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Ice Remained Even When Earth Was Hot

Ice is in retreat worldwide as glaciers melt, Arctic ice floes vanish, and Antarctic ice shelves break apart. Will all of it eventually disappear as the globe warms?

Not necessarily, say André Bornemann of the University of Leipzig in Germany and several colleagues. From sediment cores drilled out of the Atlantic seafloor, they retrieved fossils of tiny, shell-encased marine organisms called foraminifers that lived 91 million years ago during the Cretaceous Thermal Maximum, when tropical seas were about 12 Fahrenheit degrees warmer than they are today.

The fossils' shells contained a high proportion of oxygen-18, an isotope that increases in the ocean relative to oxygen-16 when water evaporates from the sea and gets trapped on land as ice. The isotope data suggest that even during the hot spell, an ice sheet half the size of the current Antarctic ice cap existed — but where?

It couldn't have been near the North Pole, which the fossil record shows was then home to heat-loving crocodiles. Instead, Bornemann thinks the ice cap covered high mountain ranges near the South Pole. The warm weather prevailing elsewhere would have injected plenty of moisture into the air to fall as snow on high altitudes way down south.

But make no mistake. Compared with today, a lot of ice was missing back then, and sea levels were much higher — a history that seems to be on the verge of repeating itself.

The research was detailed earlier this year in the journal Science.