net losses per year) and this 2014 study in Geophysical Research Letters (160 gigatons of net losses per year). It also contradicts assertions by the leading consensus body of climate science, the U.N.’s Intergovernmental Panel on Climate Change, which stated in 2013 that Antarctica is “losing mass” and that this process is accelerating. That statement was itself based on multiple studies showing Antarctic ice loss.
Not only does the new research fly in the face of all of this — if true, it also raises serious questions about our current understanding of sea level rise. If Antarctica is actually gaining ice, that means that a significant percentage of the current rise of the seas, estimated at roughly 3.22 millimeters per year by NASA itself, must be coming from elsewhere. (It takes 360 gigatons of ice to raise seas by 1 millimeter).
No wonder, then, that a number of researchers have been quoted expressing skepticism about the new research, even as climate change doubters have had a field day — adding the study to an argumentative arsenal that previously included misleading claims about growing Antarctic sea ice.
So what’s going on here — and what should you make of the new claim that Antarctica isn’t losing ice or raising our seas?
Measurement disputes and burdens of proof. The new NASA study uses satellite data from NASA’s ICEsat (Ice, Cloud and land Elevation Satellite) and the European Remote-sensing Satellite to derive its results, which are based on precision measurements of the elevation of the ice sheet and how that is changing over time. The key finding is that snow-related mass gain atop the ice sheet is more than compensating for the flow of ice outward in glaciers that reach the sea.
“Our interpretation is that this has been going on since the beginning of the last ice age when the snowfall over the continent doubled, the accumulation over the continent doubled, as shown in ice cores,” says H. Jay Zwally, the lead author and a longtime NASA expert on the planet’s ice sheets and methods for studying them with satellites.
More specifically, the research asserts that a “dynamic thickening” of ice over time has occurred as a result of this snowfall. Or as a NASA explanation puts it, “This small thickening, sustained over thousands of years and spread over the vast expanse of these sectors of Antarctica, corresponds to a very large gain of ice – enough to outweigh the losses from fast-flowing glaciers in other parts of the continent and reduce global sea level rise.”
However, this contradicts many other results, including those derived using a different NASA tool — the GRACE satellites, twin measurement devices that orbit the Earth and measure the changing mass of ice based on differential tugs of gravity on the spacecraft as they pass over it. Accordingly, numerous scientists have expressed skepticism, to varying degrees, about the new research.
Andrew Shepherd, a glaciologist at the University of Leeds in the UK who was an author of one of the recent studies cited above finding net Antarctic mass loss, puts it this way in an e-mail to the Post:
Zwally and his team have tried to account for snowfall, which masks changes in the thickness of the polar ice sheets. It’s right to attempt this, but in places where nothing much happens – like the interior of Antarctica, which is a vast desert – it’s really quite difficult to be sure that snowfall can be simulated with enough precision to detect ice imbalance. Fortunately we now have many different ways to examine Earth’s ice sheets – from space and on foot – and I’m confident that we can get to the bottom of this contradiction by taking everything into account.
I also reached an author of the other study cited above finding a net mass loss using GRACE — geoscientist Christopher Harig of Princeton University. Harig defended the GRACE measurements and the finding that Antarctica is losing mass, and said that a key part of the difference between his research and the new study involves how researchers handle something called “glacial isostatic adjustment” or GIA, which refers to the rising of land as the weight of ice has been removed from it since the last ice age.
As Harig put it by e-mail:
GRACE gives us the most direct measurement of mass changes that we have currently. This paper, which uses laser altimetry, claims the discrepancy between our results is due to recent GIA model corrections being incorrect, and that GRACE is more sensitive to the error. If we add back the GIA corrections, and compare our results then, their estimates should agree with ours because we measure mass directly. Instead they are still very far away. Arguing that because their results are different, they must be better, is unsustainable.
Other scientists have also defended the GRACE approach, and criticized the new research. Al Jazeera, for instance, quoted two other top Antarctic researchers — NASA’s Eric Rignot and the National Snow and Ice Data Center’s Ted Scambos — expressing arguably stronger criticisms of the new work than those cited above. Rignot has published some of the most cited work on the rapid retreat and unsettling vulnerability of West Antarctica, and Scambos is an author of a recent study finding that strong winds are wearing down East Antarctica and likely adding 80 gigatons per year to sea level rise — results that pretty clearly trend in a very different direction from the new NASA study.
