Melting Antarctic ice, rising seas: What can we expect?
Like a triple-whammy from the ice-bound southern continent, two new studies present the same dreadful conclusions:
Antarctic ice is melting unexpectedly fast
The melt — and its effect on sea level — will accelerate with passing decades and centuries
There is no apparent mechanism to stop the ice from sliding into the ocean
In other words, the long-feared melting of the giant Antarctic ice sheet has begun. Abandoning the caution that befogged previous analyses, scientists used the headline-friendly term “collapse” to describe conditions on the West Antarctic Ice Sheet.
“We wanted to know, is this really instability kicking in?” said Ian Joughin, at the University of Washington Polar Science Center. Joughin was first author of a study1 of the future of two glaciers feeding the Amundsen Sea in West Antarctica. “The [computer] model indicates that there is not much of a stabilizing influence, and as it evolves it only gets worse,” he told us.
We asked how those glaciers, called Thwaites and Pine Island, rank among the factors that can cause sea-level rise. “Pretty near the top, if not the top,” he said.
The basis for concern about ice sheets is simple: The planet’s two ice sheets are both looking unstable, and even a partial melting could raise sea level by several meters, obliterating coastal cities and settlements. A 2011 study 2, based on a temperature rise of 4°C., anticipated a sea level rise of 0.5 to 2 meters, which would displace 72 million to 187 million people.
According to a 2010 report in Environment@Harvard, “An added 0.5 meters (20 inches) of ocean water by the year 2050 would put $28 trillion in assets at risk in the world’s 136 port megacities, according to a 2009 report of scientists and insurance experts assembled by World Wide Fund/Allianz, a global investment and insurance company.”
The two studies brought a renewed sense of gloom and doom to the on-again, mostly off-again discussion of climate change. The word “collapse” ensured attention, especially after an ominous U.S. government report on the harm due to climate-change.
Yet even as the melting accelerates, its short term effects will be minor, contributing about one-sixth of the total sea level rise of 3.3 millimeters per year (1.3 inches in 10 years). Any rise, however gradual, concerns coastal cities from London to New York to Miami to Tokyo and Shanghai — all built on low-lying land near natural harbors.
The effects of sea level rise are most dramatic during storms like Hurricane Sandy (2102) in New York and New Jersey, when the wind piles up water, causing floods.
Both studies affirmed that the glaciers are speeding up, and forecast further acceleration. Some of the Antarctic glaciers under study were advancing by 2.5 kilometers per year in the 1970s; today they cover four kilometers in a year. “Some glaciers in Greenland move up to 15 kilometers per year,” Eric Rignot of NASA and the University of California – Irvine told us, “so it’s easy to conceive that these Antarctic glaciers could speed up by a factor of four.”
Rignot was first author of a new, satellite-radar study3, of movement on the West Antarctic Ice Sheet.
Ice on the move
The ice under study contain enough water to raise global sea levels by 4 feet, or 1.2 meters. The glaciers help restrain the entire West Antarctic Ice Sheet, which, if it melts, would raise sea level by three or four meters.
That’s unlikely to happen for centuries at least, but the recent U.N. Intergovernmental Panel on Climate Change (IPCC) report estimates sea-level rise from almost one foot to about three feet (26 to 98 centimeters) by 2100. These projections, however, largely ignored the possibility of major ice loss in Antarctica.
Rignot expects that sea level rise by 2100 will be closer to the high end of the IPCC range.
An unpublished study4 figured that the entire white continent is losing about 160 billion tons of ice per year.
Time for Noah? Seas on the rise
Sea level is more difficult to measure than you might expect: it’s affected by wind, atmospheric pressure, water temperature, and the configuration of the coast. In places like New Orleans, the land is sinking, amplifying the effect rate of a rising ocean. And in parts of Alaska, the land is rising after immensely heavy glaciers have melted, counteracting the rising sea.
Globally, the sea is rising about 3.3 millimeters per year:
1 millimeter from melting ice shelves on Antarctica and Greenland
1 millimeter from melting mountain glaciers
1.3 millimeters from the expansion of warming ocean water
New, improved observations have clarified the factors driving sea level rise, says Joughin. “In previous IPCC [Intergovernmental Panel on Climate Change] reports, all the sources did not add up, but I believe they do now, within the uncertainties in the data.”
Ice at the precipice
Rather than rely on models, Rignot’s study focused on satellite radar measurements that showed the exact surface contours of six ice streams flowing into the Amundsen Sea between 1992 and 2011.
The Rignot study focused on the “grounding line,” the last point where a glacier touches land and begins to float. As more surface is exposed to relatively warm water, all resistance to forward motion disappears.
Radar pinpoints the grounding line by identifying the line where the now-floating ice starts to rise and fall on the tide.
At the center of Pine Island glacier, the grounding line retreated 31-kilometers in 20 years; at Thwaites glacier, it retreated 14 kilometers.
More is yet to come: “Upstream of the 2011 grounding line positions,” Rignot’s group concluded. “We found no major bed obstacle that would prevent the glaciers from further retreat.”
Study leader Eric Rignot minced no words: “The collapse of this sector of West Antarctica appears to be unstoppable. The fact that the retreat is happening simultaneously over a large sector suggests it was triggered by a common cause, such as an increase in the amount of ocean heat beneath the floating parts of the glaciers. At this point, the end appears to be inevitable.”
Grounding line movement on Pine Island Glacier
The warming climate gets the blame, indirectly, in this melting-disintegration-collapse. Once transferred to the ocean, the heat melts ice at the grounding line, causing the majority of melting in Antarctic ice.
The “mass conservation” technique that Rignot used to sharpen his results reminds us of basic accounting. “We look at the flux of ice and make sure as it moves downstream, we conserve the mass,” says Rignot. “It’s a way of refining our reconstruction of the bed beneath the ice sheet to a higher level of detail, and removing potential errors.”
Melting starts a self-reinforcing process, Joughin told us. When the ice thins, it loses weight and lifts off at the grounding line, exposing more itself to more water while simultaneously loosening the grip of its rocky base — all of which tend to speed up the flow. “The melting causes thinning, and that causes more ice to float,” says Joughin. “If there wasn’t this feedback, we would not see a huge amount of change.”
All this motion came as a bit of a surprise, Joughin said. “Ten or 20 years ago, when I started measuring ice flow, everybody said if you measure the speed once, you are done, glaciers respond on a much slower time scale. So it was pretty extraordinary when all these observations from several groups showed that a glacier could change rapidly. Pine Island glacier could accelerate from 2,500 meters a year to 4,000 meters in a decade or two.”
When glaciologists say “collapse,” they are talking on a geological time scale, and yet Rignot sees immediate significance in his results. “We have reached one of those tipping points in climate change, when things change in a way that makes it difficult or impossible to change back. This is one sign that we need to care about these issues, and do something about them.”
– David J. Tenenbaum