Extraordinary runoff from the Greenland Ice Sheet in 2012 amplified by hypsometry and depleted firn-retention by A. B. Mikkelson et al., The Cryosphere Discuss., 9 (2015) 4625-4660; doi: 10.5194/tcd-9-4625-2015

The Cryosphere Discuss., 9 (2015) 4625-4660; doi: 10.5194/tcd-9-4625-2015

Extraordinary runoff from the Greenland Ice Sheet in 2012 amplified by hypsometry and depleted firn-retention

A. B. Mikkelsen1,2, A. Hubbard3,4, M. MacFerrin5, J. Box6, S. Doyle4, A. Fitzpatrick4, B. Hasholt1, and H. Bailey4
1Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
2Centre for Permafrost (CENPERM), University of Copenhagen, Øster Voldgade 10, Copenhagen, 1350, Denmark
3Centre for Arctic Gas Hydrate, Environment and Climate, Department of Geology, University of Tromsø, Dramsveien 201, 9037, Norway
4Department of Geography and Earth Sciences, Aberystwyth University, Aberystwyth, SY23 3DB, UK
5Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
6Department of Marine Geology and Glaciology, Geological Survey of Denmark and Greenland, Copenhagen, Denmark

Received: 21 Jul 2015 – Accepted: 29 Jul 2015 – Published: 03 Sep 2015

Abstract

It has been argued that the infiltration and retention of meltwater within firn across the percolation zone of the Greenland ice sheet has the potential to buffer up to ~3.6 mm of global sea level rise (Harper et al., 2012). Despite evidence confirming active refreezing processes above the equilibrium line, their impact on runoff and proglacial discharge has yet to be assessed. Here we compare meteorological, melt, firn-stratigraphy and discharge data from the extreme 2010 and 2012 summers to determine the relationship between atmospheric forcing and runoff across the Kangerlussuaq catchment of the Greenland ice sheet, which drains into Watson River. The bulk discharge in 2012 of 6.8 km3 exceeded that of 2010 of 5.3 km3 by 28%, despite only a 3% difference in net energy available for melt between the two summers. This large disparity in discharge response can be explained by a 24% contribution of runoff originating from above the long-term equilibrium line in 2012, triggered by diminished firn retention that culminated in three days of record discharge from 11 July of 3,100 m3 s−1 (0.27 km3 d−1) that washed-out the Kangerlussuaq bridge. 

Throughout the 2010 melt-season, there was a steady increase in the residual difference between integrated melt over the catchment and cumulative proglacial discharge that by mid-September equated to 21% (~1.1 km3) of the total melt generated being retained within the catchment. In 2012, a similar pattern is observed until 11 July, after which the residual fell by 50% and further diminished so that less than 0.4 km3 (~5 %) of the total melt was retained by the end of the summer. Cumulative energy receipts versus bulk discharge further indicate a marked contrast between the two melt seasons, such that in 2012 there was a notably higher discharge response per unit energy forcing after the 11 July. 

Density profiles from cores and pits within the accumulation area acquired in April 2012 reveal an extensive, dense, ice-layer between 0.9 to 1.4 m snow depth that extended from the equilibrium line to at least 1,840 m elevation. This perched superimposed ice layer can be attributed to melt refreezing during previous summers and we hypothesize that in July 2012, it provided a barrier to further infiltration rendering the underlying pore space inaccessible thereby forcing extensive runoff from the accumulation zone. Discharge was further amplified by catchment hypsometry, leading to a disproportionate increase in the area contributing to runoff as the melt-level rose above the ice sheet plateau in July 2012. Satellite imagery and oblique aerial photographs confirm an active network of supraglacial rivers extending 140 km from the ice margin providing strong support for the hypothesis. 

Our findings substantiate active infiltration processes across the percolation zone of the Greenland ice sheet, though the resulting patterns of refreezing are complex and can lead to spatially extensive, perched, superimposed layers within the firn. In 2012, such layers extended to 1,840 m, providing a low-permeable obstruction to further meltwater storage, thereby promoting runoff into the hydrological system that contributed directly to sea-level rise.

Citation: Mikkelsen, A. B., Hubbard, A., MacFerrin, M., Box, J., Doyle, S., Fitzpatrick, A., Hasholt, B., and Bailey, H.: Extraordinary runoff from the Greenland Ice Sheet in 2012 amplified by hypsometry and depleted firn-retention, The Cryosphere Discuss., 9 (2015) 4625-4660; doi:10.5194/tcd-9-4625-2015.

http://www.the-cryosphere-discuss.net/9/4625/2015/tcd-9-4625-2015.html

Greenland ice sheet has lost over 0.6 meters of ice in crucial areas in the past 11 months

http://polarportal.dk/fileadmin/polarportal/surface/SMB_map_LA_acc_EN_20150724.png

IceSat data confirm that Greenland's ice sheet melting has been underestimated, and it is accelerating

The most detailed study yet of the Greenland ice sheet illustrates the complex process that is causing billions of tonnes to melt every year

by Tim Radford, Climate News Network, December 27, 2014

LONDON − Greenland’s ice sheet shrank by an average of 243 billion tonnes a year between 2003 and 2009 – a rate of melting that is enough to raise the world’s sea levels by 0.68 mm per year.

