PARIS • Raising the earth's average surface temperature by another degree Celsius will lock in 2.5m of sea level rise from Antarctica alone, and an extra 3 deg C will see the frozen continent lift oceans by 6.5m, scientists have warned.
These devastating increases in the global waterline - enough to cripple coastal cities from Mumbai to Miami and displace hundreds of millions of people - would unfold over hundreds to thousands of years.
But the man-made greenhouse gas emissions that could guarantee such an outcome are on track to occur on a timescale measured in decades, the scientists reported in the journal Nature.
One of the study's most alarming conclusions is that sea level hikes caused by a disintegrating Antarctic ice sheet - which holds enough frozen water to boost oceans by 58m - would become dramatically larger with each additional degree of warming.
Sea level rise, for example, would average about 1.3m for each of the first two degrees above pre-industrial levels.
The earth's average surface temperature has already gone up 1.1 deg C since the late 19th century, enough to enhance the severity of deadly heatwaves, droughts and tropical cyclones.
But from 2 deg C to 6 deg C above that benchmark, the increase in sea level would double to 2.4m for each degree of warming.
At the upper end of that range, climate change would devastate civilisation and redraw the map of the world's coastlines, scientists say.
Beyond that, each added degree would result in 10m more, pushing the entire ice sheet past the point of no return and lifting oceans to levels not seen for millions of years.
"In the end, it is our burning of coal and oil that determines if and when critical temperature thresholds in Antarctica are crossed," Professor Anders Levermann, a climate scientist at the Potsdam Institute for Climate Impact Research and a co-author of the study, said in a statement.
"And even if the ice loss happens on long time scales, the respective carbon dioxide levels can already be reached in the near future."
The ice sheet atop West Antarctica will be the first to go, eroded not so much by warm air but rather warm sea water seeping below the underbelly of glaciers and eroding their ocean-facing edges, known as ice shelves.
"That makes glaciers the size of Florida slide into the ocean," noted Dr Torsten Albrecht, another co-author and also a researcher at the Potsdam Institute.
Once global warming crosses the 6 deg C threshold, the dynamic changes.
"As the gigantic mountains of ice" - up to 5km thick - "slowly sink to lower heights where the air is warmer, this leads to more melt at the ice surface," Dr Albrecht added.
This process has already created raging rivers of meltwater on the Greenland ice sheet, which saw a net loss of more than half a trillion tonnes in mass last year alone.
"This very important and timely study makes clear the urgent need to stabilise surface temperature rise in line with Paris Agreement targets so as to limit the total committed sea level rise to a few metres," said Dr Matt Palmer, a British Met Office sea level rise scientist who did not take part in the research.
The 2015 Paris Climate Agreement enjoins nations to cap global warming at "well below" 2 deg C, and 1.5 deg C if possible.
Even 2 deg C "represents an existential threat to entire nation states", glaciology professor Jonathan Bamber of the University of Bristol told the Science Media Centre, commenting on the study.
"We're looking at removing nations from the map - it doesn't get much more serious than that."