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Greenland’s Ice Sheet More Stable Than Once Believed

The enormous sheets of ice that lie atop Greenland may not be as prone to catastrophic melting as many scientists thought, even if the planet continues to warm and temperatures remain high for hundreds of years. But while that may sound like good news, new evidence also suggests that parts of the even vaster ice sheets that lie atop Antarctica could be more unstable than once believed.

That’s the conclusion of scientists who have been drilling deep into the Greenland ice sheet since 2007, in a Danish-led project known as the North Greenland Eemian Ice Drilling (NEEM). Their results, published Wednesday in a landmark paper in Nature, show that local temperatures rose some 8°C (14.5°F) higher than they are today during the so-called Eemian period, a stretch of natural global warming that occurred between about 115,000 and 130,000 years ago.

Mt. Erebus rising above an ice-covered Antarctica. New evidence suggests that the vast ice sheets that lie atop Antarctica could be more unstable than once believed.
Credit: NSIDC.
 

“This is higher than most paleo-climate models have suggested,” said Dorthe Dahl-Jensen of the University of Copenhagen, the project leader and lead author of the new study, in an interview.

In one sense, the surprisingly high temperatures could seem reassuring: even the most pessimistic climate models project only eight or nine degrees Fahrenheit of global average warming by the end of this century, and while Arctic temperatures are likely to rise more than the global mean, Greenland’s ice is more likely to remain partially intact than some scientists have feared. But combined with other research, the new paper also suggests that the Antarctic contributed correspondingly more to sea level that was up to 25 feet higher than we see today. 

The ancient warming, say the scientists, also caused extensive melting on what was the surface of the ice sheet more than a thousand centuries ago.

Extensive laboratory analysis of ancient ice samples show that the result of these soaring temperatures, significantly higher than the 9°F of warming climate scientists project at worst by the end of this century, caused extensive melting on what was the surface of the ice sheet more than a thousand centuries ago.

The same sort of thing happened last summer, when 97 percent of the Greenland’s surface ice had undergone some melting by mid-July — something Dahl-Jensen experienced first-hand. “We had an event where it rained at the NEEM site,” she said. “That’s very unusual; we’d never seen it before.” When that happens, she said, “the rain doesn’t run off; it penetrates into the snowpack and refreezes into an ice layer.”

The scientists studying ice from the NEEM drilling cores saw exactly the same sort of refrozen layers in ice from the Eemian period. They also sampled tiny air bubbles trapped by snowflakes that fell during the Eemian, and which were eventually compressed into glacial ice.

Drilling Back to the Future: Climate Clues from Ancient Ice

In July of 2009, Climate Central senior research scientist Heidi Cullen traveled to
Greenland with a production team from StormCenter Communications to visit the North Greenland Eemian Ice Drilling Project, or NEEM.

The amount and the relative abundance of oxygen, nitrogen and other gases in the bubbles told scientists how high the ice surface was above sea level when the Eemian began and when it ended — since then, as now, air has a different composition and density at different altitudes. The conclusion: 128,000 years ago, shortly after the Eemian started, the ice sheet at the NEEM site in northern Greenland was 650 feet higher than it is today. Six thousand later it had melted back by more than a thousand feet, ending up at about 425 feet lower than the present level.

That sounds like a lot, but as Jensen said, “it’s not like the whole ice sheet disappeared.” In fact, she said 75 percent of the ice remained intact, even after 6,000 years of very high global temperatures. This would have driven global sea level up by no more than 2 meters, or about 6 and a half feet. For those who recently experienced the storm surge from Hurricane Sandy, that may seem pretty awful, given that Sandy’s destructive power was amplified by a mere foot or so of sea-level rise since 1900.

But studies of ancient shorelines all over the world have shown that the actual sea-level rise during the Eemian period was as much as 8 meters, or 26 feet, an almost incomprehensively destructive change. Back then, said Hubertus Fischer, at Switzerland’s University of Bern, one of the study’s co-authors, “there were very few humans, and no coastal cities. If we saw such a huge increase now, the consequences would be drastic even if it took a couple of thousand years to unfold.”

It might not take that long, however. If Greenland only accounted for a quarter of that long-ago inundation, Dahl-Jensen said, “this means Antarctica must have contributed the rest.”

As the planet warms under the thickening blanket of greenhouse gases humans are pumping into the atmosphere, Antarctic ice may play a major factor again. “West Antarctica, and parts of East Antarctica are unstable, meaning that the bed of the ice sheet lies as much as a mile below sea level,” Dahl-Jensen said. “If you reduce the thickness of this ice, it could pop up like an iceberg and start floating in the ocean. This would give you rapid sea-level rise, which would be much more dangerous than slow changes.”

While this scenario doesn’t come directly from the NEEM analysis, but rather from the chain of reasoning that leads from it, evidence from other observations suggests there could be something to the idea. A paper that appeared in Nature in April of 2012 noted a thinning in some of Antarctica’s ice shelves tied to warm ocean currents eating at the ice from underneath. A paper in Nature Geoscience a few weeks later projected even more and more rapid, thinning in the future, again thanks to deterioration from below.

None of this necessarily changes the best current projections of sea level rise by the end of this century, which still stand at about 1 to 2 meters, or 3 to 6 feet, by 2100 — especially since temperatures were significantly higher during the Eemian. That’s going to be plenty destructive, agree scientists and urban planners.