In what is claimed as the first detailed study, geologist Beata Csatho, of the University of Buffalo in the US, and colleagues report in the Proceedings of the National Academy of Sciences that they used satellite and aerial data to reconstruct changes in the ice sheet at 100,000 places, and to confirm that the process of losing 277 cubic kilometres of ice a year is more complex than anyone had predicted.

The Greenland ice sheet is the second biggest body of ice on Earth − second only to Antarctica − and its role in the machinery of the northern hemisphere climate is profound.

Careful measurements

It has been closely studied for decades, but such are the conditions in the high Arctic that researchers have tended to make careful measurements of ice melt and glacier calving in fixed locations – in particular, at four glaciers − and then try to estimate what that might mean for the island as a whole.

“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,” Dr Csatho said.

The study looked at readings from NASA’s ice, cloud and land elevation satellite ICESat, and from aerial surveys of 242 glaciers wider than 1.5 km at their outlets, to get a more complete picture of melting, loss and – in some cases – thickening of the ice sheet as a whole.

Previous studies have focused on the four glaciers. One of them, Jakobshavn, has doubled its speed of flow since 2003, and closer studies have begun to reveal more about the dynamics of individual flows.

But the real strength of the study is that it establishes the pattern of ice melt in more detail, and suggests that climate models may not give a clear enough picture of the future of the ice cap. To put it crudely, Greenland could lose ice faster in the future than any of today’s predictions suggest.

Meanwhile, a team from the UK has been trying to work out what is happening on the surface of the ice sheet. Each summer, of course, some of the ice melts. Some of this gets to the sea, but some freezes again in the natural seasonal order of things.

But glaciology researcher Amber Leeson, of the University of Leeds, and colleagues report in Nature Climate Change that the “supraglacial” lakes that form each summer could also affect ice flow.

Their computer simulations suggest that these lakes will migrate further inland as the century wears on and the world continues to warm. Ice reflects heat, water absorbs it. So the process could trigger further melting. Some of this extra meltwater could slide or drain to the base of the glacier, lubricating its flow and accelerating the process yet again.

Thin pancake

“Our research shows that, by 2060, the area of Greenland covered by them will double,” Dr Leeson said. “When you pour pancake batter into a pan, if it rushes quickly to the edge of the pan, you end up with a thin pancake. It’s similar to what happens with ice sheets. The faster it flows, the thinner it will be.

“When the ice sheet is thinner, it is at a slightly lower elevation and at the mercy of warmer air temperatures than it would be if it were thicker, increasing the size of the melt zone around the edge of the ice sheet.”

In the last 40 years, the band in which such supraglacial lakes can form has crept 56 km inland. By 2060, the simulations now suggest, it could reach 110 km inland, doubling the area of coverage and delivering yet more meltwater to fuel further warming.

Once again, the research suggests that current models underestimate the rate of ice loss. 

Greenland Ice Stores Liquid Water Year-Round

POTENTIAL FOR STORING MELTWATER IMPORTANT FOR CALCULATING SEA-LEVEL RISE

Univ. of Utah, Dec. 22, 2013 – Researchers at the University of Utah have discovered a new aquifer in the Greenland Ice Sheet that holds liquid water all year long in the otherwise perpetually frozen winter landscape. The aquifer is extensive, covering 27,000 square miles.

The reservoir is known as a “perennial firn aquifer” because water persists within the firn – layers of snow and ice that don’t melt for at least one season. Researchers believe it figures significantly in understanding the contribution of snowmelt and ice melt to rising sea levels.

The study was published online Sunday, Dec. 22, 2013, in the journal Nature Geoscience.

“Of the current sea level rise, the Greenland Ice Sheet is the largest contributor – and it is melting at record levels,” says Rick Forster, lead author and professor of geography at the University of Utah. “So understanding the aquifer’s capacity to store water from year to year is important because it fills a major gap in the overall equation of meltwater runoff and sea levels.”

Forster’s team has been doing research in southeast Greenland since 2010 to measure snowfall accumulation and how it varies from year to year. The area they study covers 14% of southeast Greenland yet receives 32% of the entire ice sheet’s snowfall, but there has been little data gathered.

In 2010, the team drilled core samples in three locations on the ice for analysis. Team members returned in 2011 to approximately the same area, but at lower elevation. Of the four core samples taken then, two came to the surface with liquid water pouring off the drill while the air temperatures were –4 F. The water was found at about 33 feet below the surface at the first hole and at 82 feet in the second hole.

“This discovery was a surprise,” Forster says. “Although water discharge from streams in winter had been previously reported, and snow temperature data implied small amounts of water, no one had yet reported observing water in the firn that had persisted through the winter.”

