A ÌìÃÀÓ°ÊÓ´«Ã½ scientist is co-author on a new paper that tracks down why the ice sheet is darkening. The , led by Columbia University, was published March 3 in The Cryosphere.

“According to the satellites, Greenland is darkening by about 2 percent per decade since 1996,” said second author , a research scientist at the UW’s Joint Institute for the Study of the Atmosphere and Ocean. “That seems really small, but it’s actually climatically significant.”

Wildfires have been recently proposed as the of Greenland’s ice sheet, but historical records of fires during that period could not explain the changing reflectivity since the mid-90s, Doherty said.

The authors find instead that most of the darkening is a side effect of warming.

“The lion’s share of the darkening is driven by feedback to the snowpack optical properties,” Doherty said. The study shows that as the glacier melts, the snow crystals get larger and impurities surface – both processes familiar to anybody who has seen old, spring snow.

With warming each year’s snow melts completely to expose the darker glacier ice underneath, and there are more melt pools, which also darkens the surface.

“You don’t necessarily have to have a ‘dirtier’ snowpack to make it dark,” said lead author , a research professor at Columbia University.

Looking forward, the study concludes that continued warming will cause Greenland to absorb about 8 percent more sunlight by the end of this century, with bigger changes along its western edge.

The research draws on the by Doherty and , a UW professor of atmospheric sciences. The team collected some 1,200 snow samples from Northern landscapes, including Greenland. Their follow-up looked at how impurities tend to accumulate at the surface as the snow melts.

By combining the on-the-ground observations with satellite images from 1981 to 2012 and computer models of glaciers, the new study shows the past two decades of darkening can be explained by larger crystals that reflect less sunlight and exposed impurities at the ice surface.

The research also draws on Warren’s earlier studies of and how affects polar landscapes.

For Greenland today it appears that warming, not deposited air pollution, is the primary culprit.

“Using satellites, models and other information to put some bounds on what is happening to the glacier’s surface and what’s causing it, our conclusion is that it’s not an increase in wildfires that is causing the darkening,” Doherty said.

That distinction matters, she said, because it means that regulating particulate emissions from diesels or reducing wildfire smoke would not save Greenland’s ice. Instead, the darkening is being driven by increased melting, which is caused by climate warming.

“There’s a potential for the Greenland ice sheet to contribute significantly to sea-level rise in the next 50 to 100 years,” Doherty said. When calculating future sea-level rise, she added, scientists need to be able to model all the processes that are significant.

“We now have a better understanding of what’s causing the darkening, and we know that it’s significant enough that we need to include it in our models.”

Other co-authors are Xavier Fettweis at Belgium’s University of Liege, Patrick Alexander at NASA’s Goddard Institute of Space Studies, Jeyavinoth Jeyaratnam at the City College of New York, and Julienne Stroeve at the University of Colorado. The research was funded by the National Science Foundation and NASA.

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For more information, contact Doherty at 206-543-6674 or sdoherty@uw.edu.

See also a Columbia University .

Now ÌìÃÀÓ°ÊÓ´«Ã½ atmospheric scientists have estimated that up to half of the recent warming in Greenland and surrounding areas may be due to climate variations that originate in the tropical Pacific and are not connected with the overall warming of the planet. Still, at least half the warming remains attributable to global warming caused by rising carbon dioxide emissions. The is published May 8 in .

Greenland and parts of neighboring Canada have experienced some of the most extreme warming since 1979, at a rate of about 1 degree Celsius per decade, or several times the global average.

“We need to understand why in the last 30 years global warming is not uniform,” said first author , a UW research scientist in atmospheric sciences. “Superimposed on this global average warming are some regional features that need to be explained.”

The study used observations and advanced computer models to show that a warmer western tropical Pacific Ocean has caused atmospheric changes over the North Atlantic that have warmed the surface by about a half-degree per decade since 1979.

“The pattern of the changes in the tropical Pacific that are responsible for remarkable atmospheric circulation changes and warming in Greenland and the Canadian Arctic are consistent with what we would call natural variability,” said co-author , a UW professor of atmospheric sciences.

Researchers say it’s not surprising to find the imprint of natural variability in an area famous for its melting ice. In many of the fastest-warming areas on Earth, global warming and natural variations both contribute to create a “perfect storm” for warming, said co-author , a UW professor of atmospheric sciences.

The natural variations in the new study related to an unusually warm western tropical Pacific, near Papua New Guinea. Since the mid-1990s the water surface there has been about 0.3 degrees hotter than normal. Computer models show this affects the regional air pressure, setting off a stationary wave in the atmosphere that arcs in a great circle from the tropical Pacific toward Greenland before turning back over the Atlantic.

“Along this wave train there are warm spots where the air has been pushed down, and cold spots where the air has been pulled up,” Wallace said. “And Greenland is in one of the warm spots.”

