Researchers have created the most realistic reconstruction yet of how a vast ice sheet advanced across northwestern Europe starting about 31,000 years ago and then retreated into oblivion, exposing landmasses that today are Great Britain and Ireland. The detailed chronology could improve forecasts of melting ice sheets in Antarctica and Greenland, which could devastate coastal regions around the world.

The detailed new history of the ice sheet is “extremely useful and valuable,” says Frank Pattyn, a glaciologist at the Free University of Brussels. The geological history, he adds, provides a way to test the accuracy of modern simulations of melting ice sheets, which could help researchers predict how much and how fast oceans will rise this century.

At the peak of the last major ice age, about 24,000 years ago, the Scottish Highlands were covered in ice 1 kilometer thick, and mammoths and woolly rhinoceroses roamed southern England. The immense ice sheet extended over Ireland and across the North Sea into Scandinavia. Over the next 15 millennia, the ice sheet broke up, retreated, and melted away. In the process, its meltwater raised global sea levels by 1.8 meters. If other ice sheets melt the same amount this century, it will be more than enough to submerge much of Miami.

To chart the movements of this ancient ice sheet, geologists assembled scattered clues from the shape of the landscape—features of which were carved by massive glaciers—as well as debris dumped by the ice sheet. For example, researchers pinpointed and dated the edge of the ice sheet by recognizing the hilly terrain made of mud, gravel, and rocks left behind by the retreating ice. Radiocarbon dating of animal remains such as seashells found in these hills revealed when the ice sheet departed from various locations.
Over the past few years, a group of about 40 scientists amassed these data to refine the history of what’s known as the British-Irish Ice Sheet. Climate researchers consider that ice sheet to be particularly relevant for understanding modern climate change because in places it resembled several modern ice sheets, including the massive ice sheet in Greenland today.

In the new study, the scientists created a computer model that estimated how huge amounts of ice would have deformed under pressure and flowed over the ancient land between 31,000 and 15,000 years ago. Then they adjusted these to match known facts about the ice sheet’s history.

The researchers assembled data from their research expeditions, including analyses of about 15 tons of rock and sediment samples, tripling the data points for where the age of the ice sheet could be established. The data—which included three times as many dated locations of the ice sheet’s movement as past simulations—came from land-based fieldwork and submarine-captured sediment that once lay below the ice sheet.

“This work is important because it brings together many lines of evidence to provide a history of how the British-Irish Ice Sheet retreated at unprecedented detail,” says Natalya Gomez, a glaciologist at McGill University who was not involved.

The team created maps of the ice sheet for each millennium, then animated its growth and decline. The new animation (seen above) was made from maps that debuted last month in the journal Boreas. According to the new calculations, at its maximum extent about 24,000 years ago, the ice sheet was about 30% larger than previously estimated—a “large surprise,” says Christopher Clark, a glacial geomorphologist at the University of Sheffield, who led the 5-year project.

Oddly, the ice sheet started to shrink even before the climate began to warm. Moreover, its collapse was surprisingly abrupt, the researchers say. This presented a mystery until scientists realized the ice sheet’s weight must have been enough to press the land down into the ocean. After the landmasses began to sink, the ice sheet that had once extended offshore to the west and into the North Sea no longer rested on the sea floor, but instead began to float. This destabilized it and it gradually broke up as icebergs snapped off into the sea. “You see it retreat dramatically fast,” Pattyn says. “Suddenly it’s all gone.”

Finally, the warming climate melted the ice that remained on land, making Great Britain and Ireland hospitable enough for human occupation. How much of those islands will remain dry land depends greatly on the stability of today’s ice sheets. The possibility that warming temperatures could cause huge chunks of land-based ice to slide into the ocean and raise sea levels is “definitely a big concern,” Pattyn says.

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