Massive study provides first detailed look at how Greenland’s ice is vanishing

The surface of the Greenland Ice Sheet. A new study uses NASA data to provide the first detailed reconstruction of how the ice sheet and its many glaciers are changing. The research was led by University at Buffalo geologist Beata Csatho. -  Beata Csatho
The surface of the Greenland Ice Sheet. A new study uses NASA data to provide the first detailed reconstruction of how the ice sheet and its many glaciers are changing. The research was led by University at Buffalo geologist Beata Csatho. – Beata Csatho

The Greenland Ice Sheet is the second-largest body of ice on Earth. It covers an area about five times the size of New York State and Kansas combined, and if it melts completely, oceans could rise by 20 feet. Coastal communities from Florida to Bangladesh would suffer extensive damage.

Now, a new study is revealing just how little we understand this northern behemoth.

Led by geophysicist Beata Csatho, PhD, an associate professor of geology at the University at Buffalo, the research provides what the authors think is the first comprehensive picture of how Greenland’s ice is vanishing. It suggests that current ice sheet modeling studies are too simplistic to accurately predict the future contributions of the entire Greenland Ice Sheet to sea level rise, and that Greenland may lose ice more rapidly in the near future than previously thought.

“The great importance of our data is that for the first time, we have a comprehensive picture of how all of Greenland’s glaciers have changed over the past decade,” Csatho says.

“This information is crucial for developing and validating numerical models that predict how the ice sheet may change and contribute to global sea level over the next few hundred years,” says Cornelis J. van der Veen, PhD, professor in the Department of Geography at the University of Kansas, who played a key role in interpreting glaciological changes.

The project was a massive undertaking, using satellite and aerial data from NASA’s ICESat spacecraft and Operation IceBridge field campaign to reconstruct how the height of the Greenland Ice Sheet changed at nearly 100,000 locations from 1993 to 2012.

Ice loss takes place in a complex manner, with the ice sheet both melting and calving ice into the ocean.
The study had two major findings:

  • First, the scientists were able to provide new estimates of annual ice loss at high spatial resolution (see below).

  • Second, the research revealed that current models fail to accurately capture how the entire Greenland Ice Sheet is changing and contributing to rising oceans.

The second point is crucial to climate change modelers.

Today’s simulations use the activity of four well-studied glaciers — Jakobshavn, Helheim, Kangerlussuaq and Petermann — to forecast how the entire ice sheet will dump ice into the oceans.

But the new research shows that activity at these four locations may not be representative of what is happening with glaciers across the ice sheet. In fact, glaciers undergo patterns of thinning and thickening that current climate change simulations fail to address, Csatho says.

“There are 242 outlet glaciers wider than 1.5 km on the Greenland Ice Sheet, and what we see is that their behavior is complex in space and time,” Csatho says. “The local climate and geological conditions, the local hydrology — all of these factors have an effect. The current models do not address this complexity.”

The team identified areas of rapid shrinkage in southeast Greenland that today’s models don’t acknowledge. This leads Csatho to believe that the ice sheet could lose ice faster in the future than today’s simulations would suggest.

The results will be published on Dec. 15 in the Proceedings of the National Academy of Sciences, and the study and all information in this press release are embargoed until 3 p.m. Eastern Time that day.

Photos, data visualizations and video are available by contacting Charlotte Hsu at the University at Buffalo at chsu22@buffalo.edu.

How much ice is the Greenland Ice Sheet losing?

To analyze how the height of the ice sheet was changing, Csatho and UB research professor and photogrammetrist Anton Schenk, PhD, developed a computational technique called Surface Elevation Reconstruction And Change detection to fuse together data from NASA satellite and aerial missions.

The analysis found that the Greenland Ice Sheet lost about 243 metric gigatons of ice annually — equivalent to about 277 cubic kilometers of ice per year — from 2003-09, the period for which the team had the most comprehensive data. This loss is estimated to have added about 0.68 millimeters of water to the oceans annually.

The figures are averages, and ice loss varied from year to year, and from region to region.

Why are today’s projections of sea level rise flawed, and how can we fix them?

Glaciers don’t just gradually lose mass when the temperature rises. That’s one reason it’s difficult to predict their response to global warming.

