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    21 Jul 2016

    Greenland loses a trillion tonnes of ice in four years - compared to 90 billion tonnes a year during the last century

    It’s no news that Greenland is in serious trouble — but now, new research has helped quantify just how bad its problems are. Asatellite study, published last week in the journal Geophysical Research Letters, suggests that the Greenland ice sheet lost a whopping 1 trillion tonnes of ice between the years 2011 and 2014 alone. And a big portion of it came from just five glaciers, about which scientists now have more cause to worry than ever.  
    It’s the latest story in a long series of increasingly worrisome studies on ice loss in Greenland. Research already suggests that the ice sheet has lost at least 9 trillion tonnes of ice in the past century and that the rate of loss has increased over time. Climate scientists are keeping a close eye on the region because of its potentially huge contributions to future sea-level rise (around 20 feet, or six metres, if the whole thing were to melt) — not to mention the damage it’s already done. Ice loss from Greenland may have contributed as much as a full inch of sea-level rise in the last 100 years and up to 10 percent of all the sea-level rise that’s been documented since the 1990s.   
    The new study takes a detailed look at ice loss in Greenland between 2011 and 2014 using measurements from the CryoSat-2, an environmental research satellite launched by the European Space Agency in 2010. It relied on a type of measurement known as altimetry — basically, measuring how the surface of Greenland’s altitude changed over time in response to ice gains or losses.   
    “Simplistically, if the ice sheet’s going up, we can find that as evidence that the ice sheet is growing,” said lead author Malcolm McMillan, a research fellow at the University of Leeds. “And where we see that the ice sheet surface is lowering, we can find that the ice sheet is losing ice.”
    But he cautioned that this is something of a simplification. The researchers also had to consider how other factors such as snowfall — which would be difficult to differentiate by satellite — might be affecting changes on the surface of the ice sheet.
    “Snow and ice are at different densities, so they’re associated with a different amount of mass loss,” McMillan explained. “We used a regional climate model and a model of the surface of the ice sheet to really inform us and tell us about the nature of the changes that we’re seeing.”
    Using this method — combining the satellite observations with modeling — the researchers found that the Greenland ice sheet lost mass at an average rate of about 269 billion tonnes per year from January 2011 through December 2014. Altogether, this comes to about 1 trillion tonnes of ice loss over the four-year period.  
    That said, there were some major fluctuations from one year to the next — an observation that University of Sheffield climate expert Edward Hanna (who was not involved in the new study) said is one of the paper’s most notable findings. The biggest losses were observed in 2012, when an unusually warm summer helped bring about a loss of more than 400 billion tonnes of ice. The next year, 2013, saw a comparatively modest loss of just over 100 billion tonnes.  
    “There’s not so many studies that do these sort of trend analyses or time studies for the latest few years,” Hanna noted. “So it’s really trying to assess how the ice sheet is responding to ongoing climate variability or change.” 
    Overall, the ice loss was particularly prevalent in the southwest, but the scientists noted that there were also losses observed in the cooler, northern parts of the ice sheet. Notably, the researchers also found that a solid 12 percent of all the ice loss came from just a handful of glaciers composing less than 1 percent of the ice sheet’s total area.
    Each of these five glaciers flows outward into the sea, so that a combination of both rising air temperatures and ocean temperatures likely play a part in their ongoing retreat. Among these was the iconic Jakobshavn glacier, a well-studied location now famous for its recent massive ice losses. It’s been known to calve blocks of ice boasting several square miles in surface area, as measured from above.  
    Scientists were already fairly well aware of the massive losses being suffered by these glaciers, McMillan acknowledged. But the finding helps reinforce previous observations and drive home their disproportionate role in the ice sheet’s contributions to sea-level rise. “Also … it means that going forward, we’re able to kind of develop long-term and systematic records that we can [use to] regularly monitor these glaciers and see how they’re changing into the future,” he said.
    In fact, the study’s results match up reasonably well with measurements taken by certain other satellites. After doing some comparisons, the researchers found that data from NASA’s GRACE satellites, for instance, suggest that Greenland is losing ice at a rate of about 287 billions tonnes per year. And according to Hanna, the results stand well with scientists’ overall estimates of recent ice loss in Greenland, which he says are consistently suggested to be around 250 billion tonnes annually for the past few years. 
    In this way, the study reinforces many beliefs that were already widely held about Greenland’s precarious condition. But the techniques used to do so may strengthen future measurements, which will be used to inform the climate models that help scientists make predictions about how the ice sheet will behave in the future — a crucial step in determining the amount of sea-level rise we might expect over any given time period.

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