The aquifer is extensive, covering 27,000 square miles — larger than the state of West Virginia.  It is similar in form to a groundwater aquifer on land that can be used for drinking water. “Here instead of the water being stored in the airspace between subsurface rock particles, the water is stored in the air space between the ice particles, like the juice in a snow cone,” Forster adds. “The surprising fact is the juice in this snow cone never freezes, even during the dark Greenland winter. Large amounts of snow fall on the surface late in the summer and quickly insulates the water from the subfreezing air temperatures above, allowing the water to persist all year long.”    

Why Studying Ice in Greenland is Important

The Greenland Ice Sheet is vast, covering roughly the same area as the states of California, Nevada, Arizona, New Mexico, Colorado and Utah combined. The average thickness of the ice is 5,000 feet. In 2012, the ice sheet lost volume of 60 cubic miles – a record for melt and runoff.

The consequences of losing the ice sheet could be catastrophic. If all the water retained in the ice sheet melted, it is estimated that the global sea level would rise about 21 feet, says Forster. Although no one is predicting a total meltoff all at once, keeping an eye on ice formation, runoff amounts and how the water is moving is critical to accurately predicting sea level changes.

Until now, calculations of the ice sheet mass changes did not include a year-round storage mechanism for liquid water. Models predicted that water either flowed into rivers and lakes on the ice surface, into crevasses and subglacial streams that eventually run into the sea, or was refrozen within the ice sheet.

Discovery of the perennial aquifer will help scientists predict the movement and temperature of water within the ice sheet with more precision.

Forster says the reservoir’s exact role is unknown. “It might conserve meltwater flow and thus help slow down the effects of climate change. But it may also have the opposite effect, providing lubrication to moving glaciers and exacerbating ice velocity and calving increasing the mass of ice loss to the global ocean.”

As for whether climate change caused the aquifer to form, Forster says that’s not clear, but simulations of the Greenland Ice Sheet going back to the early 1970s would suggest it has been around for some time.

How the Study was Conducted

The previously unknown storage mode was found in the southeast section of Greenland, where conditions combine to provide sufficient rain and snowmelt to fill the firn with water, as well as high levels of snow accumulation that insulate the water from freezing during the winter.

The team used data collected by airborne and ground-penetrating radar to pinpoint the aquifer, and then took core samples on the ground.

Airborne radar imagery was collected in the area by NASA Operation IceBridge, which is a program directed at collecting images of Earth’s polar ice in unprecedented detail to better understand the processes that connect polar regions with climate change. Ground-penetrating radar and a roving Global Positioning System navigation unit also were towed across the ice in the same area via snowmobile, collecting data every five seconds.

Researchers found that the radar images from air and ground corresponded on both the depth of a bright horizon, indicating where there is a change in consistency of the ice, as well as the undulations of the horizon across distance of about 15 miles. This was confirmation that the airborne radar could map the aquifer just as well as the ground-based radar.

Core samples were taken with a 4-inch-diameter drill. Two segments were extracted that were saturated with liquid water – one from a depth of about 33 feet and another the following day about a mile east and at a depth of more than 80 feet.

Temperatures in the spring of 2011 were below average. Forster notes that, “because air temperatures were minus 4 degrees Fahrenheit during drilling and because surface melting in the area did not begin until June in 2011, there is no doubt that the water found in the firn had persisted through the winter.”

This research is an international collaboration among researchers at the University of Utah, the Geological Survey of Denmark and Greenland, Byrd Polar Research Center at the Ohio State University, Institute for Marine and Atmospheric Research Utrecht, Utrecht University, the NASA Goddard Space Flight Center, the Center for Remote Sensing of Ice Sheets at the University of Kansas, and the Desert Research Institute at the University of Nevada, Reno. Forster and the Utah team were supported by the National Science Foundation and NASA.

http://unews.utah.edu/news_releases/greenland-ice-stores-liquid-water-year-round/

"Observations of Pronounced Greenland Ice Sheet Firn Warming and Implications for Runoff Production," by Chris Polashenski et al., GRL (2014); DOI: 10.1002/2014GL059806

Geophysical Research Letters, (2014); DOI: 10.1002/2014GL059806

Observations of Pronounced Greenland Ice Sheet Firn Warming and Implications for Runoff Production^†

Chris Polashenski^1,2,*, 

Zoe Courville^2,3,

Carl Benson⁴, 

Anna Wagner¹, 

Justin Chen⁵, 

Gifford Wong⁶, 

Robert Hawley⁶and

Dorothy Hall⁷

Abstract


Field measurements of shallow borehole temperatures in firn across the northern Greenland Ice Sheet (GIS) are collected during May 2013. Sites first measured in 1952-1955 are re-visited, showing long term trends in firn temperature. Results indicate a pattern of substantial firn warming (up to +5.7 C) at mid-level elevations (1,400-2,500 m) and little temperature change at high elevations >2,500  m). We find that latent heat transport into the firn due to meltwater percolation drives the observed warming. Modeling shows that heat is stored at depth for several years and energy delivered from consecutive melt events accumulates in the firn. The observed warming is likely not yet in equilibrium with recent melt production rates, but captures the progression of sites in the percolation facies toward net runoff production.