In the study, scientists found that some of Greenland’s glaciers thickened even when the temperature rose. Others exhibited accelerated thinning. Some displayed both thinning and thickening, with sudden reversals.

As a step toward building better models of sea level rise, the research team divided Greenland’s 242 glaciers into 7 major groups based on their behavior from 2003-09.

“Understanding the groupings will help us pick out examples of glaciers that are representative of the whole,” Csatho says. “We can then use data from these representative glaciers in models to provide a more complete picture of what is happening.”

In a new project, she and colleagues are investigating why different glaciers respond differently to warming. Factors could include the temperature of the surrounding ocean; the level of friction between a glacier and the bedrock below; the amount of water under a glacier; and the geometry of the fjord.

“The physics of these processes are not well understood,” Csatho says.

The NASA missions: A colossal undertaking

The study combined data from various NASA missions, including:

  • NASA’s Ice, Cloud and Land Elevation Satellite (ICESat), which measured the ice sheet’s elevation multiple times a year at each of the nearly 100,000 locations from 2003-09.

  • NASA’s, massive aerial survey that employs highly specialized research aircrafts to collect data at less frequent intervals than ICESat. These missions began measuring the Greenland Ice Sheet’s elevation in 1993. Operation IceBridge was started in 2009 to bridge the time between ICESat-1 and ICESat-2, and will continue until at least 2017, when NASA’s next generation ICESat-2 satellite is expected to come online.

Csatho says the new study shows why careful monitoring is critical: Given the complex nature of glacier behavior, good data is crucial to building better models.

Collaborators

Besides Csatho, Schenk and van der Veen, the project included additional researchers from the University at Buffalo, Utrecht University in The Netherlands, the Technical University of Denmark and Florida Atlantic University.

New study finds Antarctic Ice Sheet unstable at end of last ice age

This is one of many icebergs that sheared off the continent and ended up in the Scotia Sea. -  Photo courtesy of Michael Weber, University of Cologne
This is one of many icebergs that sheared off the continent and ended up in the Scotia Sea. – Photo courtesy of Michael Weber, University of Cologne

A new study has found that the Antarctic Ice Sheet began melting about 5,000 years earlier than previously thought coming out of the last ice age – and that shrinkage of the vast ice sheet accelerated during eight distinct episodes, causing rapid sea level rise.

The international study, funded in part by the National Science Foundation, is particularly important coming on the heels of recent studies that suggest destabilization of part of the West Antarctic Ice Sheet has begun.

Results of this latest study are being published this week in the journal Nature. It was conducted by researchers at University of Cologne, Oregon State University, the Alfred-Wegener-Institute, University of Hawaii at Manoa, University of Lapland, University of New South Wales, and University of Bonn.

The researchers examined two sediment cores from the Scotia Sea between Antarctica and South America that contained “iceberg-rafted debris” that had been scraped off Antarctica by moving ice and deposited via icebergs into the sea. As the icebergs melted, they dropped the minerals into the seafloor sediments, giving scientists a glimpse at the past behavior of the Antarctic Ice Sheet.

Periods of rapid increases in iceberg-rafted debris suggest that more icebergs were being released by the Antarctic Ice Sheet. The researchers discovered increased amounts of debris during eight separate episodes beginning as early as 20,000 years ago, and continuing until 9,000 years ago.

The melting of the Antarctic Ice Sheet wasn’t thought to have started, however, until 14,000 years ago.

“Conventional thinking based on past research is that the Antarctic Ice Sheet has been relatively stable since the last ice age, that it began to melt relatively late during the deglaciation process, and that its decline was slow and steady until it reached its present size,” said lead author Michael Weber, a scientist from the University of Cologne in Germany.

“The sediment record suggests a different pattern – one that is more episodic and suggests that parts of the ice sheet repeatedly became unstable during the last deglaciation,” Weber added.

The research also provides the first solid evidence that the Antarctic Ice Sheet contributed to what is known as meltwater pulse 1A, a period of very rapid sea level rise that began some 14,500 years ago, according to Peter Clark, an Oregon State University paleoclimatologist and co-author on the study.

The largest of the eight episodic pulses outlined in the new Nature study coincides with meltwater pulse 1A.

