Heat waves, droughts and mass extinctions are all potential threats from climate change. But the scariest risk has always been that of rapid sea-level rise caused by the collapse of the massive ice sheets in Greenland and Antarctica. There is enough water locked on Greenland alone to raise global sea levels by 23 ft. (7 m) if it melted, which would swamp coastal cities like London and Shanghai and all but wipe away small island states like the Maldives and Tuvalu. We can likely adapt, expensively, to higher temperatures and changing precipitation patterns, but it's difficult to imagine how we could cope with the oceans literally erasing some of our most valuable real estate.

While the geologic record shows the earth has experienced rapid sea-level rise in the past, during the sharp warm-ups that follow the end of ice ages, those big melts have occurred when the world had much less ice than it does now. Scientists are unsure of how quickly rising temperatures from global warming could destabilize and melt our existing sheets — the working assumption has been that such major melting and subsequent sea-level rise would take centuries, if not longer, even in a warmer world. (See TIME's special report on the environment.)

But a new study published in the April 16 issue of Nature argues that our ice sheets may be far more vulnerable than we believe, and that it may be a matter of decades before cities like New York are turned into swampland. Scientists led by Paul Blanchon of the National Autonomous University of Mexico examined sea-level fluctuations during the planet's last inter–ice age warm period, about 121,000 years ago, and found that the water rose as much as 10 ft. (3 m) in a matter of decades thanks to melting ice sheets. That conclusion indicates that, in the current interglacial period, we could well be facing rapidly rising tides by the end of the century if warming continues unchecked.

Blanchon examined fossil coral reefs about 40 miles south of Cancún on the east coast of Mexico's Yucatan peninsula. (The fossils had been exposed during the construction of a new seaside resort.) Working with his co-authors at Germany's Leibniz Institute of Marine Science, Blanchon calculated the age of the samples by measuring isotopes of thorium in the fossils, a process similar to carbon-dating. The patterns of the fossils indicated points where the coral died when the seas rose too fast for the organisms to adapt; each time the seas stabilized, the corals grew back, but at higher elevations and further inland, a process geologists call backstepping. The result is something like the ascending rings on a bathtub that indicate rising water levels.

Blanchon confirmed the age of the Mexican fossils at different elevations by comparing them to similar reefs in the Bahamas, and determined that the seas might have risen by 6.5 to 10 ft. (2 to 3 m) over the course of 50 to 100 years — far faster than scientists had assumed. Only rapidly melting ice sheets could explain sea-level rise occurring that swiftly, which would indicate that the ice locked away in Greenland and Antarctica today might not be as safe as we had thought.

There are caveats: the interglacial period during which the Mexican coral fossils were deposited was warmer than the world today, and sea levels were as much as 20 ft. (6 m) higher. And other scientists caution that Blanchon's work should still be viewed as preliminary and in need of independent confirmation at other, similar sites where old coral fossils have been deposited. (One obstacle is that only a few places on the earth — the Yucatan peninsula among them — have been seismically calm enough over the past several hundred thousand years to allow for such measurements.) But in the wake of the surprise breakaway of the Wilkins Ice Shelf in Antarctica, which won't raise sea levels but will speed the melting of the remaining Antarctic ice, the Nature study is a grim warning of a potentially flooded future.