Polar ice sheets are synchronized swimmers

Glaciers in the Northern and Southern Hemispheres advance and retreat together

When the last ice age was coming to a close about 19,000 years ago, the ice sheets that cover Antarctica began to shrink to their current sizes at about the same time as those in the Arctic, a new study finds.

Evidence from deep sea sediments suggests the East Antarctic ice sheet, seen jutting into the Weddell Sea, may be more susceptible to global changes in ocean level and temperature than previously believed. Courtesy of Michael Weber

That implies that the East Antarctic Ice Sheet, long thought to be relatively stable compared with the western  side of the ice-clad continent, “is in fact, not so stable,” says geologist Michael Weber of the University of Cologne, Germany. And if the much larger East Antarctic Ice Sheet were to break off into the Weddell Sea, the resulting sea level rise would be tremendous, says Weber, who, with his colleagues, reports the findings in the Dec. 2 Science.

“Before our study, people thought Antarctic ice sheets began their retreat around 14,000 years ago,” says Weber. After gathering data from samples of deep-sea sediment plumbed from the Weddell Sea off  East Antarctica, the researchers now suggest that Antarctic glaciers started to shrink 5,000 years earlier, around the same time as their counterparts in the Northern Hemisphere.

The sediments the researchers studied contained fossils made of carbonate, a chemical compound that’s easy to calculate the age of but rarely found in ice cores used to establish a timeframe for glacial growth and retreat. “This is some of the first carbonate found at high latitudes,” says geochemist Jess Adkins of Caltech, who calls the study a “fantastic breakthrough.”

Because the seafloor sediments accumulated as varves, or repeating layers that reflect the passage of time much like tree rings, Weber’s team calculated the age of Antarctic carbonate shells at a certain layer, and then simply counted the layers downward to reconstruct the history of glacial activity.

Although the researchers feel confident suggesting the advance and retreat of the Arctic and Antarctic ice sheets may have occurred simultaneously, they still don’t fully understand the mechanism that may have provoked the synchronized melt. Escalating atmospheric temperatures, increased circulation of warm surface or deep-sea water, and rising sea levels are all potential culprits for the global glacial retreat 19,000 years ago.

“The oceans didn’t heat up until later,” says Andrew Mackintosh of the Arctic Research Centre at Victoria University in Wellington, New Zealand. He suggests that melting of the Northern Hemisphere ice sheet caused oceans to rise across the globe, shifting Southern Hemisphere ice into deeper water, which then spurred the Antarctic ice sheet’s retreat.

The new findings also suggest more broadly that melting or freezing ice sheets at one pole affects the ice and ocean waters at the other end of the world. If a melting cycle accelerates again, coastal populations could be endangered by rising sea levels.

“Warming ocean water is a problem for the future,” says Mackintosh. “A change of one degree can make a huge difference to the ice sheet.” He believes that understanding how polar ice sheets responded to environmental factors in the past may help predict how changing climate conditions will affect glacial masses over the next century.    

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