“During that time, the sea level on a global basis rose about 50 feet in just 350 years – or about 20 times faster than sea level rise over the last century,” noted Clark, a professor in Oregon State’s College of Earth, Ocean, and Atmospheric Sciences. “We don’t yet know what triggered these eight episodes or pulses, but it appears that once the melting of the ice sheet began it was amplified by physical processes.”

The researchers suspect that a feedback mechanism may have accelerated the melting, possibly by changing ocean circulation that brought warmer water to the Antarctic subsurface, according to co-author Axel Timmermann, a climate researcher at the University of Hawaii at Manoa.

“This positive feedback is a perfect recipe for rapid sea level rise,” Timmermann said.

Some 9,000 years ago, the episodic pulses of melting stopped, the researchers say.

“Just as we are unsure of what triggered these eight pulses,” Clark said, “we don’t know why they stopped. Perhaps the sheet ran out of ice that was vulnerable to the physical changes that were taking place. However, our new results suggest that the Antarctic Ice Sheet is more unstable than previously considered.”

Today, the annual calving of icebergs from Antarctic represents more than half of the annual loss of mass of the Antarctic Ice Sheet – an estimated 1,300 to 2,000 gigatons (a gigaton is a billion tons). Some of these giant icebergs are longer than 18 kilometers.

Network for tracking earthquakes exposes glacier activity

Alaska’s seismic network records thousands of quakes produced by glaciers, capturing valuable data that scientists could use to better understand their behavior, but instead their seismic signals are set aside as oddities. The current earthquake monitoring system could be “tweaked” to target the dynamic movement of the state’s glaciers, suggests State Seismologist Michael West, who will present his research today at the annual meeting of the Seismological Society of America (SSA).

“In Alaska, these glacial events have been largely treated as a curiosity, a by-product of earthquake monitoring,” said West, director of the Alaska Earthquake Center, which is responsible for detecting and reporting seismic activity across Alaska.

The Alaska seismic network was upgraded in 2007-08, improving its ability to record and track glacial events. “As we look across Alaska’s glacial landscape and comb through the seismic record, there are thousands of these glacial events. We see patterns in the recorded data that raise some interesting questions about the glaciers,” said West.

As a glacier loses large pieces of ice on its leading edge, a process called calving, the Alaska Earthquake Center’s monitoring system automatically records the event as an earthquake. Analysts filter out these signals in order to have a clear record of earthquake activity for the region. In the discarded data, West sees opportunity.

“We have amassed a large record of glacial events by accident,” said West. “The seismic network can act as an objective tool for monitoring glaciers, operating 24/7 and creating a data flow that can alert us to dynamic changes in the glaciers as they are happening.” It’s when a glacier is perturbed or changing in some way, says West, that the scientific community can learn the most.

Since 2007, the Alaska Earthquake Center has recorded more than 2800 glacial events along 600 km of Alaska’s coastal mountains. The equivalent earthquake sizes for these events range from about 1 to 3 on the local magnitude scale. While calving accounts for a significant number of the recorded quakes, each glacier’s terminus – the end of any glacier where the ice meets the ocean – behaves differently. Seasonal variations in weather cause glaciers to move faster or slower, creating an expected seasonal cycle in seismic activity. But West and his colleagues have found surprises, too.

In mid-August 2010, the Columbia Glacier’s seismic activity changed radically from being relatively quiet to noisy, producing some 400 quakes to date. These types of signals from the Columbia Glacier have been documented every single month since August 2010, about the time when the Columbia terminus became grounded on sill, stalling its multi-year retreat.

That experience highlighted for West the value of the accidental data trove collected by the Alaska Earthquake Center. “The seismic network is blind to the cause of the seismic events, cataloguing observations that can then be validated,” said West, who suggests the data may add value to ongoing field studies in Alaska.

Many studies of Alaska’s glaciers have focused on single glacier analyses with dedicated field campaigns over short periods of time and have not tracked the entire glacier complex over the course of years. West suggests leveraging the data stream may help the scientific community observe the entire glacier complex in action or highlight in real time where scientists could look to catch changes in a glacier.

“This is low-hanging fruit,” said West of the scientific advances waiting to be gleaned from the data.

