The ice shelf on Antarctica鈥檚 Pine Island Glacier lost about one-fifth of its area from 2017 to 2020, mostly in three dramatic breaks. The timelapse video incorporates satellite images from January 2015 to March 2020. For most of the first two years, the satellite took high-resolution images every 12 days; then for more than three years it captured images of the ice shelf every six days. Images are from the Copernicus Sentinel-1 satellites operated by the European Space Agency on behalf of the European Union.
Credit: Joughin et al./Science Advances

For decades, the ice shelf helping to hold back one of the fastest-moving glaciers in Antarctica has gradually thinned. Analysis of satellite images reveals a more dramatic process in recent years: From 2017 to 2020, large icebergs at the ice shelf鈥檚 edge broke off, and the glacier sped up.

Since floating ice shelves help to hold back the larger grounded mass of the glacier, the recent speedup due to the weakening edge could shorten the timeline for Pine Island Glacier鈥檚 eventual collapse into the sea. The from researchers at the 天美影视传媒 and British Antarctic Survey was published June 11 in the open-access journal Science Advances.

鈥淲e may not have the luxury of waiting for slow changes on Pine Island; things could actually go much quicker than expected,鈥� said lead author , a glaciologist at the UW Applied Physics Laboratory. 鈥淭he processes we鈥檇 been studying in this region were leading to an irreversible collapse, but at a fairly measured pace. Things could be much more abrupt if we lose the rest of that ice shelf.鈥�

Pine Island Glacier contains approximately 180 trillion tons of ice 鈥� equivalent to 0.5 meters, or 1.6 feet, of global sea level rise. It is already responsible for much of Antarctica鈥檚 contribution to sea-level rise, causing about one-sixth of a millimeter of sea level rise each year, or about two-thirds of an inch per century, a rate that鈥檚 expected to increase. If it and neighboring Thwaites Glacier speed up and flow completely into the ocean, releasing their hold on the larger West Antarctic Ice Sheet, global seas could rise by several feet over the next few centuries.

These glaciers have attracted attention in recent decades as their ice shelves thinned because warmer ocean currents melted the ice鈥檚 underside. From the 1990s to 2009, Pine Island Glacier鈥檚 motion toward the sea accelerated from 2.5 kilometers per year to 4 kilometers per year (1.5 miles per year to 2.5 miles per year). The glacier鈥檚 speed then stabilized for almost a decade.

Results show that what鈥檚 happened more recently is a different process, Joughin said, related to internal forces on the glacier.

From 2017 to 2020, Pine Island鈥檚 ice shelf lost one-fifth of its area in a few dramatic breaks that were captured by the Copernicus Sentinel-1 satellites, operated by the European Space Agency on behalf of the European Union. The researchers analyzed images from January 2015 to March 2020 and found that the recent changes on the ice shelf were not caused by processes directly related to ocean melting.

鈥淭he ice shelf appears to be ripping itself apart due to the glacier鈥檚 acceleration in the past decade or two,鈥� Joughin said.

Two points on the glacier鈥檚 surface that were analyzed in the paper sped up by 12% between 2017 and 2020. The authors used an ice flow model developed at UW to confirm that the loss of the ice shelf caused the observed speedup.

鈥淭he recent changes in speed are not due to melt-driven thinning; instead they’re due to the loss of the outer part of the ice shelf,鈥� Joughin said. 鈥淭he glacier鈥檚 speedup is not catastrophic at this point. But if the rest of that ice shelf breaks up and goes away then this glacier could speed up quite a lot.鈥�

It鈥檚 not clear whether the shelf will continue to crumble. Other factors, like the slope of the land below the glacier鈥檚 receding edge, will come into play, Joughin said. But the results change the timeline for when Pine Island鈥檚 ice shelf might disappear and how fast the glacier might move, boosting its contribution to rising seas.

鈥淭he loss of Pine Island鈥檚 ice shelf now looks like it possibly could occur in the next decade or two, as opposed to the melt-driven subsurface change playing out over 100 or more years,鈥� said co-author , an ocean physicist at British Antarctic Survey. 鈥淪o it’s a potentially much more rapid and abrupt change.鈥�

Pine Island鈥檚 shelf is important because it鈥檚 helping to hold back this relatively unstable West Antarctic glacier, the way the curved buttresses on Notre Dame cathedral hold up the cathedral鈥檚 mass. Once those buttresses are removed, the slow-moving glacier can flow more quickly downward to the ocean, contributing to rising seas.