In previous studies, Wallace and Battisti have documented the existence of decades-long climate variations in the Pacific Ocean that resemble the well-known shorter-range El Niño variations.

This particular location in the tropical Pacific may be a sweet spot for generating global atmospheric waves. A series of led by co-author , a UW professor of Earth and space sciences, working with Ding and Battisti, showed that waves starting in the same place but radiating southward are warming West Antarctica and melting the Pine Island Glacier.

Researchers can’t say for how long the tropical Pacific will remain in this state.

“Our work shows that about half of the warming signal in Greenland comes from the predictable part – forcing of climate by anthropogenic greenhouse gases – but about half comes from the unpredictable part,” Steig said.

This makes shorter-term forecasts difficult, but helps scientists to make more accurate long-range projections.

“Nothing we have found challenges the idea that globally, glaciers are retreating,” Battisti said. “We looked at this place because the warming there is really remarkable. Our findings help us to understand on a regional scale how much of what you see is human-induced by the buildup of CO2, and how much of it is natural variability.”

The dramatic message of “Chasing Ice” remains true, authors say.

“There’s nothing in this paper that negates the message in the movie,” Wallace said. “Ice appears to be exquisitely sensitive to the buildup of greenhouse gases, more than we ever would have thought.” Natural variations could either accelerate or decelerate the melting rate of Greenland’s glaciers in coming decades, he said, but “in the long run, the human-induced component is likely to prevail.”

The research was funded by the National Science Foundation, UW’s Quaternary Research Center, the National Basic Research Program of China and the APEC Climate Center. Other co-authors are at the UW; Ailie Gallant at Australia’s Monash University; and Hyung-Jin Kim at South Korea’s APEC Climate Center.

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For more information, contact Ding at qinghua@uw.edu, Wallace at 206-543-7390 or wallace@atmos.washington.edu, Battisti at 206-543-2019 or battisti@uw.edu, and Steig at 206-685-3715 or steig@uw.edu.

NSF grant numbers: OPP 1043092, ATM 1122989.

“So far, on average we’re seeing about a 30 percent speedup in 10 years,” said Twila Moon, a ÌìÃÀÓ°ÊÓ´«Ã½ doctoral student in Earth and space sciences and lead author of a paper documenting the observations published May 4 in Science.

The faster the glaciers move, the more ice and meltwater they release into the ocean. In a previous study, 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 double their velocity between 2000 and 2010 and then stabilize at the higher speed, and another scenario in which the speeds would increase tenfold and then stabilize.

At the lower rate, Greenland ice would contribute about four inches to rising sea level by 2100 and at the higher rate the contribution would be nearly 19 inches by the end of this century. But the researchers who conducted that study had little precise data available for how major ice regions, primarily in Greenland and Antarctica, were behaving in the face of climate change.

In the new study, the scientists created a decadelong 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.

“In some sense, this raises as many questions as it answers. It shows there’s a lot of variability,” said Ian Joughin, a glaciologist in the UW’s Applied Physics Laboratory who is a coauthor of the Science paper and is Moon’s doctoral adviser.

Other coauthors are Benjamin Smith of the UW Applied Physics Laboratory and Ian Howat, an assistant professor of earth sciences at Ohio State University. The research was funded by NASA and the National Science Foundation.

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.

“There’s the caveat that this 10-year time series is too short to really understand long-term behavior,” Howat said. “So there still may be future events – tipping points – that could cause large increases in glacier speed to continue. Or perhaps some of the big glaciers in the north of Greenland that haven’t yet exhibited any changes may begin to speed up, which would greatly increase the rate of 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.

“We can’t look at one glacier for 100 years, but we can look at 200 glaciers for 10 years and get some idea of what they’re doing,” Joughin said.

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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For more information, contact Moon at 406-600-2793 or twilap@uw.edu, or Joughin at 206-221-3177 or irj@uw.edu.

Lubricating meltwater that makes its way from the surface down to where a glacier meets bedrock turns out to be only a minor reason why Greenland’s outlet glaciers accelerated their race to the sea 50 to 100 percent in the 1990s and early 2000s, according to ÌìÃÀÓ°ÊÓ´«Ã½’s Ian Joughin and Woods Hole Oceanographic Institution’s Sarah Das. The two are lead co-authors of two papers posted this week on Science magazine’s Science Express.

The report also shows that surface meltwater is reaching bedrock farther inland under the Greenland Ice Sheet, something scientists had speculated was happening but had little evidence.

“Considered together, the new findings indicate that while surface melt plays a substantial role in ice sheet dynamics, it may not produce large instabilities leading to sea level rise,” says Joughin, a glaciologist with the UW’s Applied Physics Laboratory. Joughin goes on to stress that “there are still other mechanisms that are contributing to the current ice loss and likely will increase this loss as climate warms.”