Antarctic ice core sheds new light on how the last ice age ended

Brian Bencivengo, assistant curator of the National Ice Core Laboratory, in Lakewood, Colo., holds a one-meter-long section of the West Antarctic Ice Sheet (WAIS) Divide Ice Core. -  Geoffrey Hargreaves, National Science Foundation
Brian Bencivengo, assistant curator of the National Ice Core Laboratory, in Lakewood, Colo., holds a one-meter-long section of the West Antarctic Ice Sheet (WAIS) Divide Ice Core. – Geoffrey Hargreaves, National Science Foundation

Analysis of an ice core taken by the National Science Foundation- (NSF) funded West Antarctic Ice Sheet (WAIS) Divide drilling project reveals that warming in Antarctica began about 22,000 years ago, a few thousand years earlier than suggested by previous records.

This timing shows that West Antarctica did not “wait for a cue” from the Northern Hemisphere to start warming, as scientists had previously supposed.

For more than a century scientists have known that Earth’s ice ages are caused by the wobbling of the planet’s orbit, which changes its orientation to the sun and affects the amount of sunlight reaching higher latitudes.

The Northern Hemisphere’s last ice age ended about 20,000 years ago, and most evidence had indicated that the ice age in the Southern Hemisphere ended about 2,000 years later, suggesting that the South was responding to warming in the North.

But research published online Aug. 14 in the journal Nature shows that Antarctic warming began at least two, and perhaps four, millennia earlier than previously thought.

Most previous evidence for Antarctic climate change had come from ice cores drilled in East Antarctica, the highest and coldest part of the continent. However, a U.S.-led research team studying the West Antarctic core found that warming there was well underway 20,000 years ago.

WAIS Divide is a large-scale and multi-year glaciology project supported by the U.S. Antarctic Program (USAP), which NSF manages. Through USAP, NSF coordinates all U.S. science on the southernmost continent and aboard vessels in the Southern Ocean and provides the necessary logistics to make the science possible.

The WAIS Divide site is in an area where there is little horizontal flow of the ice, so the data are known to be from a location that remained consistent over long periods.

The WAIS Divide ice core is more than two miles deep and covers a period stretching back 68,000 years, though so far data have been analyzed only from layers going back 30,000 years. Near the surface, one meter of snow is equal to a year of accumulation, but at greater depths the annual layers are compressed to centimeters of ice.

“Sometimes we think of Antarctica as this passive continent waiting for other things to act on it. But here it is showing changes before it ‘knows’ what the North is doing,” said T.J. Fudge, a University of Washington doctoral student in Earth and Space Sciences and lead corresponding author of the Nature paper. Fudge’s 41 co-authors are other members of the WAIS project.

Fudge identified the annual layers by running two electrodes along the ice core to measure higher electrical conductivity associated with each summer season. Evidence of greater warming turned up in layers associated with 18,000 to 22,000 years ago, the beginning of the last deglaciation.

“This deglaciation is the last big climate change that we’re able to go back and investigate,” he said. “It teaches us about how our climate system works.”

West Antarctica is separated from East Antarctica by a major mountain range. East Antarctica has a substantially higher elevation and tends to be much colder, though there is recent evidence that it too is warming.

Rapid warming in West Antarctica in recent decades has been documented in previous research by Eric Steig, a professor of Earth and Space Sciences at the University of Washington who serves on Fudge’s doctoral committee and whose laboratory produced the oxygen isotope data used in the Nature paper. The new data confirm that West Antarctica’s climate is more strongly influenced by regional conditions in the Southern Ocean than East Antarctica is.

“It’s not surprising that West Antarctica is showing something different from East Antarctica on long time scales, but we didn’t have direct evidence for that before,” Fudge said.

He noted that the warming in West Antarctica 20,000 years ago is not explained by a change in the sun’s intensity. Instead, how the sun’s energy was distributed over the region was a much bigger factor. It not only warmed the ice sheet but also warmed the Southern Ocean that surrounds Antarctica, particularly during summer months when more sea ice melting could take place.

Changes in Earth’s orbit today are not an important factor in the rapid warming that has been observed recently, he added. “Earth’s orbit changes on the scale of thousands of years, but carbon dioxide today is changing on the scale of decades so climate change is happening much faster today,” Fudge said.