鈥淪ediment records in front of and beneath the Pine Island ice shelf indicate that the glacier front has remained relatively stable over a few thousand years,鈥� Dutrieux added. 鈥淩egular advance and break-ups happened at approximately the same location until 2017, and then successively worsened each year until 2020.鈥�

Other co-authors are and at the UW; and Mark Barham at British Antarctic Survey. The study was funded by the U.S. National Science Foundation, NASA and the U.K. Natural Environment Research Council.

For more information, contact Joughin at ian@apl.washington.edu and Dutrieux at pitr1@bas.ac.uk

NSF: OPP-1643285, NASA grant: NNX17AG54G

天美影视传媒 researchers used detailed topography maps and computer modeling to show that the collapse appears to have already begun. The fast-moving Thwaites Glacier will likely disappear in a matter of centuries, researchers say, raising sea level by nearly 2 feet. That glacier also acts as a linchpin on the rest of the ice sheet, which contains enough ice to cause another 10 to 13 feet (3 to 4 meters) of global sea level rise. The is published May 16 in . (A separate , to be published in Geophysical Research Letters, comes to a similar conclusion.)

“There’s been a lot of speculation about the stability of marine ice sheets, and many scientists suspected that this kind of behavior is under way,” said lead author , a glaciologist at the UW’s Applied Physics Laboratory. “This study provides a more quantitative idea of the rates at which the collapse could take place.”

The good news is that while the word “collapse” implies a sudden change, the fastest scenario is 200 years, and the longest is more than 1,000 years. The bad news is that such a collapse may be inevitable.

“Previously, when we saw thinning we didn’t necessarily know whether the glacier could slow down later, spontaneously or through some feedback,” Joughin said. “In our model simulations it looks like all the feedbacks tend to point toward it actually accelerating over time; there’s no real stabilizing mechanism we can see.”

Earlier warnings of collapse had been based on a simplified model of ice sitting in an inward-sloping basin. The topography around Antarctica, however, is complex. The new study used , developed at the University of Kansas with funding from the National Science Foundation, to image through the thick ice and map the topography of the underlying bedrock, whose shape controls the ice sheet’s long-term stability. The mapping was done as part of NASA鈥檚 Operation , and included other instruments to measure the height of the ice sheet鈥檚 rapidly thinning surface. In some places Thwaites Glacier has been losing tens of feet, or several meters, of elevation per year.

UW researchers combined that data with their own satellite measurements of ice surface speeds. Their computer model was able to reproduce the glacier’s ice loss during the past 18 years, and they ran the model forward under different amounts of ocean-driven melting.

The place where the glacier meets land, the grounding line, now sits on a shallower ridge with a depth of about 2,000 feet (600 meters). Results show that as the ice edge retreats into the deeper part of the bay, the ice face will become steeper and, like a towering pile of sand, the fluid glacier will become less stable and collapse out toward the sea.

“Once it really gets past this shallow part, it’s going to start to lose ice very rapidly,” Joughin said.

The study considered future scenarios using faster or slower melt rates depending on the amount of future warming. The fastest melt rate led to the early stages lasting 200 years, after which the rapid-stage collapse began. The slowest melt rate kept most of the ice for more than a millennium before the onset of rapid collapse. The most likely scenarios may be between 200 and 500 years, Joughin said.

“All of our simulations show it will retreat at less than a millimeter of sea level rise per year for a couple of hundred years, and then, boom, it just starts to really go,” Joughin said.

Researchers did not model the more chaotic rapid collapse, but the remaining ice is expected to disappear within a few decades.

The thinning of the ice in recent decades is most likely related to climate change, Joughin said. More emissions would lead to more melting and faster collapse, but other factors make it hard to predict how much time we could buy under different scenarios.

The other co-authors are at the UW and , who did her doctorate at the UW and is now at NASA’s Goddard Space Flight Center. The research was funded by the National Science Foundation and NASA.

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For more information, contact Joughin at 206-221-3177 or ian@apl.washington.edu.

Grant numbers: NSF: ANT-0424589 and ANT-0631973, NASA NNX09AE47G

“We are now seeing summer speeds more than four times what they were in the 1990s, on a glacier which at that time was believed to be one of the fastest, if not the fastest, glacier in Greenland,” said lead author , a glaciologist at the UW’s Polar Science Center.