But even if the planet never reaches Eemian-level temperatures, Fischer said, “we expect about 5 degrees Celsius [9°F] of warming if we keep blowing out carbon dioxide for the next hundred years.”

If the mercury stays that high for any significant length of time, it will inevitably send sea level even higher, and the message from the NEEM project is clear.

“The West Antarctic ice sheet is clearly more sensitive to warming than we thought,” Fischer said. “This is really a huge message.” 

Related Content
The Story Behind Record Ice Loss in Greenland
Sandy's Storm Surge Explained and Why It Matters
Antarctic Ice Shelves Melting from Below, Study Finds
The Bad News Continues to Flow About Antarctica's Ice

Comments

By Andrew Revkin (Garrison, NY)
on January 23rd, 2013

Meshes with what Richard Alley told me in 2008 about the vision of a stable, if somewhat reduced, Greenland ice sheet even in a warmer world:

“It’s basically a big lump of ice sitting on this bedrock,” Dr. Alley said in describing Greenland’s behavior in warm conditions. “What it tries to do is snow more in the middle and melt more on the edges. If it pulls its edges back, then there’s less area to melt, and that helps it survive. That’s why you can have a stable ice sheet in a warmer climate.”
http://www.nytimes.com/2008/01/08/science/earth/08gree.html

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By David Snällfot (Malmö, Sweden)
on January 24th, 2013

The temperature mentioned for the specific NEEM site on Greenland 8°C (14.5°F) should not be confused with an average temperature increase for the entire planet, say 5°C (9°F) by 2100 .

Typically the average polar region temperatures are twice that of the average planetary temperatures, thus a global increase of 5°C (9°F) would effectively mean 10°C (18°F) for the polar regions.

Peak global temperatures during the Eemian is estimed to be less than 1°C above current temperatures, for example (Hansen & Sato, 2012). http://link.springer.com/chapter/10.1007/978-3-7091-0973-1_2?LI=true

 

 

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By William Hughes-Games (Waipara New Zealand 7447)
on January 27th, 2013

On the other hand, Gaia may have another little surprise for us.
http://mtkass.blogspot.co.nz/2012/11/greenland-melting.html

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By Mike Lemonick (Princeton, NJ 08540)
on January 27th, 2013

David Snällfot raises an important point, leading me to go back and correct the misimpression created by the original (Ben Strauss raised it as well!).

Andy Revkin shows two things, once again: first, that the instincts of experienced climate scientist are often (though not always (right on point). Second, that experienced climate journalists are often way ahead of the curve.

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By Geoff Thomas (Malanda Qld. Australia 4885)
on January 30th, 2013

It is great that the NEEM project has released it’s (preliminary?) reports, and they do substantiate what most informed folk perceived as the likely situation.
Always the most worrying situation was the West Antarctic Ice Shield. (WAIS)
I will explain below, however I would first like to comment on the “believers” mentioned in the article title, as I think that folk who jump on the latest bit of climate “news” and hurriedly spout it to all and sundry, typically never checking the sources nor context, do more harm to the effort of us who try to work against Global Warming, each in our various ways, than the warming Deniers, into whose welcoming laps fall the consequences of these believed falsities.
Back to the WAIS etc, - the Earth has had cooling and warming periods before, and some say that over 50 metres of sea level rise are locked into ice on the poles, - most of it on the East antarctic ice shield.
However in the Pleistocene, our last big warming, the East Antarctic did not melt, nor I gather from the NEEM report, did Greenland entirely, they are on land, so not as much affected by ocean temperatures. paradoxically the WAIS also did not melt entirely but it should have, as it is almost entirely resting on the sea bed.
The WAIS contains a possible 6 metres of sea level rise, probably more like 4 metres in the shorter term, and should the surface sea currents continue bringing warmer water to that area, a collapse such as happened to the Antarctic Peninsular, is possible, - particularly two glacier systems which drain a large percentage of the WAIS, - the Thwaites and Pine Creek glaciers, which flow into the Amundsen Sea, - currently partially blocked by an ice plug which is showing significant instability, - these are worth paying attention to and there may be some reports coming out of the ice drilling currently occurring in Antarctica, after Summer, which is now in Antarctica.
Should that plug let go it would probably, in the very short term, (couple of years ) result in a 2 metre sea level rise.
The Earth has also enormous thermal reserve in the deep ocean currents, some of which come to the surface around Antarctica, - bringing up nutrients from the deep so creating situations for whales etc to breed, and it will be quite a long time before the warmer waters now sinking due to their salt content to join these huge slow currents, will reach such destinations as Antarctica, so the situation is not clear with the balance between the warmer surface currents and the currently colder deeper currents.
If the surface currents win, we will have big problems in our lifetime, if the deeper, more pity our grandchildren as those deep currents are unstoppable.

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By Mike Lemonick (Princeton, NJ 08542)
on January 31st, 2013

Geoff, thanks for that extremely thoughtful comment.

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By Alex J (Portland, OR)
on February 11th, 2013

In addition to David’s point, we also have the black carbon influence on top of the greenhouse gas forcing. How does that factor in to Greenland’s melt, either directly or via sea ice reduction and Arctic ocean warming?

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