More than 100,000 years ago mass melting of Antarctic ice sheets caused ocean temperatures to rise and become the major cause of an extreme rise in sea level.
A recent expedition by an international team of scientists led by University of New South Wales Earth and Climate Science Professor Chris Turney discovered a gap in the ice sheet record immediately prior to the Last Interglacial.
This period of missing ice coincides with extreme sea level increase, suggesting rapid ice loss from the West Antarctic Ice Sheet.
Volcanic ash, trace gas samples and ancient DNA from bacteria trapped in the ice all support this finding.
Professor Turney and his team had travelled to the Patriot Hills Blue Ice Area, a site located at the periphery of the West Antarctic Ice Sheet.
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.
The researchers used a different method from the conventional downward drilling to retrieve their samples, called horizontal ice core analysis.
Prof Turney said instead of drilling kilometres into the ice, we can 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 said.
They found the extreme ice loss caused a multi-metre rise in global mean sea levels – and it took less than 2 degree Celsius 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."
The West Antarctic sheet rests on the seabed and is fringed by large areas of floating ice, called ice shelves, that protect the central part of the sheet.
The severity of the ice loss suggests that the West Antarctic Ice Sheet is highly sensitive to future ocean warming.
“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 said.
“The West Antarctic Ice Sheet is sitting in water, and this water is getting warmer and warmer,” said Professor Turney.
“This study shows that we would lose most of the West Antarctic Ice Sheet in a warmer world."
The researchers are also concerned that persistent high sea surface temperatures would prompt the East Antarctic Ice Sheet to melt, driving global sea levels even higher.
Director of The Institute for Sustainable Futures at the UK University of Keele Professor Christopher Fogwill said recent projections suggest that the Antarctic contribution may be up to ten times higher than the Intergovernmental Panel on Climate Change 2013 report forecast which is deeply worrying."
The report suggests that global sea level will rise between 40cm and 80cm over the next century, with Antarctica only contributing around 5cm of this.
“Our study highlights that the Antarctic Ice Sheet may lie close to a tipping point, which once passed may commit us to rapid sea level rise for millennia to come. This underlines the urgent need to reduce and control greenhouse gas emissions that are driving warming today.”
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.”
The research was published in Proceedings of the National Academy of Sciences (PNAS).