The show that in summer of 2012 the glacier reached a record speed of more than 10.5 miles (17 kilometers) per year, or more than 150 feet (46 meters) per day. These appear to be the fastest flow rates recorded for any glacier or ice stream in Greenland or Antarctica, researchers said.

The scientists note that the summer speeds are temporary, with the glacier flowing more slowly over the winter months. But they point out that even the glacier’s average annual speed over the past couple of years is nearly three times its average annual speed in the 1990s.

This speedup of Jakobshavn means that the glacier is adding more and more ice to the ocean, contributing to sea-level rise.

“We know that from 2000 to 2010 this glacier alone increased sea level by about 4/100 of an inch (1 millimeter). With the additional speed it likely will contribute a bit more than this over the next decade,” Joughin said.

Jakobshavn Glacier,聽which is widely believed to be the glacier that produced the large iceberg that sank the Titanic in 1912, drains the Greenland ice sheet into a deep-ocean fjord on the west coast of the island. At its calving front, where the glacier effectively ends as it breaks off into icebergs, some of the ice melts while the rest is pushed out, floating into the ocean. Both of these processes contribute about the same amount to sea-level rise from Greenland.

As the Arctic region warms, Greenland’s glaciers have been thinning and calving icebergs farther and farther inland. This means that even though the glacier is flowing toward the coast and carrying more ice into the ocean, its calving front is actually retreating. In 2012 and 2013, Jakobshavn’s front retreated around 0.6 miles (1 km) each year compared to its position the previous summer.

In Jakobshavn’s case, the thinning and retreat coincide with the increase in speed. The calving front of the glacier is now located in a deeper area of the fjord, where the underlying rock bed is about 0.8 miles (1.3 km) below sea level, which the scientists say explains the record speeds.

“As the glacier’s calving front retreats into deeper regions, it loses ice 鈥� the ice in front that is holding back the flow 鈥� causing it to speed up,” Joughin said.

The team used satellite data to measure the glacier as part of studies funded by NASA and the U.S. National Science Foundation.

“We used computers to compare pairs of images acquired by the German Space Agency’s . As the glacier moves we can track changes between images to produce maps of the ice flow velocity,” Joughin said.

The researchers believe Jakobshavn is unstable, meaning it will continue to retreat further inland. By the end of this century its calving front could retreat as far back as the head of the fjord through which the glacier flows, about 31 miles (50 km) upstream from where it is today.

“The thing that’s remarkable about the Jakobshavn Glacier is that even after all the mass that it has already lost, it is able to keep doing it, year after year,” said co-author , a glaciologist at the UW’s Polar Science Center. “A smaller glacier would settle down after losing that much mass. Jakobshavn’s ability to drain ice from the ice sheet is really exceptional among all of the glaciers in Greenland.”

The other co-authors are , a UW doctoral student in Earth and space sciences, and Dana Floricioiu at the German Aerospace Center.

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For more information, contact Joughin at 206-221-3177 or ian@apl.washington.edu and Smith at 206-616-9176 or bsmith@apl.washington.edu.

Adapted from a by the European Geosciences Union.

Dozens of climate scientists have reconciled their measurements of ice sheet changes in Antarctica and Greenland during the past two decades. The results, published Nov. 29 in the journal , roughly halve the uncertainty and discard some conflicting observations.

“We are just beginning an observational record for ice,” said co-author , a glaciologist in the 天美影视传媒’s Applied Physics Laboratory who is lead author on an accompanying review article. “This creates a new long-term data set that will increase in importance as new measurements are made.”

The examined three methods that had been used by separate groups and established common places and times, allowing researchers to discard some outlying observations and showing that the results agree to within the uncertainties of the methods.

“It provides a simpler picture,” said co-author , a research scientist at the UW’s Applied Physics Laboratory. “In the 1990s, not very much was happening. Sometime around 1999, the ice sheets started losing more mass, and probably have been losing mass more rapidly over time since then.”

The effort, led by Andrew Shepherd at the University of Leeds in the UK, reconciles three existing ways to measure this loss. The first method takes an accounting approach, combining climate models and observations to tally up the ice gain or loss. Two other methods use special satellites to precisely measure the height and gravitational pull of the ice sheets to calculate how much ice is present.

Each method has strengths and weaknesses. Until now scientists using each method released estimates independent from the others. This is the first time they have all compared their methods for the same times and locations.

