Researchers from the University of Cambridge and the British Antarctic Survey have discovered for the first time direct evidence that the West Antarctic Ice Sheet shrank suddenly and dramatically during the last Ice Age, about 8,000 years ago. Evidence in ice cores shows that in one location, the ice thinned by 450 meters in less than 200 years, which is taller than the height of the Empire State Building.

Evidence in ice cores shows that in one location, the West Antarctic ice sheet thinned by 450 meters in just under 200 years, exceeding the height of the Empire State Building. Source: University of Cambridge/British Antarctic Survey

This is the first evidence of rapid ice loss found anywhere in Antarctica. Scientists worry that today's rising temperatures could destabilize parts of the West Antarctic Ice Sheet in the future, potentially crossing tipping points and triggering runaway collapses. The new research, published today (February 8) in the journal Nature Geoscience, reveals how quickly Antarctic ice will melt if temperatures continue to soar.

Professor Eric Wolff, from the Department of Earth Sciences at the University of Cambridge and senior author of the new study, said: "We now have direct evidence that this ice sheet has suffered rapid ice loss in the past. This is not unique to our model predictions and could happen again if parts of the ice sheet become unstable."

Inside the drilling tent, engineers and scientists separate the inner and outer barrels of the drill bit between drills. Image credit: University of Cambridge/British Antarctic Survey

The Antarctic ice sheet contains enough fresh water from west to east to raise global sea levels by about 57 meters. The West Antarctic Ice Sheet is considered particularly vulnerable because much of it rests on bedrock below sea level. Model projections suggest that much of the West Antarctic Ice Sheet could disappear within the next few centuries, causing sea levels to rise. However, the exact timing and speed of the ice loss is uncertain.

Historical background and modern significance

One way to train ice sheet models to make better predictions is to feed them data on ice loss during warming periods in Earth's history. At the peak of the last ice age 20,000 years ago, Antarctic ice covered a much larger area than it does now. As the Earth thawed and temperatures slowly rose, the West Antarctic Ice Sheet shrank to its current extent.

Study co-author Dr Isobel Rowell from the British Antarctic Survey said: "We wanted to know what happened to the West Antarctic Ice Sheet at the end of the last Ice Age, when the Earth's temperatures were rising, albeit at a slower rate than current anthropogenic warming. Using the ice cores, we can go back to that time and estimate the thickness and extent of the ice sheet."

Drilling and living tents. Photo credit: Eric Wolf

Ice cores are made up of layers of ice that form when snow falls and are then buried and compacted into ice crystals over thousands of years. Each ice sheet contains bubbles of ancient air and pollutants that mix with annual snowfall, providing clues to changes in climate and ice extent.

Researchers drilled a 651-meter-long ice core in 2019. The ice mound is located on the edge of the ice sheet, near where grounded ice flows into the floating Longne Ice Shelf.

After the ice cores were shipped back to Cambridge at minus 20 degrees Celsius, researchers analyzed them to reconstruct the ice thickness. First, they measured stable water isotopes, which indicate the temperature at which the snow fell. The higher the altitude, the cooler the temperature, so they were able to equate higher temperatures with lower, thinner ice.

They also measured the pressure of air bubbles trapped in the ice. Like temperature, air pressure changes systematically with altitude. Lower, thinner ice contains air bubbles with higher pressure.

These measurements told them that the ice thinned rapidly 8,000 years ago. "Once the ice thins, it shrinks rapidly. This is clearly a tipping point - a runaway process," Wolf said.

They believe that the West Antarctic Ice Sheet typically rests on bedrock, and warm water entering beneath the edge of the ice sheet may be responsible for the thinning of the ice. This likely caused part of the ice sheet to break away from the bedrock, causing it to suddenly float and form the current Longne Ice Shelf. This allows adjacent ice rises to no longer be constrained by ground ice and rapidly thin.

The researchers also found that the amount of sodium in the ice (derived from salt in sea fog) increased about 300 years after the ice thinned. This told them that as the ice thinned, the ice shelves shrank back, bringing the ocean hundreds of kilometers closer to their study site.

"We already knew from models that the ice was thinning around this time, but the date was uncertain," Rowell said. "Ice sheet models place the timing of the ice retreat at between 12,000 and 5,000 years ago, but can't tell how fast it was retreating. We now have a very precise observational date for the date that can be incorporated into improved models."

Although the West Antarctic Ice Sheet retreated rapidly 8,000 years ago, it stabilized by the time it reached its current approximate extent. "The key now is to figure out whether the additional warmth will destabilize the ice and cause it to start retreating again," Wolf said.

Compiled source: ScitechDaily