http://onlinelibrary.wiley.com/doi/10.1002/2014GL059806/abstract

NASA data shed new light on changing Greenland ice

by George Hale, phys.org, March 10, 2014

This is the calving front of Greenland's Jakobshavn Glacier seen during an IceBridge survey flight in 2012. Credit: NASA / Jefferson Beck

Research using NASA data is giving new insight into one of the processes causing Greenland's ice sheet to lose mass. A team of scientists used satellite observations and ice thickness measurements gathered by NASA's Operation IceBridge to calculate the rate at which ice flows through Greenland's glaciers into the ocean. The findings of this research give a clearer picture of how glacier flow affects the Greenland Ice Sheet and shows that this dynamic process is dominated by a small number of glaciers.

Over the past few years, Operation IceBridge measured the thickness of many of Greenland's glaciers, which allowed researchers to make a more accurate calculation of ice discharge rates. In a new study published in the journal Geophysical Research Letters, researchers calculated ice discharge rates for 178 Greenland glaciers more than one kilometer (0.62 miles) wide.

Ice sheets grow when snow accumulates and is compacted into ice. They lose mass when ice and snow at the surface melts and runs off and when glaciers at the coast discharge ice into the ocean. The difference between yearly snowfall on an ice sheet and the sum of melting and discharge is called a mass budget. When these factors are equal, the mass budget is balanced, but for years the Greenland Ice Sheet has had a negative mass budget, meaning the ice sheet is losing mass overall.

For years the processes of surface melt and glacier discharge were roughly equal in size, but around 2006 surface melt increased and now exceeds iceberg production. In recent years, computer model projections have shown an increasing dominance of surface melt, but a limited amount of glacier thickness data made pinpointing a figure for ice discharge difficult.

Ice discharge is controlled by three major factors: ice thickness, glacier valley shape and ice velocity. Researchers used data from IceBridge's ice-penetrating radar – the Multichannel Coherent Radar Depth Sounder, or MCoRDS, which is operated by the Center for Remote Sensing of Ice Sheets at the University of Kansas, Lawrence, Kan. – to determine ice thickness and sub-glacial terrain, and images from satellite sources such as Landsat and Terra to calculate velocity. The team used several years of observations to ensure accuracy. "Glacier discharge may vary considerably between years," said Ellyn Enderlin, glaciologist at the University of Maine, Orono, Maine and the study's lead author. "Annual changes in speed and thickness must be taken into account."

Being able to study Greenland in such a large and detailed scale is one of IceBridge's strengths. "IceBridge has collected so much data on elevation and thickness that we can now do analysis down to the individual glacier level and do it for the entire ice sheet," said Michael Studinger, IceBridge project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "We can now quantify contributions from the different processes that contribute to ice loss."

With data on glacier size, shape and speed, researchers could calculate each glacier's contribution to Greenland's mass loss and the total volume of ice being discharged from the Greenland Ice Sheet. Of the 178 glaciers studied, 15 accounted for more than three-quarters of ice discharged since 2000, and four accounted for roughly half. Considering the large size of some of Greenland's glacier basins, such as the areas drained by the Jakobshavn, Helheim and Kangerdlugssuaq glaciers, this was not exactly surprising.

What they also found was that the size of these basins did not necessarily correlate with glacier discharge rate, shuffling the order of Greenland's largest glaciers. Previously Helheim Glacier was thought to be Greenland's third largest glacier, but this study puts it in fifth place and adds two southeast Greenland glaciers, Koge Bugt and Ikertivaq South to the list of big ice-movers.

Glacier thickness measurements and this study's calculation methods have the potential to improve future computer model projections of the Greenland Ice Sheet. And with a new picture of which glaciers contribute most to mass loss, IceBridge will be able to more effectively target areas in future campaigns, promising more and better data to add to the research community's body of knowledge.

 Explore further: Mathematician uses skills to study Greenland's retreating glaciers (w/ Video)

Journal reference: Geophysical Research Letters  

Provided by NASA's Goddard Space Flight Center

http://phys.org/news/2014-03-nasa-greenland-ice.html

Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities in the wet snow zone, Sermeq Avannarleq, West Greenland

Journal of Geophysical Research: Earth Surface, Vol. 18, No. 3, pp. 1241-1256 (September 2013); doi: 10.1002/jgrf.20079

Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities in the wet snow zone, Sermeq Avannarleq, West Greenland

