A hundred years from now, humans may remember 2014 as the year that wefirst learnedthat we may have irreversibly destabilized the great ice sheet of West Antarctica, and thus set in motionmore than 10 feet of sea level rise.
Meanwhile, 2015 couldbe the year of the double whammy when we learned the same about one gigantic glacier of East Antarctica, which could set in motion roughly the same amount all over again. Northern Hemisphere residents and Americans in particular should take note when the bottom of the world loses vast amounts of ice, those of us living closer to its top getmoresea level rise than the rest of the planet,thanks to the law of gravity.
The findings about East Antarctica emerge from anew paperjust out inNature Geoscience byan international team of scientists representing the United States, Britain, France and Australia. They flew a number of research flights over the Totten Glacier of East Antarctica the fastest-thinning sectorof the worlds largest ice sheet and tooka variety of measurements to try to figure out the reasons behind its retreat. And the news wasnt good: It appears that Totten, too, is losing ice because warm ocean water is getting underneath it.
The idea of warm ocean water eroding the ice in West Antarctica, what were finding is that may well be applicable in East Antarctica as well, saysMartin Siegert, a co-author of the study and who is based at the Grantham Institute at Imperial College London.
The floating ice shelf of the Totten Glaciercovers an area of 90 miles by 22 miles. Itit is losing an amount of ice equivalent to 100 times the volume of Sydney Harbour every year,notesthe Australian Antarctic Division.
Thats alarming, because the glacier holds back a much more vast catchment of ice that, were its vulnerable parts to flow into the ocean, could produce a sea level rise of more than 11 feet whichis comparable to the impact from a lossof the West Antarctica ice sheet.And thats a conservative lower limit, says lead study author Jamin Greenbaum, a PhD candidate at the University of Texas at Austin.
In its alignment with the land and the sea, the Totten Glacier is similar to the West Antarctic glaciers, which also featureice shelvesthat slope out from the vast sheet of ice on land and extendinto the water. These ice shelves are a key source of instability, because if ocean waters beneath them warm, theycan lose ice rapidly, allowing the ice sheet behind them to flow more quickly into the sea.
The researchersused three separate types of measurements taken during their flights gravitational measurements, radar and laser altimetry to get a glimpseof what might be happening beneath the massive glacier, whose ice shelves are more than 1,600 feet thick in places. Using radar, they could measure the ices thickness. Meanwhile, by measuring the pull of the Earths gravity on the airplane in different places, the scientists were able to determine just how far below that ice the seafloor was.
The result was the discovery of two undersea troughs or valleys beneath the ice shelf regions where the seafloor slopes downward, allowing a greater depth of water beneath the floating ice. These cavities or subsea valleys, theresearchers suggest, may explain the glaciers retreat they could allow warmer deep waters to get underneath the ice shelf, accelerating its melting.
Aerial photo of Totten Glaciers ice shelf edge taken during one of the teams geophysical survey flights. (Jamin Greenbaum)
In this particular area of Antarctica, Greenbaum says, a warmer layer of ocean water offshoreis actually deeper than thecolder layers above it, because of the saltwater content of the warm water (which increases its density). And the canyons may allow that warm water access to the glacier base.What we found here is that there are seafloor valleys deeper than the depth of the maximum temperature measured near the glacier, Greenbaum says.
One of these canyons is three miles wide, in a region that was previously believed to simply hold icelying atop solid earth. On the contrary, the new study suggests the ice is instead afloat.
The availability of warm water, and the observed melting, notes the study, support the idea that the behaviour of Totten Glacier is an East Antarctic analogue to ocean-driven retreat underway in the West Antarctic Ice Sheet (WAIS). The global sea level potential of 3.5 m flowing through Totten Glacier alone is of similar magnitude to the entire probable contribution of the WAIS.
For Richard Alley, a glaciologist at Penn State University, the new research hints at a possible solution to a question that scientists have long had about the planets past and in particular thePliocene epoch, beginning 5.3 million years ago, when sea levels weredramatically higher, by as much as 40 meters.
The sea-level indicators from the Pliocene have suggested that an important amount of ice came out of East Antarctica into the ocean, says Alley. Sedimentary records offshore pointed in the same way, and recent modelingshows the strong potential for this to have happened. This new paper adds to the evidence the pieces are fitting together.
One limitation of the study is that the scientists were not able to directly measure the temperature of ocean water that is reaching the glacier itself. While this could be done with robotic underwater vehicles or other methods, that wasnt part of the study at this time. Thus, the conclusions are more focused on inferring the vulnerability of the glacier based on a number of different pieces of evidence topped off by the fact that the glacier is, indeed, retreating.
What we need now is a confirmation of the findings of the paper from oceanographic data, because it is one thing to find potential pathways for warm water to intrude the cavity, it is another to show that thisis actually happening, observesEric Rignot, an Antarctica expert at the University of California, Irvine. This paper comes short of the latter, but other research efforts are underway toget critical oceanographic information near Totten.
Maximum Antarctic sea ice 2014(0:42)
For residents of the United States and indeed, the entire Northern Hemisphere the impact of major ice loss from Antarctica could be dire. If Antarctica loses volumes of ice that would translate into major contributions to sea level rise,that risewould not be distributed evenly around the globe. The reason is the force of gravity. Antarctica is so massive that it pulls the ocean toward it, but if it loses ice, that gravitational pull will relax, and the ocean will slosh back toward the Northern Hemisphere which will experience additional sea level rise.
For the United States, the amount of sea level rise could be 25 percent or more than the global average.
Much as with the ocean-abutting glaciers of West Antarctica, just because a retreat has been observed and because the entirety of the region implies a sea level rise of 11 or more feet were all ice to end up in the ocean does not mean that well see anything near that much sea level rise in our lifetimes. These processes generally are expected to play out over hundreds of years or more. They would reshape the face of the Earth but we may never see it.
The problem, then, is more the world were leaving to our children and grandchildren because once such a gigantic geophysical process begins, its hard to see how it comes to a halt. With warming oceans, its difficult to see how a process that starts now would be reversed, or reversible, in a warming world, Siegert says.
Sea rise predicted at 11 feet.