Outlet glaciers are rapid flows of ice that start in the Greenland Ice Sheet and extend all the way to the ocean, where their fronts break apart in the water as icebergs, a process called calving. While most of the ice sheet moves less than one tenth a mile a year, some outlet glaciers gallop along at 7.5 miles a year, making outlet glaciers a concern because of their more immediate potential to cause sea level rise.

If surface meltwater lubrication at the intersection of ice and bedrock was playing a major role in speeding up the outlet glaciers, one could imagine how global warming, which would create ever more meltwater at the surface, could cause Greenland’s ice to shrink much more rapidly than expected — even catastrophically. Glacial ice is second only to the oceans as the largest reservoir of water on the planet and 10 percent of the Earth’s glacial ice is found in Greenland.

It turns out, however, that when considered over an entire year, surface meltwater was responsible for only a few percent of the movement of the six outlet glaciers monitored, says Joughin, lead author of “Seasonal Speedup along the Western Flank of the Greenland Ice Sheet.” Even in the summer it appears to contribute at most 15 percent, and often considerably less, to the total annual movement of these fast-moving outlet glaciers.

Calculations were made both by digitally comparing pairs of images acquired at different times from the Canadian RADARSAT satellite and by ground-based GPS measurements in a project funded by the National Science Foundation and National Aeronautics and Space Administration.

But while surface meltwater plays an inconsequential role in the movement of outlet glaciers, meltwater is responsible for 50 to 100 percent of the summer speed up for the large stretches near the edge of the ice sheet where there are no major outlet glaciers, a finding consistent with, but somewhat larger than, earlier observations.

“What Joughin, Das and their co-authors confirm is that iceflow speed up with meltwater is a widespread occurrence, not restricted to the one site where previously observed. But, they also show that the really fast-moving ice doesn’t speed up very much with this. So we can expect the ice sheet in a warming world to shrink somewhat faster than previously expected, but this mechanism will not cause greatly faster shrinkage,” says Richard Alley, professor of geosciences at Pennsylvania State University, who is not connected with the papers.

So what’s behind the speed up of Greenland’s outlet glaciers? Joughin says he thinks what’s considerably more significant is when outlet glaciers lose large areas of ice at their seaward ends through increased calving, which may be affected by warmer temperatures. He’s studied glaciers such as Jakobshavn Isbrae, one of Greenland’s fastest-moving glaciers, and says that as ice calves and icebergs float away it is like removing a dam, allowing ice farther uphill to stream through to the ocean more quickly. At present, iceberg calving accounts for approximately 50 percent of the ice loss of Greenland, much of which is balanced by snowfall each winter. Several other studies recently have shown that the loss from calving is increasing, contributing at present rates to a rise in sea level of 1 to 2 inches per century.

“We don’t yet know what warming temperatures means for increased calving of icebergs from the fronts of these outlet glaciers,” Joughin says.

Until now scientists have only speculated if, and how, surface meltwater might make it to bedrock from high atop the Greenland Ice Sheet, which is a half-mile or more thick in places. The paper “Fracture Propagation to the Base of the Greenland Ice Sheet During Supraglacial Lake Drainage,” with Woods Hole Oceanographic Institution’s glaciologist Das as lead author, presents evidence of how a lake that disappeared from the surface of the inland ice sheet generated so much pressure and cracking that the water made it to bedrock in spite of more than half a mile of ice.

The glacial lake described in the paper was 2 to 2 ½ miles at its widest point and 40 feet deep. Researchers installed monitoring instruments and, 10 days after leaving the area, a large fracture developed, a crack spanning nearly the full length of the lake. The lake drained in 90 minutes with a fury comparable to that of Niagara Falls. (The researchers were ever so glad they hadn’t been on the lake in their 10-foot boat with its 5-horsepower engine and don’t plan future instrument deployments when the lakes are full of water. They’ll get them in place only when the lakes are dry.)

Measurements after the event suggest there’s an efficient drainage system under the ice sheet that dispersed the meltwater widely. The draining of multiple lakes each could explain the observed net regional summer ice speedup, the authors write.

Along with Das and Joughin other authors on the two papers are Matt King, Newcastle University, UK; Ben Smith, Ian Howat (now at Ohio State) and Twila Moon of the UW’s Applied Physics Laboratory; Mark Behn and Dan Lizarralde of Woods Hole Oceanographic Institution; and Maya Bhatia, Massachusetts Institute of Technology/WHOI Joint Program.

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For more information:
Joughin, (206) 221-3177, irj@u.washington.edu
Das, sdas@whoi.edu, or contact PIO Mike Carlowicz, (508) 289-3340, mcarlowicz@whoi.edu
Alley, (814) 863 1700, ralley@geosc.psu.edu