Julie Palais, the Antarctic Glaciology Program director in NSF’s Division of Polar Programs, said new findings will help scientists to “better understand not only what happened at the end of the last ice age but it should also help inform our understanding of what might be happening as the climate warms and conditions begin to change in and around the Antarctic continent.”

She added, “West Antarctica is currently experiencing some of the largest changes on the continent, such as the large calving events in the Amundsen Sea Embayment linked to warm ocean currents undercutting the outlet glaciers. The recent changes are consistent with the WAIS Divide results that show West Antarctica is sensitive to changes in ocean conditions in the past.”

Melting water’s lubricating effect on glaciers has only ‘minor’ role in future sea-level rise

Scientists had feared that melt-water which trickles down through the ice could dramatically speed up the movement of glaciers as it acts as a lubricant between the ice and the ground it moves over.

But in a paper published today in PNAS, a team led by scientists from the University of Bristol found it is likely to have a minor role in sea-level rise compared with other effects like iceberg production and surface melt.

The results of computer modelling, based on fieldwork observations in Greenland, revealed that by the year 2200 lubrication would only add a maximum of 8mm to sea-level rise – less than 5 per cent of the total projected contribution from the Greenland ice sheet.

In fact in some of their simulations the lubricating effect had a negative impact on sea-level rise – in other words it alone could lead to a lowering of sea-level (ignoring the other major factors).

Lead author, Dr Sarah Shannon, from the University of Bristol, said: “This is an important step forward in our understanding of the factors that control sea-level rise from the Greenland Ice Sheet. Our results show that melt-water enhanced lubrication will have a minor contribution to future sea-level rise. Future mass loss will be governed by changes in surface melt-water runoff or iceberg calving.”

Previous studies of the effects of melt-water on the speed of ice movement had assumed the water created cavities at the bottom of ice masses. These cavities lifted the ice slightly and acted as a lubricant, speeding up flow.

This theory had led scientists to think that increased melt-water would lead directly to more lubrication and a consequent speeding up of the ice flow.

But the Bristol-led study took into account recent observations that indicate larger amounts of melt-water may form channels beneath the ice that drain the water away, reducing the water’s lubricating effect. The scientists found that no matter whether more melt-water increases or decreases the speed of ice flow, the effect on sea level is small.

Dr Shannon said: “We found that the melt-water would lead to a redistribution of the ice, but not necessarily to an increase in flow.”

The findings are part of research undertaken through the European funded ice2sea programme. Earlier research from the programme has shown that changes in surface melting of the ice sheet will be the major factor in sea-level rise contributions from Greenland.

Professor David Vaughan, ice2sea co-ordinator based at the British Antarctic Survey in Cambridge, said: “This is important work but it’s no reason for complacency. While this work shows that the process of lubrication of ice flow by surface melting is rather insignificant, our projections are still that Greenland will be a major source of future sea-level rise. As we have reported earlier this year, run-off of surface melt water directly into the ocean and increased iceberg calving are likely to dominate.”

Sea level rise: New iceberg theory points to areas at risk of rapid disintegration

In events that could exacerbate sea level rise over the coming decades, stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean, according to new iceberg calving simulations from the University of Michigan.

“If this starts to happen and we’re right, we might be closer to the higher end of sea level rise estimates for the next 100 years,” said Jeremy Bassis, assistant professor of atmospheric, oceanic and space sciences at the U-M College of Engineering, and first author of a paper on the new model published in the current issue of Nature Geoscience.

Iceberg calving, or the formation of icebergs, occurs when ice chunks break off larger shelves or glaciers and float away, eventually melting in warmer waters. Although iceberg calving accounts for roughly half of the mass lost from ice sheets, it isn’t reflected in any models of how climate change affects the ice sheets and could lead to additional sea level rise, Bassis said.

“Fifty percent of the total mass loss from the ice sheets, we just don’t understand. We essentially haven’t been able to predict that, so events such as rapid disintegration aren’t included in those estimates,” Bassis said. “Our new model helps us understand the different parameters, and that gives us hope that we can better predict how things will change in the future.”

