“It’s better to sit in your chair before watching the results,” warns French glaciologist Eric Rignot. With several international researchers, he has written a report on the ice melt in Greenland for 47 years. The measurement is relatively accurate in 2019, thanks to an arsenal of satellites, weather stations and sophisticated climate models.
Scientists knew how to do this 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 phenomenon that affects all of Greenland
In their study published Monday, April 22 in the Proceedings of the American Academy of Sciences (PNAS), researchers have recalculated the loss of ice since 1972, date of the orbiting of the first Landsat satellites having regularly photographed Greenland.
“It’s also something that affects the four corners of Greenland, not just the warmer parts of the South,” says Rignot.
Three methods to measure glacial melt
Scientists 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.
- Measure earth gravity variations: if the mountains do not move, it must be due to the movements and transformations of water.
- Glaciologists have developed models called mass balance: they compare what accumulates on Greenland (rain, snow) to what comes out (rivers of ice), and calculate what is left.
These models, confirmed with field measurements, have become very reliable since the mid-2000s, says Eric Rignot.
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 little data they had for this period (medium-resolution satellite photos, aerial photos, snow cores and other field observations) helped 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 47 gigatonnes (47 billion tonnes) of ice a year on average, before losing an equivalent volume in the 1980s.
The ice melts six times faster today
Melting continues at this rate in the 1990s, before a sharp 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.
“It’s an excellent job, by a well-established research team using new methods to extract more information from the available data,” commented Colin Summerhayes of the Scott Polar Research Institute in Cambridge.
As a similar work of the same team for Antarctica, the new study provides a longer context to the rapid melting observed in Greenland in recent years.
“The glacial melting observed over the past eight years is equivalent to that of the previous four decades,” says Amber Leeson of Lancaster University.
