Shrinking ice sheet made a surprising comeback

The ice sheets near earth's poles have been constantly shrinking for the past 20,000 years. At least, that's what scientists used to think. But according to a study published in Nature, the West Antarctic Ice Sheet has regrown in recent history - and the process was driven by its own shrinking.

Since the peak of the last glacial period about 20,000 years ago, the planet has been warming, the seas rising, and the ice sheets generally getting smaller. But the new study, co-led by glaciologist Jonathan Kingslake from Columbia's Lamont-Doherty Earth Observatory, found that the West Antarctic Ice Sheet shrank more than anyone suspected.

Around 10,000 years ago, its leading edge had retreated 200 kilometers farther inland than its present-day location. Yet it didn't collapse. It bounced back, and Kingslake and his team think they know how.

Unfortunately, the mechanism behind the ice sheet's regrowth probably won't work fast enough to save today's ice sheets from melting and causing seas to rise. However, the findings could help to refine predictions about how today's warming climate will impact polar ice and sea-level rise.

Accidental Discoveries
The team stumbled upon two key parts of its discovery by accident. First, during a trip to Antarctica to study ancient ice flows, Kingslake and a colleague were towing a radar device over the ice near the Weddell Sea. Just before they turned around to head back to camp, the radar spotted something strange hundreds of meters below, where the ice sheet meets solid ground.

Normally, ice layers look like smooth, undulating waves. But this feature was a series of what looked like cracks slicing up through the layers chaotically. "It was just bizarre," says Kingslake. "We hadn't seen these kinds of structures near the base of an ice sheet before."

Later, an in-depth survey turned up many more of these bizarre structures in the area. They resembled areas with rapid melting, which could have happened only if the ice had been in contact with the ocean. But that's not the way ice sheets are supposed to work - in most places they only flow in one direction: from land to the ocean.

The second discovery came when Reed Scherer and his team from Northern Illinois University performed a novel analysis on sediments recovered from the base of the ice sheet on the other side of the continent.

The team found substantial amounts of carbon-14 under the ice. That was surprising, because carbon-14 comes from living organisms and their remains. That meant the deposits under the ice sheet had to have come from marine life.

"There are no fish where the ice is grounded on the sea floor," explains Scherer, "but radiocarbon in sediments 200 kilometers upstream tells us that the sea had been much further back before. Ocean creatures left behind a radiocarbon clock."

The team concluded that this part of the ice sheet had also been in contact with the ocean sometime within the past 40,000 years, probably much more recently.

So, What Happened?
Team members led by Torsten Albrecht from the Potsdam Institute for Climate Impact Research used a detailed computer model to investigate several possible explanations for these strange findings. They knew that as the climate warmed after the last glacial maximum, the sea level rose.

Their computer model simulated those rising seas lifting the ice shelf, like a boat stranded on a beach when the tide comes in. The uplift caused the grounding line - where the ice sheet's leading edge stops making contact with the seafloor and instead turns into a floating ice shelf - to fall back. Seawater flowed under the lifted ice, exposing some 350,000 square kilometers of the base to the ocean, according to the simulations.

But the sea level never went back down, and there wasn't enough snowfall to make the ice shelf sit back down on the seafloor - so there was no obvious reason for the grounding line to move back out another 200 kilometers or so, to its present day location.

Instead, the Potsdam team's model found that the earth rose to meet the ice.

"If you pile up a bunch of ice on the earth's crust, it bends down," Kingslake explains. "Remove it, and it pops back up." The team thinks that over thousands of years, as the ice sheet shrank, the crust in this area rebounded by hundreds of meters and the grounding line moved back out.

"When I observed the re-growth in our numerical computer simulations of Western Antarctica, I first thought this might be a flaw," says Albrecht. "It looked so different from what you find in the text books. So I started figuring out the involved interactions between the ice, ocean and Earth and their typical time scales."

The position of the grounding line matters because it separates ice that is resting on the ground from ice that is floating. If the grounding line advances, the grounded portion of the ice sheet grows and water is removed from the ocean. This causes global sea levels to fall.

The new findings are the opposite of what people thought before, says Kingslake. "People thought the ice sheet just retreated to its current position. We didn't think it could completely overshoot and then readvance to its current position."

What Does It Mean For Us?
The researchers caution that these findings do not mean that we should stop worrying about the world's melting ice sheets. The crustal uplift that helped the ice to readvance takes thousands of years, whereas large portions of the present-day ice sheets could disappear within centuries.

"If the ice sheet does collapse as predicted," says Kingslake, "this uplift is unlikely to have a noticeable effect this century. But over thousands of years it could be very important."

In addition, the team doesn't know how common this rebounding process is. Although two sections of Antarctica's western ice sheet seem to have regrown within the past 10,000 years, the simulations suggest the rapidly melting Thwaites Glacier has been unaffected by this phenomenon.

Whether or not the rebound will stop ice retreat in the future depends on the shape of the local seafloor and how easily the crust pops back up. Predicting those variables requires more accurate mapping of the ground beneath the ice, as well as higher resolution ice sheet models.

Going forward, Kingslake and his colleagues will continue trying to work out details of the rebounding process. Eventually, they also hope to find a way to incorporate more detailed seafloor information into ice sheet models without making them too unwieldy.

The team's new line of thinking could help to make climate models more realistic and reliable, improving predictions about how ice sheets will melt and seas will rise in the coming decades.

"Generally, people talk about predicting how fast the ice sheet is going to change," says Kingslake. "We're going further and asking in which direction it's going to change - whether it's going to grow or continue to shrink over the next few thousand years."

Research Report: "Extensive retreat and re-advance of the West Antarctic ice sheet during the Holocene,"

Related Links
The Earth Institute at Columbia University
Beyond the Ice Age

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