“It brought everyone together,” Joughin said. 鈥淚t鈥檚 comparing apples to apples.鈥�

Since 1998, scientists have published at least 29 different estimates of how much ice sheets have contributed to sea-level rise, ranging from 1.9 mm (0.075 inches) a year to 0.2 mm (0.0079 inches) drop per year. The new, combined estimate is that ice sheets have since 1992 contributed on average 0.59 mm (0.023 inches) to sea-level rise per year, with an uncertainty of 0.2 mm per year. Overall sea levels have risen by about 3.3 mm per year during that time period, much of which is due to expansion of warmer ocean waters.

鈥淓stablishing more consistent estimates for the contribution from ice sheets should reduce confusion, both among the scientific community and among the public,鈥� Joughin said.

Understanding why the ice sheets have been shedding mass faster in the last decade is an area of intense research. The accelerated ice loss was not predicted by the models, leading the latest Intergovernmental Panel on Climate Change report to place no upper limit on its estimate for future ice-sheet loss.

Joughin is lead author of an that reviews factors that cause ice sheets to lose more mass. In particular, it looks at what happens when warmer ocean waters reach the underside of large floating Antarctic ice sheets or abut glaciers in Greenland鈥檚 fjords.

Joughin and his co-authors, Richard Alley of Pennsylvania State University and David Holland of New York University, suggest ways to better monitor and understand those changes: Create finer-grained ocean models that could include narrow fjords, develop more models to study the interaction between ice sheets and ocean water, and improve ice sheet monitoring.

Taking measurements at ice edges is perilous, they write, because skyscraper-sized chunks of ice can topple on floating instruments with no notice, and outgoing glaciers can scour any instruments moored to the ocean floor.

Understanding ice sheets is central to modeling global climate and predicting sea-level rise. Even tiny changes to sea level, when added over an entire ocean, can have substantial effects on storm surges and flooding in coastal and island communities.

The West Antarctic Ice Sheet could trigger abrupt changes globally if it were to become unstable, and although Greenland is thought to be more stable, the recent calving of glaciers has led to some alarm.

Joughin believes the recent activity is a reason to pay attention, but not to panic.

鈥淲e don鈥檛 fully understand why it鈥檚 accelerating,鈥� Joughin said. “But the longer-term observations we have, the more solid predictions we will be able to make.”

The UW portions of the research were funded by the National Science Foundation and the National Aeronautics and Space Administration.

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For more information, contact Joughin at 206-221-3177 or ian@apl.washington.edu and Smith at 206-616-9176 or bsmith@apl.washington.edu.

“So far, on average we’re seeing about a 30 percent speedup in 10 years,” said Twila Moon, a 天美影视传媒 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 鈥� tipping points 鈥� 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 天美影视传媒’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 陆 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

composite images down to where its leading edge meets an ice-filled bay, the

surface of which looks whiter than anything else in the images. Conditions
were such in 2005 that the glacier lost considerable amount of ice off its
leading edge or, as scientists say, it had retreated so the area of the bay
filled with ice is greater. Then the edge re-established itself in 2006.

Two of Greenland’s largest glaciers shrank dramatically and dumped twice as much ice into the sea during a period of less than a year between 2004 and 2005. And then, less than two years later, they returned to near their previous rates of discharge.

The variability over such a short time, reported online Feb. 9 on Science magazine’s Science Express, underlines the problem in assuming that glacial melting and sea level rise will necessarily occur at a steady upward trajectory, according to lead author Ian Howat, a post-doctoral researcher with the 天美影视传媒’s Applied Physics Laboratory and the University of Colorado’s National Snow and Ice Data Center. The paper comes a year after a study in the journal Science revealed that discharge from Greenland’s glaciers had doubled between 2000 and 2005, leading some scientists to speculate such changes were on a steady, upward climb.

“While the rates of shrinking of these two glaciers have stabilized, we don’t know whether they will remain stable, grow or continue to collapse in the near future,” Howat says. That’s because the glaciers’ shape changed greatly, becoming stretched and thinned.

“Our main point is that the behavior of these glaciers can change a lot from year to year, so we can’t assume to know the future behavior from short records of recent changes,” he says. “Future warming may lead to rapid pulses of retreat and increased discharge rather than a long, steady drawdown.”