Thomas Phillips^1,2,*, 

Harihar Rajaram³,

William Colgan^1,4, 

Konrad Steffen^1,5,

Waleed Abdalati¹

Abstract

[1] Wintertime satellite-derived ice surface velocities, from 2001 through 2007, suggest an increase in ice velocity in the wet snow zone of Southwest Greenland. We present a thermomechanical model to evaluate the influence of surface meltwater runoff on englacial temperatures, via cryo-hydrologic warming (CHW), as a possible mechanism to explain this velocity increase at Sermeq Avannarleq. The model incorporates CHW through a previously published dual-column parameterization. We compare model simulations with (i) CHW active over the entire ice thickness (“base case CHW”), (ii) CHW active only in the surface 80 m of the ice sheet (“surface CHW”), and (iii) “no CHW” to represent a traditional thermomechanical model. The horizontal extent of CHW is prescribed based on equilibrium line altitude position and thus incorporates the upstream expansion of the ablation zone over the past decade. The base case CHW simulations reproduce the observed increase in inland ice velocity between 2001 and 2007 reasonably well. The no CHW and surface CHW simulations significantly underestimate observed ice surface velocities in both epochs. The higher ice velocities in the base case CHW simulations are attributable to both decreased basal ice viscosities associated with increased basal ice temperatures and an increase in the extent of basal sliding permitted by temperate bed conditions. Only the temperate bed extent predicted by the base case CHW simulation is consistent with independent observations of basal sliding. Based on our sensitivity analysis of CHW, we evaluate alternative explanations for an increase in inland ice velocity and suggest CHW is the most plausible mechanism.

http://onlinelibrary.wiley.com/doi/10.1002/jgrf.20079/abstract

Meltwater warming interior of Greenland's Ice Sheet causing accelerating outflow

New Research Explains Acceleration of Greenland’s Inland Ice

July 24, 2013

Science

By developing a new model investigate the effects of meltwater on Greenland glaciers, researchers discovered that meltwater warms the ice sheet, which then softens, deforms, and flows faster.

This animation shows how ice is naturally transported from interior topographic divides to the coast via glaciers. The colors represent the speed of ice flow, with areas in red and purple flowing the fastest at rates of kilometers per year. The vectors indicate the direction of flow.

Surface meltwater draining through cracks in an ice sheet can warm the sheet from the inside, softening the ice and letting it flow faster, a new NASA-funded study finds.

During the last decade, researchers have captured compelling evidence of accelerating ice flow at terminal regions, or “snouts,” of Greenland glaciers as they flow into the ocean along the western coast. Now, the new research shows that the interior regions are also flowing much faster than they were in the winter of 2000-2001, and the study authors propose a reason for the speedup.

“Through satellite observations, we determined that an inland region of the Sermeq Avannarleq Glacier, 40 to 60 miles from the coast, is flowing about 1.5 times faster than it was about a decade ago,” said Thomas Phillips, lead author of the new paper and a research associate at the time of the study with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado, Boulder.

The researchers used ice-sheet-wide velocity maps for Greenland from a NASA program called Making Earth System Data Records for Use in Research Environments. Studying the velocity maps, the researchers saw that in 2000-2001 the inland segment of the Sermeq Avannarleq Glacier was flowing at about 130 feet (40 meters) per year. In 2007-2008, that speed was closer to 200 feet (60 meters) per year.

“At first, we couldn’t explain this rapid interior acceleration,” Phillips said. “We knew it wasn’t related to what was going on at the glacier’s terminus. The speedup had to be due to changes within the ice itself.”

To shed light on the observed acceleration, Phillips and his team developed a new model to investigate the effects of meltwater on the ice sheet’s physical properties. The team found that percolating meltwater carries heat from the sun and warms the ice sheet, which then—like a warm stick of butter—softens, deforms and flows faster.

Previous studies estimated that it would take centuries to millennia for new climates to increase the temperature deep within ice sheets. But when the influence of meltwater is considered, warming can occur within decades and, thus, produce rapid accelerations. The paper has been accepted for publication in the Journal of Geophysical Research: Earth Surface, a journal of the American Geophysical Union.

The researchers were tipped off to this mechanism by the massive amount of meltwater they observed on the ice sheet’s surface during their summer field campaigns, and they wondered if it was affecting the ice sheet. During the last several decades, atmospheric warming above the Greenland Ice Sheet has caused an expanding area of the surface to melt during the summer, creating pools of water that gush down cracks in the ice. The meltwater eventually funnels to the interior and bed of the ice sheet.

“The sun melts ice into water at the surface, and that water then flows into the ice sheet carrying a tremendous amount of latent energy,” said William Colgan, a coauthor and adjunct research associate with the University of Colorado’s Cooperative Institute for Research in Environmental Sciences. “The latent energy then heats the ice.”

The new model shows that this speeds up ice flow in two major ways: One, the retained meltwater warms the bed of the ice sheet and preconditions it to accommodate a basal water layer, making it easier for the ice sheet to slide by lubrication. Two, warmer ice is also softer (less viscous), which makes it flow more readily.

“Basically, the gravitational force driving the ice sheet flow hasn’t changed over time, but with the ice sheet becoming warmer and softer, that same gravitational force now makes the ice flow faster,” Colgan said.

