"The implication of nonradiative energy fluxes dominating Greenland ice sheet exceptional ablation area surface melt in 2012," by Robert Fausto et al., GRL 43 (2016); doi: 10.1002/2016GL067720

Geophysical Research Letters, 43 (March 2016), DOI: 10.1002/2016GL067720

The implication of nonradiative energy fluxes dominating Greenland ice sheet exceptional ablation area surface melt in 2012

Robert Fausto, Dirk van As, Jason E. Box and Ruth Mottram

Abstract

During two exceptionally large, July 2012, multi-day, Greenland ice sheet, melt episodes, non-radiative energy fluxes (sensible, latent, rain, and subsurface collectively) dominated the ablation area surface energy budget of the southern and western ice sheet. On average, the non-radiative energy fluxes contributed up to 76% of daily melt energy at nine automatic weather station sites in Greenland. Comprising 6% of the ablation period, these powerful melt episodes resulted in 12–15% of the south and west Greenland automatic weather station annual ablation totals. Analysis of high resolution (~5 km) HIRHAM5 regional climate model output indicates widespread dominance of non-radiative energy fluxes across the western ablation area during these episodes. Yet HIRHAM5 still underestimates melt by up to 56% during these episodes due to a systematic underestimation of turbulent energy fluxes typical of regional climate models. This has implications for underestimating future melt, when exceptional melt episodes are expected to occur more frequently.

Introduction 

Understanding the Greenland ice sheet surface climate response is crucial for reducing uncertainties in future predictions of both magnitude and rate of global sea level change [Dutton et al., 2015] and freshwater flux [Lenaerts et al., 2015]. The rate of Greenland ice sheet mass loss has accelerated over the past decades [Tedesco et al., 2013; Khan et al., 2015], and in recent years, the surface components of the ice sheet’s mass budget have become the dominant source of ice loss, outpacing the ice dynamic component [Enderlin et al., 2014; Andersen et al., 2015]. Partly due to two exceptional melt episodes in July 2012, new records for ice sheet surface melt area and ice mass loss were set [Tedesco et al., 2013]. Satellite observations revealed more than 98% of the ice sheet surface was melting on 12 July 2012, which was unprecedented in the 1978 to present satellite record [Nghiem et al., 2012]. This widespread melt in the accumulation area was enhanced by low-level liquid clouds [Bennartz et al., 2013] promoted by the advection of anomalously warm and moist air over Greenland [Neff et al., 2014], which decreased the firn’s ability to retain meltwater [Machguth et al., 2016]. Deposition of wildfire black carbon further promoted melt through enhanced sunlight absorption [Keegan et al., 2014]. Projections suggest that such melt episodes will become increasingly frequent in coming decades [Collins et al., 2013; McGrath et al., 2013].

Melting of the surface of Greenland's ice sheet is adding to sea level rise faster than previously realized

by Tim Radford, Climate News Network, January 9, 2016

LONDON – Water may be flowing from the Greenland icecap and into the sea more quickly than anybody expected.

It doesn’t mean that global warming has got conspicuously worse: rather, researchers have had to revise their understanding of the intricate physiology of the Northern Hemisphere’s biggest icecap.

There is enough ice and snow packed deep over 1.7 million square kilometres of Greenland that, were it all to melt, would cause a rise in global sea levels of about six metres.

Climate calculations

Since the icecap is melting as the atmospheric levels of the greenhouse gas carbon dioxide rise, and global temperatures rise with them, as a consequence of the human combustion of fossil fuels, the rate at which summer meltwater gets into the oceans becomes vital to climate calculations.

The latest rethink begins not with the pools of water that collect on the surface each summer, or the acceleration of the glaciers as they make their way to the ocean, but with a granular layer of snow just below the surface, called firn.

This is old snow in the process of being compacted into glacier ice, and covers the island in a layer up to 80 metres thick.

Until now, researchers have understood this firn layer as a kind of sponge that absorbs meltwater and holds it, thus limiting the flow of melting ice into the sea.

But a new study in Nature Climate Change by researchers from the US, Denmark and the University of Zurich suggests that earlier assumptions may be wrong.

“Meltwater couldn’t penetrate vertically through the solid ice layer, and instead drained along the ice sheet surface towards the ocean”

However, the findings are not definitive, and they deliver a picture more of science in progress, rather than any long-term conclusion.

To work out how much meltwater might be stored within the pores of the firn, the scientists set up camp in 2012, 2013 and 2015 on the ice cap to use radar and to drill a series of holes 20 metres deep into the porous firn layer − also choosing sites where samples had been taken 20 years ago.

The conclusion was that meltwater is being released faster than anticipated.

Horst Machguth, a research associate in the Department of Geography at the University of Zurich, says: “Basically, our research shows that the firn reacts fast to a changing climate. Its ability to limit mass loss of the ice sheet by retaining meltwater could be smaller than previously assumed.”

Storage capacity

An extreme melt in 2012 left a sheet of solid ice, several metres thick, on top of the porous firn, in some places.

“In subsequent years, meltwater couldn’t penetrate vertically through the solid ice layer, and instead drained along the ice sheet surface towards the ocean,” says William Colgan, assistant professor in the Department of Earth and Space Science and Engineering at York University in Toronto, Canada.

“It overturned the idea that the firn can behave as a nearly bottomless sponge to absorb meltwater. Instead, we found that the meltwater storage capacity in the firn could be capped off relatively quickly.”

The implication is that sea level rise from Greenland’s icecap is liable to be higher than predicted. Just how much higher is unknown, and the next step is to confirm the latest findings and incorporate the research so far into climate models.

Since detailed research in a hostile environment is always a challenge, any clear answer may take a few years more to emerge. 

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 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