The researchers have found the physics at the heart of iceberg calving, and their model is the first that can simulate the different processes that occur on both ends of the Earth. It can show why in northern latitudes-where glaciers rest on solid ground-icebergs tend to form in relatively small, vertical slivers that rotate onto their sides as they dislodge. It can also illustrate why in the southernmost places-where vast ice shelves float in the Antarctic Ocean-icebergs form in larger, more horizontal plank shapes.

The model treats ice sheets-both floating shelves and grounded glaciers-like loosely cemented collections of boulders. Such a description reflects how scientists in the field have described what iceberg calving actually looks like. The model allows those loose bonds to break when the boulders are pulled apart or rub against one another.

The simulations showed that calving is a two-step process driven primarily by the thickness of the ice.

“Essentially, everything is driven by gravity,” Bassis said. “We identified a critical threshold of one kilometer where it seems like everything should break up. You can think of it in terms of a kid building a tower. The taller the tower is, the more unstable it gets.”

Icebergs do have a tendency to form before that threshold though, Bassis suspects, due to cracks that are already there-either formed when capsizing bergs crash into the water and send shockwaves through the surrounding ice, or when melted water on the surface cuts through. The former is believed to have led to the Helheim Glacier collapse in 2003. The glacier had begun to retreat slowly in 2002, but suddenly gave way the following year when the thinner ice had broken away, exposing a thicker ice coast.

The latter-melted water pools-are occurring more frequently due to climate change, and they’re believed to have played a role in the rapid disintegration of the Antarctica’s Larsen B ice shelf, which crumbled over about six weeks in 2002.

When the researchers added random cracks to their model, it could mirror both Helheim and Larsen B.

A third feature is also required for the most dramatic ice collapses to occur. Icebergs can’t float away and make room for more icebergs to break off the main sheet unless the system has access to open water. So areas that border deep, unobstructed ocean rather than fjords or other waterways are at greater risk of rapid ice loss. The researchers point to the Thwaites and Pine Island glaciers in Antarctica and the Jakobshavn Glacier in Greenland, which is already retreating rapidly, as places vulnerable to “catastrophic disintegration” because they have all three components.

“The ice in those places gets thicker as you go back. If our threshold is right, then if these places start to retreat as you expose the thicker calving font, they’re susceptible to catastrophic breakup,” Bassis said.

Retreat of the current ice coasts in these places areas could occur via melting or iceberg calving.

Researchers shed new light on supraglacial lake drainage

Supraglacial lakes – bodies of water that collect on the surface of the Greenland ice sheet – lubricate the bottom of the sheet when they drain, causing it to flow faster. Differences in how the lakes drain can impact glacial movement’s speed and direction, researchers from The City College of New York (CCNY), University of Cambridge and Los Alamos National Laboratory report in “Environmental Research Letters.”

“Knowledge of the draining mechanisms allows us to improve our understanding of how surface melting can impact sea-level rise, not only through the direct contribution of meltwater from the surface, but also through the indirect contribution on the mass loss through ice dynamics,” says Dr. Marco Tedesco, the principal investigator and lead author.

Dr. Tedesco is an associate professor in CCNY’s Department of Earth and Atmospheric Sciences at CCNY and is currently serving as temporary program director for the National Science Foundation’s Polar Cyberinfrastructure Program. The research described in the paper was funded before Dr. Tedesco accepted the position at NSF.

NSF supported the research along with NASA’s cryosphere program, the Natural Environment Research Council, the U.S. Department of Energy’s earth systems modeling program, St. Catherine’s College (Cambridge), the Scandinavian Studies Fund and the B.B. Roberts Fund.

Over the past decade, surface melting in Greenland has increased considerably.

Previous research already suggested that the water injected from the rapid draining of the supraglacial lakes controlled sliding of ice over the bed beneath it. However, there was no evidence of the impact of the slow draining mechanism, which the paper identified.

Professor Tedesco and colleagues documented that supraglacial lakes have two different drainage mechanisms that cause them to empty rapidly or slowly. The findings are based on analysis of data collected in 2011 from five GPS stations the team installed around two supraglacial lakes in the Paakistoq region of West Greenland.