This transformation from stiff to soft only requires a little bit of extra heat from meltwater. “The model shows that a slight warming of the ice near the ice sheet bed—only a couple of degrees Celsius—is sufficient to explain the widespread acceleration,” Colgan said.

The findings have important ramifications for ice sheets and glaciers everywhere. “It could imply that ice sheets can discharge ice into the ocean far more rapidly than currently estimated,” Phillips said. “It also means that the glaciers are not finished accelerating and may continue to accelerate for a while. As the area experiencing melt expands inland, the acceleration may be observed farther inland.”

The study’s results suggest that to understand future sea-level rise, scientists need to account for a previously overlooked factor — meltwater’s latent energy — and its potential role in making glaciers and ice sheets flow faster into the world’s oceans. In 2007, the Intergovernmental Panel on Climate Change wrote that one of the most significant challenges in predicting sea-level rise was “limited” understanding of the processes controlling ice flow. The panel’s next assessment is due out in 2014.

“Traditionally, latent energy has been considered a relatively unimportant factor, but most glaciers are now receiving far more meltwater than they used to and are increasing in temperature faster than previously imagined,” Colgan said. “The chunk of butter known as the Greenland Ice Sheet may be softening a lot faster than we previously thought possible.”

The study was funded through a NASA ROSES grant, NASA’s Greenland Climate Network and the National Science Foundation. Other coauthors on the paper were CIRES Director Waleed Abdalati, who is also former chief scientist for NASA; former CIRES Director Konrad Steffen; and CU-Boulder engineering professor Harihar Rajaram.

Publication: Thomas Phillips et al., “Evaluation of cryo-hydrologic warming as an explanation for increased ice velocities in the wet snow zone, Sermeq Avannarleq, West Greenland,” J. Geophys. Res.: Earth Surface, 2013; DOI: 10.1002/jgrf.20079

http://scitechdaily.com/new-research-explains-acceleration-of-greenlands-inland-ice/

DMI's Surface Mass Budget of the Greenland Ice Sheet

The link to this site will go up in the left-hand column of this blog, for our convenience.

Current Surface Mass Budget of the Greenland Ice Sheet

Here you can follow the daily surface mass balance on the Greenland Ice Sheet. The snow and ice model from one of DMI’s climate models is driven every six hours with snowfall, sunlight and other parameters from a research weather model for Greenland, Hirlam-Newsnow. We can thereby calculate the melting energy, refreezing of melt water and sublimation (snow that evaporates without melting first). The result of this is a change in the snow and ice from one day to the next and this change is shown below. All numbers are in water equivalent, that is, the amount of water the snow and ice would correspond to if it was melted.

Left: Map of the surface mass balance today (in mm water equivalent per day). Right: The average surface mass balance for today’s calendar date over the period 1990-2011.

In the above figure, we show the daily surface mass balance (on the left) and you can see where it has snowed and melted (incl. sublimate) on the ice sheet over the last 24 hours. For comparison, the map to the right shows the average value for the same calendar date over the period 1990-2011. This historical average is based on weather from a somewhat different model and the two are therefore not always strictly comparable.

 

The surface mass balance is calculated over a year from September 1st to August 31st (the end of the melt season). The figure to the right shows the sum of all the daily changes from September 1st up to today. Next year on September 1st the map will be reset and we start over. This accumulated map illustrates how much the surface mass balance has contributed in each point across the ice sheet. 

The figure below shows the total daily contribution from all points on the ice sheet (top) and the same accumulated from September 1st to now (bottom). The blue curves show this season’s surface mass balance in gigatons (Gt: 1 Gt is one billion tons and corresponds to 1 cubic kilometer of water), and for comparison the mean curves from the historical model run are shown with two standard deviations on either side. Note that the accumulated curve does not end at 0 at the end of the year. Over the year, it snows more than it melts, but calving of icebergs also adds to the total mass budget of the ice sheet. Satellite observations over the last decade show that the ice sheet is not in balance. The calving loss is greater than the gain from surface mass balance, and Greenland is losing mass at the rate of about 200 Gt/yr.

Map of the accumulated surface mass balance (in mm water equivalent) from September 1st to now.

Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass balance from September 1st to now (blue line, Gt). The dark grey line shows the corresponding average number from the climatology along with two standard deviations on each side (light grey). In the bottom panel, we also show the season 2011-12 (red) which had very high summer melt in Greenland.

 

Ice flow

Due to gravity, ice flows slowly outwards like dough on a kitchen counter. When snow falls on top of the ice sheet year after year, the layers below are slowly compressed into ice. In the central part of the ice sheet, where little if any melt occurs, new layers will therefore continually be added. The ice does not grow in height, however, since the extra ice is balanced by the flow away from the center. Further out towards the coast we find the equilibrium line, where snowfall and melt are exactly balanced. Below the equilibrium line, there is more melt than snowfall and here the net mass loss is countered by the flow coming out from the center of the ice sheet. Here it is the ice sheet itself which melts.

