Melting Antarctic ice, rising seas: What can we expect?

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Melting Antarctic ice, rising seas: What can we expect?
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kid standing in waist-deep water with a relief truck in the background
Five days after Typhoon Ondoy struck Pasig City, Philippines on Sept. 26, 2009, the water had receded by only two or three inches. High water levels, or storm surges, from hurricanes and typhoons are more dangerous than ever now that they are based on a rising sea.
Credit: Aldrich Lim

Like a triple-whammy from the ice-bound southern continent, two new studies present the same dreadful conclusions:

water droplet Antarctic ice is melting unexpectedly fast

water droplet The melt — and its effect on sea level — will accelerate with passing decades and centuries

water droplet 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.

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aerial view showing ice shelf calving into the ocean
Ice break-up! The front of an ice shelf in West Antarctica shows “calving,” the separation of ice floes as they move north and melt. Ice shelves are the floating extension of ice sheets. Scientists have thought that icebergs are the major ice-removal mechanism, but a new study by NASA and university researchers shows that warming ocean water that melts ice sheets from below accounts for 55 percent of the lost ice. More.
Photo: Operation IceBridge, 2012, NASA/GSFC/Jefferson Beck

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.

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map shows that locations of several ice shelves.
These glaciers in the West Antarctic Ice Sheet are feeding a lot of liquid water into the Southern Ocean, feeding a rise in sea level.

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.

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tourists stand on a temporary walkway in a flooded square
Temporary walkways during a flood in Venice, Italy, in November, 2007. The City of Canals rests on 118 islands, and sea level is a constant concern.

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.

Sea level is increasing steadily — and at an accelerating pace. Rollover image below to see global-warming heat storage in the oceans. The oceans hold about 90 percent of the heat added by global warming. This heat raises sea level by melting the toes of glaciers in Antarctica and Greenland, and also because water expands when heated.
Both images courtesy CSIRO and Bureau of Meteorology, State of the Climate 2014. Do not reproduce in any other publications without separate written permission from CSIRO.
Below: Rates of ice loss are overlaid on a 2009 mosaic of Antarctica, based on data from NASA spacecraft. Circular graphs show total mass loss from each ice shelf, in gigatons per year. Hatched area shows proportion of ice lost due to calving. Black area shows proportion due to underwater melting. Thin black lines show ice shelf perimeter in 2007-2008.

Melting of Antarctic ice shelves

graph showing the melt rate, the ratio between ice loss due to melting and loss due to calving, in different locations in Antarctic. In west Antarctic and Antarctic Peninsula, most loss is due to melting.
graph showing that the ice sheet constantly flows into the ocean, become ice shelf that floats upon the ocean water, and break into icebergs and sea ice.
Ocean water melts ice at the grounding line, lightening the ice sheet. Friction vanishes as the floating glacier slides into the sea. Adjusted from original graphic by GFSC/NASA

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

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Map showing the location of Amundsen Sea sector, which is on the edge of West Antarctica
Studies of ice feeding into the Amundsen Sea from the West Antarctic Ice Sheet show fast movement with significant consequences for sea level. The Amundsen Sea Sector contains enough ice to raise sea level by 1.2 meters. In 1911, Norwegian explorer Roald Amundsen was the first to reach the South Pole. Among his innovations: using dogs to pull sleds instead of the ponies used by ill-fated British explorer Robert Falcon Scott.
Graphic: NASA

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

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graph showing the movement of the grounding line
The contour of its bed affects a glacier’s velocity toward the sea. Eric Rignot’s group enhanced a contour map of the bed of Pine Island glacier by factoring in the mass of the moving ice, using the technique described below. As the thinning ice retreats toward the deeper bed at the center, it will rise, float and advance without friction.
Courtesy Eric Rignot, JPL/University of California – Irvine.

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

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Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive

Bibliography

  1. Marine Ice Sheet Collapse Potentially Underway for the Thwaites Glacier Basin, West Antarctica, Ian Joughin et al, Science 12 May 2014
  2. Sea-level rise and its possible impacts given a ‘beyond 4°C world’ in the twenty-first century, Robert J. Nicholls et al, Phil. Trans. R. Soc. A 13 January 2011 vol. 369 no. 1934 161-181
  3. Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith and Kohler glaciers, West Antarctica from 1992 to 2011. E. Rignot et al, forthcoming, Geophysical Research Letters 2014, DOI 10.1002/2014GL060140
  4. Increased ice losses from Antarctica detected by CryoSat-2, Malcolm McMillan et al, Geophysical Research Letters, DOI: 10.1002/2014GL060111
  5. Rising Seas, Imperiled Cities
  6. John Oliver: A Statistically Representative Climate Change Debate
  7. Will Sea Level Rise Drown Your Town?
  8. What Does the U.S. Look Like after 3 Meters of Sea Level Rise?
  9. Eruptions Rising Sea Levels Might Promote Increased Volcanism