The measurement of melting ice in Greenland or Antarctica is a relatively accurate exercise in 2019, thanks to the arsenal of satellites, weather stations and complex climate models.
Scientists even knew how to do it well in the 1990s and 2000s, but estimates from previous decades were so unreliable as satellites and other measurement technologies were less advanced.
In a study published Monday in the PNAS reports, researchers have recalculated the ice loss since 1972 when Landsat's first satellites were shot regularly in Greenland's filming.
"When you look back for several decades, it's better to sit in your chair before you look at the results, because it's a little scary to see how fast it's changing," said French vocalist Eric Rigot. at the University of California at Irvine, co-author of college research in California, Grenoble, Utrecht, and Copenhagen.
"This is something that affects the four corners of Greenland, not just the warmer parts of the South.
– Six times more cast iron –
Glaciologists have three methods for measuring the ice melt.
Satellites simply measure the altitude – and its variations – with a laser: if the glacier melts, the satellite sees the height drop.
A second technique, since 2002, using NASA satellites to measure the variations of terrestrial gravity: the mountains do not move (almost), but the movements and transformations of water explain them.
Finally, scientists have developed so-called mass balance models: they compare what is being accumulated in Greenland (rain, snow) with what goes out (ice rivers), and calculate what's left. These models, confirmed by field measurements, have become very reliable since the mid-2000s, says Eric Rinho – about 5 to 7% error rate, against 100% a few decades ago.
The team uses these models to go back in time and to reconstruct in detail how the Greenland ice was in the 1970s and 1980s.
The limited data available for this period (medium-resolution satellite images, aerial photographs, snow cores and other field observations) helped to refine the model.
"We added a small part of the story that did not exist," adds Eric Rinyo.
The result is that in the 1970s Greenland has accumulated an average of 47 gigatonnes per year (Gt / yr) before losing an equivalent volume in the 1980s.
Melting continued at this rate in the 1990s, before the sharp acceleration of the 2000s (187 Gt / year) and especially after 2010 (286 Gt / year).
Ice melts there six times faster today than in the 1980s, researchers say. Only Greenland's glaciers would have helped raise the oceans by 13.7 millimeters since 1972.
"This is an excellent job, from a well-established research team that uses new methods to extract more information from available data," said Colin Letterhead of the Scott Polar Research Institute in Cambridge.
Like a similar work by the same Antarctic team, the new study provides a longer context of rapid meltdown observed in Greenland in recent years.
"The ice melt observed over the last eight years is equivalent to that of the previous four decades," says Amber Leeson of Lancaster University.
? 2019 AFP