The findings come on the heels of the widely publicized Intergovernmental Panel of Climate Change’s report issued Feb. 2. Some scientists criticized the report for disregarding the surprisingly high discharges of ice from Greenland’s glaciers since 2000 when the panel estimated the amount of future sea level rise that will be caused by melting glaciers.

In the summary for policy makers (), the Intergovernmental Panel on Climate Change explained its position saying, “Dynamical processes related to ice flow and not included in current models but suggested by recent observations could increase the vulnerability of the ice sheets to warming, increasing sea level rise. Understanding of these processes is limited and there is no consensus on their magnitude.”

“I think the IPCC authors made a responsible decision in producing their estimates while noting the recent discharges are a real concern that we do not yet understand well enough to make accurate predictions,” says Ian Joughin, a glaciologist with the UW’s Applied Physics Laboratory and co-author on the Science Express article.

Getting accurate computer models of Greenland and Antarctic glaciers is important because 99 percent of the Earth’s glacial ice is found in those two places. Glacial ice is second only to the oceans as the largest reservoir of water on the planet.

Previous findings published a year ago showed that Greenland’s glaciers had doubled their discharge between 2000 and 2005, but these results were based on “snapshots” of discharge taken five years apart, Howat says.

“Did an equal amount of discharge occur every year? Did it happen all in one year? Is there a steady upward trajectory? We didn’t know,” he says.

Last week’s Science Express article adds details from Greenland’s second and third largest glaciers, Kangerdlugssuaq and Helheim, in the southwest part of Greenland. The two are known as “outlet” glaciers because their front edges reach all the way to the sea, unlike other glaciers that are landlocked. Together the two glaciers represent 35 percent of East Greenland’s total discharge. The scientists examined the glaciers’ speed, geometry and discharge between 2000 and 2006.

At Kangerdlugssuaq, roughly 80 percent of the total increase in discharge occurred in less than one year in 2005, followed by a 25 percent drop the following years, the authors say. At Helheim, discharge increased between 2000 and 2003, and then by an even greater amount between 2004 and 2005. It then dropped in 2006 to its near 2000 value.

The scientists say what they’ve learned is that the shape of these two glaciers changed as they surged toward the sea, changes that put the brakes on. The glaciers lost ice as their front edges began calving, became lighter and floated off the bottom, which led to more ice breaking off as the ice was buoyed up by water. The fronts stabilized once the ice had retreated to shallower parts of the fjords and again rested on the bottom.

They also found the pace toward the sea was faster at the front edge of the glaciers than farther up the mountain. For example Kangerdlugssuaq’s front edge increased in speed by 80 in 2005 percent while 19 miles inland the speed increased 20 percent. This caused the glaciers to thin, stretch and weigh less overall, which also slowed them down.

“All this in a matter of a few short years for these two glaciers is not the way glaciologists are used to thinking,” Howat says. “We’re used to thinking of the ice sheets in terms of millennia or centuries.”

The third co-author on the paper is Ted Scambos of the University of Colorado’s National Snow and Ice Data Center. The work was funded by the National Science Foundation and the National Aeronautics and Space Administration.

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For more information:
Howat, 206-543-8938, ihowat@apl.washington.edu

With warming temperatures as the possible underlying cause, scientists wonder what is pushing Greenland’s glaciers out to sea as much as 50 percent quicker than before.

As a glacier loses large pieces of ice on its leading edge, a process called calving, openings may be created for ice to stream through more quickly, sort of like water flooding through a sudden break in a dike or dam, suggests Ian Joughin, a glaciologist with the 天美影视传媒’s Applied Physics Laboratory.

“Greenland Rumbles Louder as Glaciers Accelerate” is Joughin’s commentary in this week’s Science on work reported by Harvard University’s Goran Ekstrom and co-authors saying that glacier quakes, one way scientists can monitor glacial activity, have increased dramatically. The Ekstrom article says that seismic data from 1993 through 2005 reveals summer glacial seismicity nearly five times greater than in winter, and a rapid increase in seismicity from 2002 onwards, with 2005 producing nearly as many events as the combined total for 1993 through 1996.

The authors of that research article hypothesize that the ice is slipping on growing pockets of meltwater, like a car hydroplaning on rain-slicked streets. The meltwater drains during the summer from the surface of the glacier to the bed through glacial conduits called moulins.