The smaller of the two lakes, Lake Half Moon, overflowed its banks and drained from the side to reach a moulin. It took approximately 45 hours to empty. The larger lake, Lake Ponting, drained through a crack in the ice beneath it and was voided in around two hours.

“At first, a crack in the ice beneath the lake may be small, but it deepens as water enters it because the pressure of the water overcomes the compressive action of the ice, which is trying to close the crack,” Professor Tedesco explains. “When the crack reaches the bed beneath the glacier, which could be 1,000 meters or more below the surface, the lake empties rapidly, like a bathtub after its plug is pulled.”

Drainage from both lakes accelerated glacial movement. However, water from Lake Ponting caused the glacier to move faster and further. While the slower drainage from Lake Half Moon caused the glacial pace to increase from baseline values of 90 – 100 meters per year to a maximum of around 420 meters a year, glacial movement in the area affected by Lake Ponting reached maximum velocities of 1,500 – 1,600 meters per year, nearly four times greater.

The drainage of the two lakes impacted the glacier’s trajectory differently, as well. The emptying of Lake Half Moon via the moulin did not change the direction of glacial movement. However, when Lake Ponting drained a slight southerly shift in the glacier’s direction was detected.

“Because the different draining mechanisms affect ice velocity, they could also affect the amount of ice lost through calving of glaciers, which results in icebergs,” Professor Tedesco points out. “Because what happens on a glacier’s surface impacts what is going on below, researchers are trying to look at glaciers as a system instead of independent components,” he adds.

“The surface is like the skin of a tissue and the subglacial and englacial channels that develop because of the surface water act like arteries or veins that redistribute this water internally.”

The contribution of the Greenland ice sheet to sea-level rise will continue to increase

New research has shown surface ice melt will be the dominant process controlling ice-loss from Greenland. As outlet glaciers retreat inland the other process, iceberg production, remains important but will not grow as rapidly.

The Greenland ice sheet is often considered an important potential contributor to future global sea-level rise over the next century or longer. In total, it contains an amount of ice that would lead to a rise of global sea level by more than seven metres, if completely melted.

Changes in its total mass are governed by two main processes – fluctuations in melting and snowfall on its surface, and changes to the number of icebergs released from a large number of outlet glaciers into the ocean.

The ice loss from the ice sheet has been increasing over the last decade, with half of it attributed to changes in surface conditions with the remainder due to increased iceberg calving – the process by which ice detaches from the glacier to become an iceberg.

Researchers from the Vrije Universiteit Brussel, funded by ice2sea, a European Union project, tackled the question of how both processes will evolve and interact in the future. This was done with a computer model, which projects the future ice sheet evolution with high accuracy using the latest available techniques and input data.

They devised a method to generalize projections made in earlier research which concerned just four of Greenland’s outlet glaciers. By doing so they could apply the earlier findings to all calving glaciers around the Greenland ice sheet. Their results indicate a total sea-level contribution from the Greenland ice sheet for an average warming scenario after 100 and 200 years of 7 and 21 cm, respectively.

The balance between the two processes by which ice is lost is, however, changing considerably in the future so that iceberg calving may only account for between 6 % and 18 % of the sea-level contribution after 200 years. This is important, because variations in outlet glacier dynamics have often been suspected to have the potential for very large sea-level contributions.

Lead author Dr Heiko Goelzer, of the Vrije Universiteit Brussel, says,

“Our research has shown that the balance between the two most important mass loss processes will change considerably in the future so that changes in iceberg calving only account for a small percentage of the sea-level contribution after 200 years with the large remainder due to changes in surface conditions.”

The limited importance of outlet glacier dynamics in the future is the result of their retreat back onto land and of strongly increasing surface melting under global warming, which removes ice before it can reach the marine margin.

Ice2sea coordinator Professor David Vaughan, of the British Antarctic Survey says,

“This scenario is no reason to be complacent. The reason the significance of calving glaciers reduces compared to surface melting is, so much ice will be lost in coming decades that many glaciers currently sitting in fjords will retreat inland to where they are no longer affected by warming seas around Greenland.”

