Ice caps, whose melting would raise the oceans by several meters, could well collapse with half a degree of additional warming of the climate, according to recent studies which shed light on their hitherto unsuspected fragilities.
The Greenland and Antarctic ice caps have lost more than 500 billion tonnes per year since the year 2000, or six Olympic swimming pools every second.
But climate models had so far underestimated their contribution to future sea level rise, accounting only for the effect of rising air temperatures on ice – and neglecting interactions complexes between the atmosphere, the oceans, the ice caps and certain glaciers.
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Researchers based in South Korea and the United States have established what sea level rise will be by 2050 according to the different scenarios of IPCC climate experts.
If current climate policies continue – including commitments made by countries under the 2015 Paris climate agreement – melting in Antarctica and Greenland would result in an increase of about one half a meter from the water level.
A figure that would rise to 1.4 meters in a worst-case scenario, in the event of a significant increase in greenhouse gas emissions.
In Antarctica, the Thwaites glacier fractures in an unsuspected way
The scientists' study, published this week in the journal Nature Communication, also pinpoints when runaway melting and runaway disintegration of these ice sheets could occur.
“Our model has thresholds between 1.5°C and 2°C of warming – 1.8°C being our best guess – for accelerating ice loss and sea level rise,” said explained to AFP Fabian Schloesser, of the University of Hawaii, co-author of the study.
Temperatures have already risen nearly 1.2°C globally since pre-industrial times.
Scientists have long known that the West Antarctic and Greenland ice sheets – which could raise sea levels by 13 meters in the long term – had “tipping points” beyond which their disintegration would be inevitable.
But the temperatures associated with this phenomenon had never been precisely identified.
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Other studies published this week in Nature also show that the Thwaites Glacier in West Antarctica is fracturing in an unsuspected way.
This glacier the size of Great Britain has already retreated 14 km since the 1990s but the phenomenon was not well understood, for lack of data.
An expedition of British and American scientists drilled a hole the equivalent of two Eiffel Towers (600 meters) deep through the thick tongue of ice pushed by Thwaites into the Admundsen Sea.
They discovered signs of accelerated erosion – with inverted staircase-like formations – as well as cracks opened by seawater.
Millions forced into exile sooner than expected
"The warm water seeps into the cracks and contributes to the wear of the glacier at its weakest point," said Britney Schmidt, author of one of the studies and professor at Cornell University in New York.
Another study, published in the publication Earth's Future, points out that the rise of the oceans will destroy arable land as well as sources of drinking water and will force millions of people into exile sooner than expected.
“The time we have to prepare for greater flood exposure may be much less than previously assumed,” the authors conclude.
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Estimates have so far relied heavily on misinterpreted data: when measuring the altitude of coastal regions using radar, the tops of trees or roofs have often been confused with ground level.
It is therefore actually much lower than we thought.
Tens of millions of people are particularly vulnerable in the coastal areas of countries such as Bangladesh, Pakistan, Egypt, Thailand, Nigeria or Vietnam.
With AFP
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