FACT SHEETS

Ancient Antarctic ice melt increased sea levels by 3+ metres – and it could happen again

13.02.20

Rising ocean temperatures drove the melting of Antarctic ice sheets and caused extreme sea level rise more than 100,000 years ago, a new international study shows.

Mass melting of the West Antarctic Ice Sheet was a major cause of high sea levels during a period known as the Last Interglacial (129,000-116,000 years ago).

The extreme ice loss caused a multi-metre rise in global mean sea levels – and it took less than 2˚C of ocean warming for it to occur.

“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.

Professor Chris Turney taking surface samples across the Patriot Hills Blue Ice Area. Image credit: AntarcticScience.com
Blue ice areas are the perfect laboratory for scientsists
– they are created by fierce, high- density katabatic winds
Ice age cycles occur approximately every 100,000 years
- due to subtle changes in Earth’s orbit around the Sun
Ice core from Patriot Hills. Image credit: AntarcticScience.com

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

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

The research was undertaken at the Patriot Hills Blue Ice Area, on the edge of the West Antarctic Ice Sheet and was published today in Proceedings of the National Academy of Sciences (PNAS).

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

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

The research was undertaken at the Patriot Hills Blue Ice Area, on the edge of the West Antarctic Ice Sheet and was published today in Proceedings of the National Academy of Sciences (PNAS).

Professor Michael Bird said the findings helped to fill a gap in understanding the Last Interglacial, when global warming triggered by changes in Earth’s orbit led to sea levels between six and nine metres higher than present day.

“That degree of sea-level rise couldn’t be fully accounted for by the melting of the Greenland Ice Sheet, and ocean expansion from warmer temperatures and melting mountain glaciers,” he says. “Evidence of significant Antarctic ice loss at that time helps us better understand the Last Interglacial, which in turn gives us a clearer view of what the future might hold.”

 

 

“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.

Blue ice areas are created by fierce, high-density winds that 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. Image credit: AntarcticScience.com

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.

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.

The severity of the ice loss suggests that the West Antarctic Ice Sheet is highly sensitive to future ocean warming.

Trace gas bubbles in the ice samples. Image credit: AntarcticScience.com

Exploring sites in West Antarctic by skidoo. Image credit: AntarcticScience.com
Professor Chris Turney climbing Flower Hills, West Antarctic. Image credit: AntarcticScience.com

“The positive feedbacks between a warming ocean, ice shelf collapse, and ice sheet melt suggests that the West Antarctic may be vulnerable to passing a tipping point,” stressed Dr Zoë Thomas, co-author and ARC Discovery Early Career Research Award (DECRA) Fellow at UNSW.

“As it reaches the tipping point, only a small increase in temperature could trigger abrupt ice sheet melt and a multi-metre rise in global sea level.”

The researchers warn that this tipping point may be closer than we think.

“The Paris Climate Agreement commits to restricting global warming to 2˚C, ideally 1.5˚C, this century,” says Professor Turney.

“Our findings show that we don’t want to get close to 2˚C warming.”

Professor Turney and his team hope to expand the research to confirm just how quickly the West Antarctic Ice Sheet responded to warming and which areas were first affected.

“We only tested one location, so we don’t know whether it was the first sector of Antarctica that melted, or whether it melted relatively late. How these changes in Antarctica impacted the rest of the world remains a huge unknown as the planet warms into the future” he says.

“Testing other locations will give us a better idea for the areas we really need to monitor as the planet continues to warm.”

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