For an ice sheet that neither grows or shrinks, there is at all points averaged over the year a balance between

• the amount of snow that falls and is compressed to ice
• the amount of snow and ice that melts or evaporates (sublimates) and
• the amount of ice that flows away due to the ice motion

The two first contributions make up the surface mass balance. For the ice sheet as a whole, there is a balance between the surface mass balance and the amount of ice that calves into the ocean as icebergs. 

If climate changes, the surface mass balance may change such that it no longer matches the calving and the ice sheet can start to gain or lose mass. This is important to keep track of, since such a mass loss will lead to global sea level rise. As mentioned, satellites measuring the ice sheet mass have observed a loss of around 200 Gt/year over the last decade.

Greenland Climate Research Centre collaborates with Danish Climate Centre at DMI on research in both atmospheric impact on the Greenland Ice Sheet and the ice flow itself and its interaction with the rest of the climate system.

Three processes determine whether the ice sheet grows or diminishes. Accumulation of snow on top increases mass. In time, the snow is transformed to ice that flows down through the ice sheet and out towards the margins. Melt in the lower regions of the ice sheet and iceberg calving from glaciers reduces the mass. If mass loss exceeds mass gain the ice sheet will shrink. Graphics Diego Winterborg.

For further information please contact climate scientist Peter L. Langen, Danish Climate Centre, iskappe@dmi.dk. 

http://beta.dmi.dk/en/groenland/maalinger/greenland-ice-sheet-surface-mass-budget/

"Why Arctic sea ice shouldn't leave anyone cold" by Neven Acropolis & Kevin McKinney

The first world in ancient Norse mythology, Niflheim, sounds just like the Arctic ice. Its snow- and ice-melt gave rise to the frost giant Ymir, from whose body the whole cosmos was formed. Today, science tells us a different story about the sea ice, but in that story, too, the ice is far older than humans. According to Polyak et al. (2010):

…sea ice became a feature of the Arctic by 47 [million years before the present], following a pronounced decline [in carbon dioxide concentrations]… Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth's overall cooler climate.

Since modern humans are just 200,000 years old, the ice might as well be eternal.

But not unchanging. We live in that part of an ice age that is termed an interglacial, when some of the ice retreats in summer. During modern human history the ice cap has melted back each summer as Earth tipped toward the sun, then grown again with new freezing as the year turned toward the Winter solstice.

Polyak et al. continues:

Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred... The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years…

This information is based upon the detailed study of ‘proxies’—indirect indicators of sea ice extent, including sea-floor sediments containing distinctive mineral, chemical and biological markers; coastal records which include things as diverse as ancient driftwood and geological beach formations; ice-cores from nearby land ice; and tree-ring records, where available.

Direct observations of sea ice go back a surprisingly long way: a Greek navigator, Pytheas of Massilia, sailed into the open Atlantic in 325 BCE, reaching:

…a land he called "Eschate Thule," where the Sun only set for three hours each day and the water was replaced by a congealed substance "on which one can neither walk nor sail." He was probably describing loose sea ice known today as "growlers," or "bergy bits." His "Thule" was probably Norway…

Since then, humans have repeatedly encountered the ice, from 11th century Novaya Zemlya, to the Norse colony in Medieval Greenland, to Henry Larsen’s 1944 transit of the Northwest Passage.  For many it proved a fatal encounter; the names of Hugh Willoughby, Willem Barents, Henry Hudson, and John Franklin would only begin a list of Arctic explorers who bartered their lives for their discoveries.  During this time, the annual melt and refreeze of the ice has been relatively consistent.

But that has changed. Since the turn of the millennium, transits of Northwest and Northeast Passages have increased dramatically. In the Northeast, millions of tons of supertanker traffic have passed already, and Russian infrastructure investments indicate that this is a mere beginning. In the Northwest, Arctic charter cruising has become a growing industry. In both, recreational boaters are increasingly common; and many of them now report that they see no pack ice at all.

Science confirms this anecdotal evidence: satellite measurements show that the ice extent has shrunk since 1979 by nearly 30%. More alarming still, the estimated volume of the sea ice is down a whopping 75%!  When the IPCC released its Fourth Assessment Report in 2007, it was generally thought that the Arctic could become ice-free somewhere near the end of this century. But ice loss has progressed at such speed that scientists now think 2030 might bring the first ice-free Arctic summer. Some say it could even happen this decade.

And if it does? Well, the Arctic sea ice is a highly efficient reflector of solar energy; where there is no ice, dark ocean water absorbs most of the sunlight. The less ice there is, then, the more the ocean heats up. This in turn melts more ice—an example of a positive feedback in action. It is a feedback chain bearing all kinds of consequences for the Arctic region. Disappearing ice can be good for some species; for instance, tiny algae may profit from the warmer waters and an extended growing season.  But those species dependent upon the sea ice for sustenance or habitat—species from microscopic sea ice diatoms, to seals and walrus, to those charismatic polar bears—may suffer, perhaps even to the point of catastrophe.