So there is plenty of skepticism out there — but at the same time, Zwally fully stands by the new research. “We have a very high confidence in our results,” he says. “We have examined the accuracy of our results quite carefully, we believe they’re accurate, and in the paper we report why they differ from other methods, and provide constructive critiques of why they differ.”
In an interview, Zwally also reiterated the notion that GRACE is more sensitive to how one treats glacial isostatic adjustment — clearly, a key point of dispute here.
What’s clear, then, is that we are looking at a significant scientific disagreement — one that turns on different technologies, methodologies, and adjustments. In such a situation, scientists will now need to hash this out and reconcile their understanding of what is happening to the gigantic snowy mass of East Antarctica in particular.
In the meantime, nonspecialists may understandably feel baffled by the technical details at play here. But here’s where they can be more confident: Since the new results contradict a lot of other findings, they will likely face a major challenge when it comes to convincing the Antarctic research community as a whole.
That’s no criticism of the scientists behind the new study — it’s simply how science works. And as the process plays out, it’s fair for scientists and outside observers alike to regard the new research as having to overcome a relatively high burden of proof, and to stay skeptical.
None of which means you should worry any less about Antarctica and sea level rise. In the meantime, the reasons for worry about Antarctica aren’t any less — and particularly when it comes to West Antarctica, a marine based ice sheet that some scientists fear has already begun a process of irreversible collapse. Indeed, a new study released Monday finds that accelerating mass loss in the glaciers of West Antarctica’s Amundsen sea region could undermine the entire ice sheet and set in motion an inevitable, if slow moving, 3.3 meter (or over 10 foot) rise in global sea levels.
All of the research groups involved here agree that a key part of West Antarctica is losing mass and that, if treated on its own rather than as part of the larger continent, is making a net positive contribution to sea level rise. The new NASA study only covers a period from 1992 to 2008, so it excludes more recent observations that other researchers have made in this area. Nonetheless, it too finds accelerating mass loss in one key part of West Antarctica that includes the closely watched Pine Island and Thwaites glaciers, growing from 60 gigatons per year between 1992 to 2001 to 97 gigatons per year between 2003 and 2008.
Zwally is skeptical about some of the more worrying takes on West Antarctic loss, but he certainly doesn’t deny that it’s happening. “There’s no reason to believe that the loss that we see now will not continue for a long time,” he says. “But there’s a serious question about whether it is still accelerating.”
Research drawing on more recent data, however, does suggest the loss rate continues to increase. Another NASA supported study of the Amundsen Sea region alone, published in late 2014, ran from 1992 all the way to 2013, and found that the melt rate had “tripled during the last decade.”
As for what’s actually happening to sea levels if Antarctica is a net gainer rather than loser of ice, Zwally says he thinks the ice is indeed coming from elsewhere — the possibilities include Greenland, the melting of smaller glaciers around the world, and more ocean heat leading to greater thermal expansion of seawater.
Any way you look at it, though, the bottom line is that we’re facing ongoing sea level rise — and an increasing mass loss from Antarctica or a decreasing mass gain (as the new study finds) would both make that problem worse. As Jamin Greenbaum, a researcher at the University of Texas at Austin who studies East Antarctica, put it by e-mail:
Unfortunately, this study doesn’t change the fact that sea level rise has been accelerating since around 1870. Actually, it suggests that Antarctica’s ability to slow that rise is decreasing due to alarming trends in coastal mass loss. For instance, in East Antarctica, Totten Glacier is losing enough mass on its own to balance mass gains in an area larger than Texas.
All in all, then, the current disagreement helps to underscore that even as humans begin to modify the biggest systems and features of the planet, scientists are still struggling to fully measure and comprehend those changes.
Until the current debate gets ironed out, then, there are several possible stances you can take, ranging from fully accepting the new study over the prior ones — as many climate change doubters seem to be doing — to dismissing it out of hand. But the most reasonable one is probably to regard these surprising new findings with a fair dose of skepticism — and to continue worrying about ice losses along the Antarctic coasts.
