Cold Case
An ice-penetrating radar system, towed behind a snowmobile, turns up strange features beneath the West Antarctic Ice Sheet. Jonathan Kingslake

Scientists have long believed the last glacial period, famously known as the Ice Age, ended with a period of continued warming which resulted in the shrinking of ice sheets and an increase in sea levels for thousands of years. The process still continues, but a new study noting three independent pieces of research has hinted at a more complicated past.

According to the new work, due to excessive warming sometime between 14,500 to 9,000 years ago, the West Antarctic Ice Sheet (WAIS) melted more rapidly than one could imagine, with its leading edge retreating 200 km farther inland than its present-day location. The sheet was on the brink of collapse, but somehow it survived, recovered to some extent over ensuing thousands of years, and came to the point we see today.

"The WAIS today is again retreating, but there was a time since the last Ice Age when the ice sheet was even smaller than it is now, yet it didn't collapse," Northern Illinois University professor Reed Scherer, one of the lead authors on the study, said in a statement. "That's important information to have as we try to figure out how the ice sheet will behave in the future."

Scherer and a team of researchers from Columbia University and Potsdam Institute for Climate Impact Research in Germany worked separately to delve into the history of Antarctica, but surprisingly, they all came to the same conclusion.

The team from northern Illinois analyzed sediments from the base of the ice sheet in Ross Sea region and found signs of left behind by marine life, which indicated the area was linked to ocean waters sometime in the last 40,000 years, which was more recent than previously thought.

"There are no fish where the ice is grounded on the sea floor,” Scherer said, noting that, “radiocarbon (carbon-14) in sediments 200 kilometers upstream tells us that the sea had been much farther back before. Ocean creatures left behind a radiocarbon clock."

Their findings were further backed when researchers from Columbia University exploring the other side of the ice sheet also found the same answer but in a different way. The group, which was on an expedition to study ancient ice flows, was using radar devices that surprisingly revealed strange features buried deep within the ice sheet. These were a series of ancient cracks filled with marine ice, which resembled the features seen in areas with rapid melting.

"It was just bizarre," Jonathan Kingslake, the author of the study said in the statement. "We hadn't seen these kinds of structures near the base of an ice sheet before, and the best explanation is that they formed as this portion of the ice sheet re-grounded."

Though the last group, led by Torsten Albrecht, wasn’t involved in an observational work, their sophisticated modeling techniques did coincide with the first two studies and noted grounding line of the ice sheet was up to 200 km inland in the Weddell Sea and 400 km in the Ross Sea region. The grounding line separates ice resting on the ground from ice that is floating. Its fall back like in this case, means melting ice sheets and sea level increase.

"It retreated inland by more than 1,000 kilometers in a period of 1,000 years in this region - on geological time-scales, this is really high-speed,” Albrecht said in another statement. “But now we detected that this process at some point got partially reversed. Instead of total collapse, the ice-sheet grew again by up to 400 kilometers. This is an amazing self-induced stabilization. However, it took a whopping 10,000 years, up until now.”

That said, it is worth noting the researchers have cautioned the processes that helped the ice sheet recover back in the day might not work in the present-day scenario of rapid melting due to human activity.

According to the group, the recovery of the ice-sheet occurred due to a process called isostatic rebound or the lift-up of ground which was depressed by the weight of ice, which disappeared after the glacial maximum.

"If you pile up a bunch of ice on the earth's crust, it bends down," Kingslake explained. "Remove it, and it pops back up."

This worked over thousands of years last time around, which was not fast enough to save ice sheets from melting and causing sea-levels to rise.