Rapidly changing conditions also have repercussions for human populations, whose income and culture depend on sea ice. Their communities literally melt and wash awaywith no sea ice to weaken wave action, and their food supplies—often still 50% ‘country food’—are at risk, as populations of traditional prey species (and frequently access to them
as well) are disrupted.

But what happens in the Arctic doesn't stay in the Arctic. When sea ice cover disappears, the changing interaction between sea and atmosphere can shift atmospheric patterns. The results may be felt all over the Northern Hemisphere. As we have seen, a smaller ice pack, combined with an ever earlier melting season, means more and more sunlight is soaked up by dark ocean waters. These warmer waters then release heat and moisture to the atmosphere during fall and winter—an effect already being observed and measured.

This change in turn may already be disturbing the jet stream, the high-altitude wind that separates southern warm air from cold Polar air. A destabilized jet stream becomes more 'wavy,' allowing frigid air to plunge farther south, a possible factor in the extreme winters that were experienced all around the Northern Hemisphere in recent years.

Another side-effect is that as the jet stream waves become larger, they slow down or even stall at times, leading to a significant increase in so-called blocking events. These cause extreme weather simply because they lead to unusually prolonged conditions of one type or another. The recent prolonged heatwave, drought and wildfires in the USA are one example of what can happen; another is the cool, dull and extremely wet first half of summer 2012 in the UK and other parts of Eurasia.

The accumulation of heat in Arctic waters also influences other frozen parts of the Arctic, such as glaciers and ice caps on Greenland and in the Canadian Archipelago. As there is less and less sea ice to act as a buffer, more energy can go into both melting glaciers from below, and warming the air above them. This has a marked effect on Greenland's marine-terminating glaciers and the Greenland Ice Sheet. Not only are glaciers flowing faster towards sea, but there is also a rapid increase in the summer surface melt Greenland experiences, leading to accelerating mass loss from the Greenland Ice Sheet. As the Arctic warms, an increased contribution to sea level rise is inevitable.

Another way Arctic warming could have worldwide consequences is through its influence on permafrost. Permanently frozen soils worldwide contain 1,400–1,700 Gigatons of carbon, about four times more than all the carbon emitted by human activity in modern times. A 2008 study found that a period of abrupt sea-ice loss could lead to rapid soil thaw, as far as 900 miles inland. Apart from widespread damage to infrastructure in northern territories (such as roads, houses and pipelines), the resulting annual carbon emissions could eventually amount to 15–35% of today’s yearly emissions from human activities.  This would make the reduction of atmospheric greenhouse gases a much more difficult task.

An even more worrying potential source of greenhouse gases is the methane in the seabed of the Arctic Ocean, notably off the coast of Siberia. These so-called clathrates contain an estimated 1,400 gigatons of methane, a more potent though shorter-lived greenhouse gas than carbon dioxide. Methane clathrate, a form of water ice that contains a large amount of methane within its crystal structure, remains stable under a combination of high pressure and low temperature.

At a depth of 50 meters or less, the East Siberian Arctic Shelf may contain the shallowest methane clathrate deposits, and thus those most vulnerable to rising water temperatures. High amounts of methane have been recently been measured over ice-free portions of the Arctic Ocean, and the waters of the East Siberian Sea have been shown to be “super-saturated” with methane; large plumes of methane bubbles have been observed there as well. The origins and significance of these emissions are not yet clear, but Arctic methane emissions in general appear to be rising: methane concentrations in the Arctic now average about 1.90 parts per million, the highest in 400,000 years.

Apart from these unrecoverable sources of fossil fuel, the Arctic is also endowed with large amounts of recoverable oil and natural gas. As the sea ice retreats, the Arctic's fossil treasures are eyed greedily by large corporations and nations bordering the Arctic Ocean. This might lead to geopolitical tensions in a world where energy is rapidly becoming more expensive.  (It is also highly ironic that the most likely cause of the disappearance of Arctic sea ice — the extraction and burning of fossil fuels — could lead to more extraction of said fuels. Another feedback loop.)

News articles on the dangers of Arctic sea ice loss are usually illustrated with pictures of polar bears.  But although many animals in the Arctic are threatened by the vanishing Arctic sea ice, homo sapiens may be the species with most at stake. While Arctic sea ice may be “out of sight, out of mind” for many, it does affect human civilization over the Northern hemisphere, and even beyond:  after thousands of years in which the sea ice played a vital role in the relatively stable conditions under which modern civilization, agriculture and a 7 billion strong world population could develop, it increasingly looks as if warming caused by the emission of greenhouse gases is bringing these stable conditions to an end. 

Whether there still is time to save the Arctic sea ice is difficult to tell, but as we've seen, serious consequences will flow from the disappearance of the sea ice.  It appears that these consequences can only be mitigated by keeping fossil fuels in the ground, and carbon out of the air. Whichever way you look at it, business-as-usual is not a sane option.

http://neven1.typepad.com/blog/2012/08/wasislac.html