Ancient Antarctic ice melt increased sea levels by 3+ meters — and it can happen again

“Not only did we lose a lot of the West Antarctic Ice Sheet, but this happened very early during the Last Interglacial,” says Chris Turney, Professor in Earth and Climate Science at UNSW Sydney and lead author of the study.

Fine layers of ancient volcanic ash in the ice helped the team pinpoint when the mass melting took place. Alarmingly, the results indicated that most ice loss occurred within the first millennia, showing how sensitive the Antarctic is to higher temperatures.

“The melting was likely caused by less than 2°C ocean warming — and that’s something that has major implications for the future, given the ocean temperature increase and West Antarctic melting that’s happening today,” Professor Turney says.

During the Last Interglacial, polar ocean temperatures were likely less than 2°C warmer than today, making it a useful period to study how future global warming might affect ice dynamics and sea levels.

“This study shows that we would lose most of the West Antarctic Ice Sheet in a warmer world,” says Professor Turney.

In contrast to the East Antarctic Ice Sheet — which mostly sits on high ground — the West Antarctic sheet rests on the seabed. It’s fringed by large areas of floating ice, called ice shelves, that protect the central part of the sheet.

As warmer ocean water travels into cavities beneath the ice shelves, ice melts from below, thinning the shelves and making the central ice sheet highly vulnerable to warming ocean temperatures.

Going back in time

To undertake their research, Professor Turney and his team travelled to the Patriot Hills Blue Ice Area, a site located at the periphery of the West Antarctic Ice Sheet, with support from Antarctic Logistics and Expeditions (or ALE).

Blue ice areas are the perfect laboratory for scientists due to their unique topography — they are created by fierce, high-density katabatic winds. When these winds blow over mountains, they remove the top layer of snow and erode the exposed ice. As the ice is removed, ancient ice flows up to the surface, offering an insight into the ice sheet’s history.

While most Antarctic researchers drill down into the ice core to extract their samples, this team used a different method — horizontal ice core analysis.

“Instead of drilling kilometres into the ice, we can simply walk across a blue ice area and travel back through millennia. By taking samples of ice from the surface we are able to reconstruct what happened to this precious environment in the past,” Professor Turney says.

Through isotope measurements, the team discovered a gap in the ice sheet record immediately prior to the Last Interglacial. This period of missing ice coincides with the extreme sea level increase, suggesting rapid ice loss from the West Antarctic Ice Sheet. The volcanic ash, trace gas samples and ancient DNA from bacteria trapped in the ice all support this finding.

Learning from the Last Interglacial

Ice age cycles occur approximately every 100,000 years due to subtle changes in Earth’s orbit around the Sun. These ice ages are separated by warm interglacial periods. The Last Interglacial is the most recent warm period to our current interglacial period, the Holocene.

While human contribution to global warming makes the Holocene unique, the Last Interglacial remains a useful research point to understand how the planet responds to extreme change.

“The future is heading far beyond the range of anything we’ve seen observed in the scientific instrumental record of the last 150 years,” says Professor Turney. “We have to look further into the past if we’re going to manage future changes.”

During the Last Interglacial, global mean sea levels were between 6m and 9m higher than present day, although some scientists suspect this could have reached 11m.

The sea level rise in the Last Interglacial can’t be fully explained by the Greenland Ice Sheet melt, which accounted for a 2m increase, or ocean expansion from warmer temperatures and melting mountain glaciers, which are thought to have caused less than a 1m increase.

“We now have some of the first major evidence that West Antarctica melted and drove a large part of this sea level rise,” says Professor Turney.