The world’s oceans could rise catastrophically as soon as 50 years from now, according to a new paper published this morning in Atmospheric Chemistry and Physics. The researchers behind the paper—Dr. James Hansen and 18 coauthors—looked back to 120,000 years ago, the last time the Earth warmed by the about the same amount it has today. (Global temperatures are now 1˚ C, or 1.8˚ F, above preindustrial levels.) Back then, natural warming unleashed nearly all of the water locked in polar ice sheets, sending sea levels surging 20-30 feet higher.

Today, over 630 million people worldwide live at or below 30 feet above sea level. If the polar ice caps began to melt as they did 120,000 years ago, the researchers determined that much of the resulting fresh water would linger around the poles. The additional layer of water could slow or even halt major ocean currents that have been helping to redistribute and partially cool the seawater’s heat, similar to how stirring a steaming cup of coffee helps it cool off faster. If the ocean currents shut down, then heat could build below the surface, the researchers think, causing even faster melting of the remaining ice that sits below the water level.

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Climate scientists agree that we’re on the path to dangerous levels of warming and that the only way to slow that warming is if we curtail our greenhouse gas emissions. Still, not all climate scientists agree with this paper’s findings. Here’s Justin Gillis, reporting for the New York Times:

“Some of the claims in this paper are indeed extraordinary,” said Michael E. Mann, a climate scientist at Pennsylvania State University. “They conflict with the mainstream understanding of climate change to the point where the standard of proof is quite high.” Among Dr. Hansen’s colleagues, some of the discomfiture about the new paper stems from his dual roles as a publishing climate scientist and, in recent years, as a political activist. He has been arrested at rallies, and he has joined with a group of young people who sued the federal government over what they said was its failure to limit global warming.

Despite these reservations, the idea of so-called tipping points suddenly altering the climate isn’t a new one. Fast melting ice sheets are a potential tipping point, for example. Another is a drying Amazon, where droughts could kill off around 80% of the rainforest, which is a significant absorber of carbon dioxide. The new paper will likely force scientists to reconsider the what might happen if the poles suddenly and rapidly melt. If the paper is accurate, then even the target of 3.6˚ F, or 2˚ C, set by the Paris climate accord might not be enough to forestall the dangerous consequences of warming. Sea levels will rise by the same amount all over the world, right? Wrong. Here’s why:

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Photo credit: Jocelyn Augustino/FEMA