Measuring melting ice in Greenland or Antarctica is a relatively accurate exercise in 2019, thanks to an arsenal of satellites, weather stations and sophisticated climate models. Scientists also knew how to do quite well for the 1990s and 2000s, but estimates from previous decades were so far unreliable because satellites and other measurement technologies were less advanced.
A change at a frightening speed
In a study published Monday, April 22, 2019 in the Proceedings of the American Academy of Sciences (PNAS), researchers have recalculated the loss of ice since 1972, when the first Landsat satellites were first photographed in Greenland. “When you look back over several decades, it's better to sit in your chair before looking at the results, because it's a bit scary to see how fast it's changing.“, told AFP the French glaciologist Eric Rignot, at the University of California Irvine, co-author of the study with colleagues in California, Grenoble, Utrecht and Copenhagen.It's also something that affects the four corners of Greenland, not just the warmer parts of the South“.
Glaciologists have three methods for measuring glacial melt. Satellites simply measure altitude – and its variations – with a laser: if a glacier melts, the satellite sees its altitude drop. A second technique, since 2002 using NASA satellites, to measure the earth's gravity variations: the mountains do not move (almost) not, it is the movements and transformations of water that explain them.
Finally, scientists have developed so-called mass balance models: they compare what accumulates on Greenland (rain, snow) to what comes out (rivers of ice), and calculate what remains. These models, confirmed with field measurements, have become very reliable since the mid-2000s, says Eric Rignot – about 5 to 7% margin of error, against 100% a few decades ago.
Ice melts six times faster
The team used these models to “go back in time” and reconstruct in detail where the ice of Greenland was in the 1970s and 1980s. The limited data they had for this period (medium resolution satellite photos, photos aerial observations, snow coring and other field observations) helped to refine the model. “We added a little piece of history that did not exist“, adds Eric Rignot.
The result is that in the 1970s, Greenland gained an average of 47 gigatonnes of ice a year (Gt / yr), before losing an equivalent volume in the 1980s. Melting continues at this rate in the 1990s , before a strong acceleration from the 2000s (187 Gt / year) and especially since 2010 (286 Gt / year). The ice melts there six times faster today than in the 1980s, say the researchers. The glaciers of Greenland alone would have helped raise the level of the oceans by 13.7 millimeters since 1972.
“C.is an excellent job, by a well-established research team that uses new methods to extract more information from the available dataColin Summerhayes of the Scott Polar Research Institute in Cambridge commented, “Like a similar work from the same team for Antarctica, the new study provides a longer context for the rapid melting observed in Greenland in recent years.”Ice melt observed over the past eight years is equivalent to that of the previous four decades“, summarizes Amber Leeson of Lancaster University.