Greenland Will Lose Mass Faster due to Ice “Lid”

January 6, 2016

Dark Snow Project Chief Scientist Jason Box contributed to a new study of Greenland mass loss that is getting attention.

Washington Post:

Rising global temperatures may be affecting the Greenland ice sheet — and its contribution to sea-level rise — in more serious ways that scientists imagined, a new study finds. Recent changes to the island’s snow and ice cover appear to have affected its ability to store excess water, meaning more melting ice may be running off into the ocean than previously thought.

That’s worrying news for the precarious Greenland ice sheet, which scientists say has already lost more than 9 trillions tons of ice in the past century — and whose melting rate only continues to increase as temperatures keep warming up. NASA estimates that the Greenland ice sheet is losing about 287 billion tons of ice every year, partly due to surface melting and partly due to the calving of large chunks of ice. Because of the ice sheet’s potential to significantly raise sea levels as it runs into the ocean, scientists have been keeping a close eye on it — and anything that might affect how fast it’s melting.

The new study, published Monday in the journal Nature Climate Change, focuses on a part of the ice sheet known as “firn” — a porous layer of built-up snow that slowly freezes into ice over time. It’s considered an important part of the ice sheet because of its ability to trap and store excess water before it’s able to run off the surface of the glacier, an essential service that helps mitigate the sea-level rise that would otherwise be caused by the runoff water.

“As this layer is porous and the pores are connected, theoretically all the pore space in this firn layer can be used to store meltwater percolating into the firn whenever melt occurs at the surface,” said the new paper’s lead author, Horst Machguth of the Geological Survey of Denmark and Greenland, in an email to The Washington Post. Over time, the percolating meltwater trickles down through the firn and refreezes.

Until recently, many scientists have assumed that most of Greenland’s firn space is still available for trapping meltwater. But the new research shows that this is likely no longer the case. Through on-the-ground observations, the scientists have shown that the recent formation of dense ice layers near the ice sheet’s surface are making it more difficult for liquid water to percolate into the firn — meaning it’s forced to run off instead.

“If you look at some of the other studies which have been arguing that you have unlimited capacity for retention of water in the firn, this study shows that that is not the case,” said Kurt Kjær, a curator and researcher at the Natural History Museum of Denmark, who has studied glacier dynamics on the Greenland ice sheet but was not involved in the study.

firn

CBC:

Scientists previously thought that melting process might be slowed down by a porous layer on the surface of the ice called firn, which is partway between ice and snow.

Climate models expected the firn layer to absorb up to 30 to 40 per cent of any meltwater that travels across the ice sheet, allowing it to refreeze instead of pouring into the ocean.

But a new international study has found that during 2012, an unusual amount of melting caused the top of the firn to freeze into solid ice. That meant it could no longer absorb the meltwater, which instead formed thousands of new rivers snaking across the surface of the ice sheet to the ocean.

“That hadn’t been seen before,” said William Colgan, a researcher at York University in Toronto who co-authored the new study.

The Intergovernmental Panel on Climate Change’s predictions of sea level rise are based on models that assume the firn would fill up gradually over the course of a century, reducing the amount of meltwater that Greenland pours into the oceans.

The new study, led by Horst Machguth at the Geological Survey of Denmark and Greenland and published this week in Nature Climate Change, suggest those predictions are underestimates.

“In a year like 2012… an extra 60 gigatonnes of water or thereabouts went into the ocean,” Colgan said.

Rising sea levels are already causing flooding in some coastal areas, and could become a more serious problem more quickly than anticipated.

While firn is theoretically renewable — it reforms from fresh snowfall — the top 40 metres of firn take 80 years to develop, and warmer years like 2012 are becoming increasingly common, Colgan said.

“Now the firn is being capped off much faster than it’s being recreated.”

And the phenomenon may not be restricted to Greenland — many other ice caps like those in Canada are covered in firn too.

“Evidence is emerging to show Canadian Arctic firn is also capping off,” Colgan said. He added that relative to their size, glaciers in Canada contribute more to sea level rise than the Greenland ice sheet.

“Greenland meltwater storage in firn limited by near-surface ice formation”- Nature Climate Change:

Abstract:

Approximately half of Greenlands current annual mass loss is attributed to runoff from surface melt1. At higher elevations, however, melt does not necessarily equal runoff, because meltwater can refreeze in the porous near-surface snow and firn2. Two recent studies suggest that all3 or most3, 4 of Greenlands firn pore space is available for meltwater storage, making the firn an important buffer against contribution to sea level rise for decades to come3. Here, we employ in situ observations and historical legacy data to demonstrate that surface runoff begins to dominate over meltwater storage well before firn pore space has been completely filled. Our observations frame the recent exceptional melt summers in 2010 and 2012 (refs 5,6), revealing significant changes in firn structure at different elevations caused by successive intensive melt events. In the upper regions (more than ~1,900m above sea level), firn has undergone substantial densification, while at lower elevations, where melt is most abundant, porous firn has lost most of its capability to retain meltwater. Here, the formation of near-surface ice layers renders deep pore space difficult to access, forcing meltwater to enter an efficient7 surface discharge system and intensifying ice sheet mass loss earlier than previously suggested3.

