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Analysis of Speed of Greenland Glaciers Gives New Insight for Rising Sea Level

Posted on 14 May 2012 by John Hartz

This is a reprint of a news release posted by the National Science Foundation on May 4, 2012.

Researchers determine that although glaciers continue to increase in velocity, the rate at which they can dump ice into the ocean is limited.

Photo of Jakobshavn Ice Fjord

The north branch of Jakobshavn Isbrae is in the upper left corner of the image, with several newly calved icebergs in front of it. The larger, faster moving, south branch is located near the upper right corner. Prior to about 2003, both branches merged to create a large floating ice tongue that extended beyond the iceberg covered area visible in this image. Since the 1990, the glacier calving front (terminus) has retreated about 18 km (11 miles). Now, it is only in the winter that both branches sometimes merge to form a much smaller seasonal ice tongue, which breaks up in the spring.

Credit: Polar Science Center, Applied Physics Laboratory, University of Washington

Changes in the speed that ice travels in more than 200 outlet glaciers indicates that Greenland's contribution to rising sea level in the 21st century could be significantly less than the upper limits some scientists thought possible.

The finding comes from a paper funded by the National Science Foundation (NSF) and NASA and published in today's (May 4, 2012) journal Science.

While the study indicates that a melting Greenland's contributions to rising sea levels could be less than expected, researchers concede that more work needs to be done before any definitive trend can be identified.

Studies like this one are designed to examine more closely and in greater detail what is actually happening with the ice sheets, often using newer and more precise tools and thereby better defining the parameters that scientists use to make predictions, such as the upper limits of sea-level rise.

"This study provides more evidence that the rate at which these glaciers can dump ice into the ocean is indeed limited," said Ian Howat, assistant professor of Earth sciences and member of the Byrd Polar Research Center at Ohio State University, a co-author on the paper. "What remains to be seen is how long the acceleration will continue--but it appears that our worst-case scenarios aren't likely."

The fate of the Earth's ice sheets and their potential contributions to sea-level rise as the globe warms are among the major scientific uncertainties cited in the Fourth Assessment of the Intergovernmental Panel on Climate Change (IPCC). This is in part because the Greenland and Antarctic ice sheets have historically been, and in large measure continue to be, relatively sparsely monitored, as compared to other parts of the globe.

The faster the glaciers move, the more ice and melt water they release into the ocean.

In previous studies, scientists trying to understand the contribution of melting ice to rising sea level in a warming world considered a scenario in which the Greenland glaciers would either double or increase by as much as ten-fold their velocity between 2000 and 2010 and then stabilize at the higher speed.

This new study shows Greenland ice would likely move at the lower rate--a doubling of its speed--and contribute about four inches to rising sea level by 2100. The previous studies used the higher speed and estimated the glaciers would contribute nearly 19 inches by the end of this century.

In the new study, the scientists extracted a decade-long record of changes in Greenland outlet glaciers by producing velocity maps using data from the Canadian Space Agency's Radarsat-1 satellite, Germany's TerraSar-X satellite and Japan's Advanced Land Observation Satellite. They started with the winter of 2000-01 and then repeated the process for each winter from 2005-06 through 2010-11 and found that the outlet glaciers had not increased in velocity as much as had been speculated.

"So far, on average we're seeing about a 30 percent speedup in 10 years [of Greenland glaciers, which gives new insight for rising sea level]," said Twila Moon, a University of Washington doctoral student in Earth and space sciences and lead author of the paper documenting the observations.

"This study is a great example of the power of high-resolution data sets in both space and time, and the importance of looking carefully at as much data as possible in helping make the best predictions we can of future changes", said Henrietta Edmonds, program director for Arctic Natural Sciences in NSF's Office of Polar Programs.

The scientists saw no clear indication in the new research that the glaciers will stop gaining speed during the rest of the century, and so by 2100 they could reach or exceed the scenario in which they contribute four inches to sea level rise.

The record showed a complex pattern of behavior. Nearly all of Greenland's largest glaciers that end on land move at top speeds of 30 to 325 feet a year, and their changes in speed are small because they are already moving slowly. Glaciers that terminate in fjord ice shelves move at 1,000 feet to a mile a year, but didn't gain speed appreciably during the decade.

In the East, Southeast and Northwest areas of Greenland, glaciers that end in the ocean can travel seven miles or more in a year. Their changes in speed varied (some even slowed), but on average the speeds increased by 28 percent in the Northwest and 32 percent in the Southeast during the decade.

Moon said she was drawn to the research from a desire to take the large store of data available from the satellites and put it into a usable form to understand what is happening to Greenland's ice. "We don't have a really good handle on it and we need to have that if we're going to understand the effects of climate change," she said. "We are going to need to continue to look at all of the ice sheet to see how it's changing, and we are going to need to continue to work on some tough details to understand how individual glaciers change."

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Comments 1 to 4:

  1. These findings, that loss of ice from the GIS are likely to contribute no more than 4 inches to sea level by 2100 stand in stark contrast to the Hansen and Sato (2011) prognosis that total ice sheet loss can be expected to double per decade resulting in SLR of 5 metres by 2100. Given the speed with which atmospheric and sea temperature is rising in the Arctic, on-going loss of albedo, and the rate of loss of land-based ice over the period 2000-2009, the conclusions reached in this article should, perhaps, be regarded with …. skepticism.
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  2. While the changes in the Arctic are indisputably very dramatic, I am hesitant to draw too simplistic an extrapolation of what is going on. I presume the long term impact of this work, if correct, would be that the current quadratic trend in the GRACE Greenland ice loss will at some point hit a maximum rate and then stop accelerating. That doesn't seem fundamentally implausible to me. When it comes to experts, while I go to Hansen first on climate modelling, when it comes to glaciology I'll certainly pay careful attention to what the specialists in that field have to say. Having said that, all new results are provisional, and many turn out to be wrong.
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  3. Agnostic, in Hansen and Sato's discussion their scenario is very explicit: "If either ice sheet [[Greenland and Antarctica]] were to lose mass at a rate with doubling time of 10 years or less, multi-meter sea level rise would occur this century." Hansen and Sato's prognosis explicitly depends on continued acceleration and the new work of Howat seems to put upper limits on this acceleration. Let's hope they are correct. Both studies however seem to agree on eventual (on centuries timescales) multi-meter sealevel rise. So the discussion here is the rate of change, not the scale of the change.
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  4. Moon et al (2012) further the examination of flow velocities of Greenland glaciers. The results in terms of flow speed changes fit previous observations. Zebras in Greenland examined the reasons why the glaciers behave differently. The sea level change noted by Moon et al (2012) is for dynamic changes only not for increased basal or surface melting. Epiq Sermia is one of the glaciers that has not accelerated dramatically but has still thinned and retreated. Upernavik Glacier did accelerate quite a bit and has lost even more area
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