Greenland: the surface of the ice sheet is melting faster than predicted
11/19/07

It is becoming difficult to deny global warming. The increase in the surface melting of Greenland's ice sheet over the past 28 years certainly confirms it. Xavier Fettweis, from ULg’s Climatology and Topoclimatology Laboratory, has been studying Greenland for the past six years. He has shown that the ice there is melting much faster than previously estimated: the surface melting of the ice sheet has increased by 45 % since 1979. What’s more, the average summer temperature in Greenland increased 2.4°C between 1979 and 2006, and 2007 already promises to be a record year.

Groenland

Since 1979, the SSM/I and SSMR satellites have been scrutinising the surface of the earth with lenses that are sensitive to microwave radiation. In particular, some of these observations are used to estimate the extent of the surface melting of the ice sheet in Greenland. The signal emitted in the microwave wavelengths by a snow-capped surface differs according to whether meltwater is present in the snow cover or not, making the satellite sensitive to melt areas. By using this data as such, no major increase in the extent of melting was previously observed, leading American researchers to believe that the surface melting of Greenland’s ice sheet had remained stable over the past few years.

But things aren’t quite that simple. Xavier Fettweis revealed discrepancies between the satellite measurements he was analysing and the predictions of his digital model of the climate when it was raining on the ice sheet, as early as 1979: “On some days, the satellite observes melting in a given place, then the next day nothing, and the following day, melting again. Such a discontinuity in time is impossible because the satellite observes the melting over a depth of one metre.

There are also spatial discontinuities. I showed that these discontinuities appeared there where my digital simulations predicted rain on the ice sheet, in other words, the satellite assimilates a day of rain with a day of no melting (because the rain clouds change the direction of the melting signal emitted by the surface)...whereas the rain actually accelerates melting. To correct this change of direction, I therefore imposed continuity in time and space to the satellite data, which allowed me to reveal this trend of an increase in melting." Just recently, observations at a frequency not biased by the presence of rain clouds confirmed Xavier Fettweis’ intuition and, in particular, the accuracy of his corrections.

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