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8 Responses to “Greenland Will Lose Mass Faster due to Ice “Lid””

  1. Sir Charles Says:

    Here an earlier study which makes the whole thing even more worrisome:

    Melting Greenland ice sheet is rapidly slowing the Gulf Stream

    • Greg Wellman Says:

      Tom, your first link explicitly says “Greenland is losing mass at about 200 Gt/yr.”

      The maps on the first half of the page do not show annual changes. The first is one specific May 1st (presumably 2015, although it says 2016, which is not possible), the second is an average of several May 1sts, and the third is from Sept 1 2015 to present (i.e. “Winter”).

      An ice sheet in balance will accumulate in the middle, flow outwards and calve/melt at the edges, with accumulation = loss. Greenland is in a state of accumulation < loss. This research about firn capping describes one mechanism by which that inequality can get stronger and probably is getting stronger. It's not claiming that accumulation by itself has stopped.

      Your second link is (a) Antarctica, not Greenland, (b) a single study (using radar altimetry) that contradicts the GRACE measurements (that use gravity) and other studies. The new study is interesting, but I wouldn't bet on it vs GRACE.

      • dumboldguy Says:

        So, Greg? Now you’ve met Tommy B! He’s a serially confused and misinformed occasional visitor to Crock. Or maybe he plays that part in the hopes that someone will believe his BS, in which case he’s a denier troll.

        Thanks for taking the time to treat him as if he was serious and knowledgeable and saving the rest of us from wasting any time. It takes a village, etc….

      • pendantry Says:

        Impressive! If only it were possible to dismiss all denierbait as convincingly as you’ve done to master Bates…


  2. Palm to forehead!!!!

    With regards to Peter L. Langen work at the Danish Climate Centre.. I think you need to read the link you provided…

    First he is discussing the calibration of MODELS….

    ” Everything shown on this site, however, is calculated with this new model, so that all curves and values are comparable.”

    NO he does not say that Greenland is gaining ice mass… quite to the contrary..
    ” As mentioned, satellites measuring the ice sheet mass have observed a loss of around 200 Gt/year over the last decade. ”

    With regards to Zally’s paper… well there are issues with this.. The study analyzed changes in the surface height of the Antarctic ice sheet measured by radar altimeters on two European Space Agency European Remote Sensing (ERS) satellites, spanning from 1992 to 2001, and by the laser altimeter on NASA’s Ice, Cloud, and land Elevation Satellite (ICESat) from 2003 to 2008. The ICESat satellite had serious tech difficulties and there is some question by the scientific community about the accuracy of the data set provided and the calibration Zally applied to the data set. Further the study only covers parts of Antarctic. This per an interview with Zally.

    ” It could take only a few decades for the ice melt in Antarctica to outweigh the ice gains, the paper’s authors say.

    “I don’t think there will be enough snowfall increase to offset these losses,” Jay Zwally, NASA glaciologist and lead author of the study, said in a press release.

    For now, the study authors say, these findings challenge current explanations for sea level rise, much of which is attributed to melting ice sheets in Greenland and Antarctica.

    “The good news is that Antarctica is not currently contributing to sea level rise, but is taking 0.23 millimeters per year away,” said Dr. Zwally.

    “But this is also bad news,” he added. “If the 0.27 millimeters per year of sea level rise attributed to Antarctica in the IPCC report is not really coming from Antarctica, there must be some other contribution to sea level rise that is not accounted for.” [Christian Science Monitor, 11/1/15]

    Further,

    “Gavin Schmidt, who directs the NASA Goddard Institute for Space Studies and was not affiliated with the study, said that data from a pair of satellites called GRACE, which measure gravity, actually points towards a net loss of ice on the Antarctic continent in more recent years.

    Schmidt said that there are two methods for measuring the mass of an ice sheet. The first measures gravity, and the second measures the elevation of the top of the ice sheet. Both methods need to take different variables into account to be accurate. The method used in this most recent study measured the ice sheet’s elevation, and the most recent time period it considered ended in 2008.

    “I would pin more weight to the GRACE data than to this latest paper,”

    One paper, still in the peer review process…. DOES NOT make a consensus!!!

    • addledlady Says:

      Who said it was consensus?

      The important point the paper raises is that scientists, cryologists and glaciologists, have been taking something for granted – the capacity of firn to absorb and retain large quantities of meltwater – when this now seems likely to be mistaken. It’s about getting people to think about a feature of the science that’s not been analysed in depth before. Unfortunately, it’s something that has been assumed to be benign or positive when it may very well be significantly negative.

      It looks to me a bit like the naive presumption that many people make about drought stricken soils. They presume that such soils will gratefully soak up any water that comes their way. Anyone who’s actually observed extremely dry soils (or soils in areas affected by fire or clear-felling) knows that the only way rainfall on such soils will be absorbed will be if that rainfall is a gentle soaking rain. If it comes in the form of a downpour the rain will wash away the loose, dusty topsoil, run off to pollute creeks and rivers, possibly causing or worsening erosion gullies in the process, and leave depleted, still fairly dry soil behind. Some soils when they dry out so completely are not just unable to absorb water, they can become water repellent.

      And it looks likely that firn can do the same thing in its very different circumstances. Instead of its open structure being ready and capable of receiving and holding meltwater, its surface can become water repellent rather than absorbent, regardless of the fact that there’s plenty of capacity beneath the surface to absorb some-most-all of the meltwater.


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