Nearly a quarter of the western Antarctic ice sheet can now be considered unstable, according to a new assessment of 25 years of satellite data.
Unstable, scientists mean that more ice is lost in the area than it is replenished by snowfall.
Some of the larger glaciers have thinned by more than 120 meters in some places.
The losses of the two largest ice currents – Pine Island and Thwaites – were multiplied by five during the period of observation of the spacecraft.
And the changes have accelerated markedly in the last decade alone.
It is thought that the driver is a warm seawater that attacks the edges of the continent where its drainage glaciers penetrate into the sea.
The study conducted by the British was presented here in Milan during the Living Planet Symposium, the largest Earth observation conference in Europe.
It was also published simultaneously in the journal Geophysical Research Letters.
The overview collects data from four satellite missions superimposed from the European Space Agency (ESA) – ERS-1, ERS-2, Envisat and Cryosat.
These spacecraft were all launched with radar altimeters to measure the change in height in the east and west sectors of the ice sheet.
Their unified data from 1992 to 2017 were then combined with meteorological models to distinguish elevation trends due to short-term changes in snowfall resulting from the long-term change in ice mass resulting from melting. and calving iceberg.
"With this unique dataset, we have been able to identify parts of Antarctica undergoing rapid and sustained thinning – regions that are moving faster than expected due to normal weather conditions," said Dr. Malcolm. McMillan from Lancaster University. British Center for Polar Observing and Modeling.
"We can now clearly see how these regions have developed over time, extending inland to some of the most vulnerable regions of Western Antarctic, which is essential for understand the contribution of the ice sheet to sea level rise, "he told BBC News.
If West Antarctica and Eastern Antarctica are considered as a whole, this contribution is 4.6 mm for the period under study. It would have been higher by one millimeter still if the area of the cap was not gaining slightly in mass during this period.
Even in this case, the losses observed in the west mean that the continent's contribution to the ever-increasing surface of the world's oceans is now moving towards the upper end of the projections.
The computer models contained in the last major assessment of the Intergovernmental Panel on Climate Change (IPCC) predicted, in the central interval, a 5 cm increase in sea level from Antarctica. here at 2100.
As it stands, Professor Andrew Shepherd of the University of Leeds said that his colleague and lead author of the CPOM project was above 10 cm.
"There are 3,000 km of coastline, including the portions of Bellingshausen, Amundsen and Marie Byrd Land – which are clearly not modeled, because that is where all the ice comes and more ice than expected" , he explained.
"So we have to go back to these models to try to understand what part of the signal they are not capturing, and of course, the elevation data, which gives a very detailed description of the imbalance, should be the first thing to do. . "
There is now a concerted international effort to investigate the most rapidly changing areas.
During the past shipping season, a United States and United Kingdom-led mission is collecting geophysical information about the ocean in front of the Thwaites Glacier. Repeated shipments to the ice surface are planned for the coming seasons.
Thwaites and its neighbor Pine Island Glacier seem to be the Achilles heel of the western Antarctic ice sheet.
Located in the sea area of Amundsen, they represent by far the biggest signal of imbalance. About 50% of their watersheds are now losing mass at an average rate since 1992 of 28 billion tonnes per year at Pine Island and 46 billion tonnes per year at Thwaites.
But it's the acceleration that speaks, say the scientists.
Between 1992 and 1997, losses were respectively 2 billion tonnes and 12 billion tonnes. During the last period of the survey (2012 to 2016), the rate rises to 55 billion tons and 76 billion tons per year.