Data show Antarctic ice stream radiating seismically





Douglas Wiens (left) and a colleague ready equipment to emplace seismographs in Antarctica during a 2001 expedition. Data gathered for this project, called TAMSEIS, provided evidence that an Antarctic ice stream radiates seismic waves twice daily that are equivalent to a magnitude seven earthquake. - Image courtesy of Doug Wiens
Douglas Wiens (left) and a colleague ready equipment to emplace seismographs in Antarctica during a 2001 expedition. Data gathered for this project, called TAMSEIS, provided evidence that an Antarctic ice stream radiates seismic waves twice daily that are equivalent to a magnitude seven earthquake. – Image courtesy of Doug Wiens

A seismologist at Washington University in St. Louis along with colleagues at Pennsylvania State University and Newcastle University in the United Kingdom have found seismic signals from a giant river of ice in Antarctica that make California’s earthquake problem seem trivial.



Douglas A. Wiens, Ph.D., professor of earth and planetary sciences in Arts & Sciences, and colleagues combined seismological and global positioning system (GPS) analyses to reveal two bursts of seismic waves from an ice stream in Antarctica every day, each one equivalent to a magnitude seven earthquake. The GPS analyses were performed by Pennsylvania State and Newcastle University researchers.



The ice stream is essentially a giant glacier 60 miles wide and one-half mile thick. The data show that the river of ice moves about 18 inches within ten minutes, remains still for 12 hours, then moves another eighteen inches. Each time it moves, it gives off seismic waves that are recorded at seismographs all around Antarctica, and even as far away as Australia.



Seismic waves from what are loosely called “glacial earthquakes,” mainly near Greenland, were originally reported in 2003, and the numbers have been increasing in recent years. Some scientists think the waves come from a phenomenon of calving, during which a big chunk of ice breaks off of a glacier and floats away in the ocean. This is a very violent activity that could generate strong seismic signals. The new results show, however, that at least some of the glacial earthquakes are produced by sudden sliding off large ice sheets.



The Antarctic signals were first detected by seismographs deployed by Wiens and his colleagues in 2001-2003 at a location about 500 miles away from the ice stream.



“At first we didn’t know where the waves were coming from, but eventually we were able to narrow down the source to the ice stream.” Wiens said.

Slower than a real earthquake



Prior to this discovery, researchers were not aware that ice streams radiated seismic waves.



“By some measures, the seismic impact is equivalent to a very large earthquake, but it doesn’t feel like it because the movement is much slower than a real earthquake,” Wiens said. “The data look an awful lot like an earthquake, but the slip lasts for 10 minutes, while on the other hand an earthquake of this size would last for just ten seconds. I guess you could call it an earthquake at glacial speed. This is very strange behavior, and we need to understand more about it.”



GPS instruments placed directly on the ice stream can detect where the slipping motion begins and where it stops. Scientists describe the motion as “stick-slip,” which is the classic motion of earthquakes. Stick-slip occurrs when the area around a fault moves slowly but the fault is stuck, remaining stationary until the stress builds up and the fault finally slips.



“The GPS shows us directly how the ice stream moves,” Wiens said. “The slip starts in a certain part of the ice stream and then it moves out, rather like a landslide might start at a certain point and then move out to envelop an entire mountainside. The GPS tells us which part moved first and what other parts moved next and so forth.”



The data show that the slip always starts from the same spot on the bed of the ice stream – what glaciologists call a “sticky” spot – which has more friction than the surrounding part of the bed.


A slip, not a creep



“Glaciologists had thought that they understood how glaciers move, and they thought the ice moves move slowly and continuously by creep, but now this indicates that they move with a fast slip, almost like an earthquake,” Wiens said.



Wiens said that it is important to understand the physics behind what is controlling this kind of slip.



“This stick-slip phenomenon may provide a clue about what makes these ice streams move faster or slower,” Wiens said. “This particular ice stream has been slowing down over the last few decades, and no one knows why. ”



Wiens plans to study seismic records of stick-slip events going back several decades to see if there are changes, and also to search for similar signals from other ice streams.



“We need to understand what controls the speed of the ice streams, because that factor will affect how fast the ice in Antarctica will go away and sea level will rise as global warming melts the West Antarctic Ice Sheet.”



The study was published in the June 5 issue of Nature Online and was funded by the National Science Foundation.