Because calving of Greenland’s fastest-moving glacier, Jakobshavn Isbrae, has an annual variability similar to its glacier quakes, Joughin writes that other explanations may revolve around calving.

“Large calving events alone might yield mass displacements sufficient to produce glacier quakes,” he writes. “Alternatively, changes in glacier geometry after a calving event introduce a force imbalance, which may yield a slip event as new force balance is established.”

Or perhaps glacier quakes are produced by stick-slip events that occur in the normal course of glacier sliding, with only hour-long periods of sticking required to build enough elastic strain to produce a detectable slip event when the ice begins to move again, he says.

Joughin, who published findings two years ago that the Jakobshavn Isbrae glacier had doubled its speed between 1997 and 2003, uses radar images from satellites to monitor the glaciers he studies.

Seismic activity has been monitored locally in Antarctica and elsewhere by placing seismometers on the glaciers. Now the Ekstrom group has determined that glacier quakes could be detected even half a world away by the existing network of seismometers that measures regular earthquakes.

“This teleseismic data provides a powerful new means for monitoring glacial activity,” Joughin says.

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For more information:
Joughin, (206) 221-3177, ian@apl.washington.edu

The world鈥檚 fastest glacier, Greenland鈥檚 Jakobshavn Isbrae, doubled its speed between 1997 and 2003. The rapid movement of ice from land into the sea provides key evidence of newly discovered relationships among ice sheets, sea level rise and climate warming.

The glacier鈥檚 sudden speed-up also coincides with very rapid thinning, up to 49 feet of ice per year after 1997, according to research published in the Dec. 2 issue of the journal Nature. Along with increased rates of ice flow and thinning, the thick ice that extends from the mouth of the glacier into the ocean, called the ice tongue, began retreating in 2000, breaking up almost completely by May, 2003.

鈥淚n many climate models glaciers are treated as responding slowly to climate change,鈥� said Ian Joughin, glaciologist at the UW鈥檚 Applied Physics Laboratory. 鈥淥ver only a few years, this glacier has begun spewing something like an additional 30 cubic kilometers of ice into the ocean, twice what it had been producing. It appears ice sheets can respond rather dramatically to climate changes.鈥�

The NASA-funded study relies on data from satellites and airborne lasers to derive ice movements. Joughin conducted much of this research while working at NASA鈥檚 Jet Propulsion Laboratory in Pasadena, Calif. Co-authors of the research are Waleed Abdalati, a senior scientist at NASA鈥檚 Goddard Space Flight Center, Greenbelt, Md., and Mark Fahnestock, a researcher at the University of New Hampshire.

Jakobshavn Isbrae is Greenland鈥檚 largest outlet glacier, draining 6.5 percent of Greenland鈥檚 ice sheet area. Data from between 1985 and 2003 showed that the glacier slowed from a velocity of 4.2 miles per year in 1985 to 3.5 miles per year in 1992. This latter speed remained somewhat constant until 1997. By 2000, the glacier had sped up to 5.8 miles per year, topping out with the last measurement in spring 2003 of 7.8 miles.

鈥淭his finding suggests the potential for more substantial thinning in other glaciers in Greenland,鈥� Abdalati said. 鈥淥ther glaciers have thinned by over a meter a year, which we believe is too much to be attributed to melting alone. We think there is a dynamic effect in which the glaciers are accelerating due to warming.鈥�

Airborne laser altimetry measurements of Jakobshavn鈥檚 surface elevation, made previously by NASA researchers, showed a thickening, or building up of the glacier from 1991 to 1997, coinciding closely with the glacier鈥檚 slowdown. Similarly, the glacier began thinning by as much as 49 feet a year just as its velocity began to increase between 1997 and 2003.

The acceleration comes at a time when the floating ice near the glacier鈥檚 calving front has shown some unusual behavior. Despite its relative stability from the 1950s through the 1990s, the glacier鈥檚 ice tongue began to break apart in 2000, leading to almost complete disintegration in 2003. The tongue鈥檚 thinning and breaking up likely reduced any restraining effects it had on the ice behind it, as several speed increases coincided with losses of sections of the ice tongue as it broke up.

鈥淲e still have a lot of work ahead of us to determine what the changes we are observing mean in terms of the long term stability of the ice sheet,鈥� Joughin said. 鈥淔or example, we don鈥檛 know if this glacier will continue to speed up and discharge more ice or if this is a short-term change that will die away within a few years.鈥