11 Mar 2016

Antarctic ice unstable in high-carbon scenario

From Our Changing World, 8:00 am on 11 March 2016

Research published today shows that the giant East Antarctic ice sheet could become unstable if carbon dioxide levels in the atmosphere exceed 600 parts per million.

New Zealand's Scott Base and the Ross Ice Shelf.

New Zealand's Scott Base and the Ross Ice Shelf. Photo: Rob McPhail

Tim Naish, director of Victoria University's Antarctic Research Centre.

Tim Naish, director of Victoria University's Antarctic Research Centre. Photo: NSF

Tim Naish, the director of Victoria University’s Antarctic Research Centre and a lead author on the publication, said the study of sediment cores that document the growth of the first continent-wide ice sheet in Antarctica 34 million years ago had identified 600ppm as a threshold beyond which the ice sheet was at risk of melting.

”The drill cores show that the first Antarctic ice sheet was quite dynamic. It advanced and retreated many times between 34 to 35 million years ago before finally stabilising at its largest extent when atmospheric carbon dioxide levels dropped below … 600 parts per million.”

The research was based on geological drill cores taken from the Ross Sea near New Zealand’s Scott Base 16 years ago by the international Cape Roberts Drilling Project.

Professor Naish said the sediment layers record advances and retreats of the East Antarctic ice sheet that are paced by changes in Earth’s orbit around the sun, known as Milankovitch cycles.

These cycles affected the distance between Antarctica and the sun and were the main reason why earth had moved in and out of ice ages and warm period naturally.

However, the cores also showed that once the ice sheet became more stable as carbon dioxide levels dropped below 600ppm, it tended to only respond to the longer Milankovitch cycles, which occured every 100,000 or 400,000 years.  

With carbon dioxide levels already at 400ppm and predicted to continue to rise unless greenhouse gas emissions were reduced quickly and dramatically, Professor Naish said the study provided insights into the potential future stability of Antarctica’s larger ice sheet.

“We know that parts of the Antarctic ice sheet that sit on bedrock below sea level are already melting in response to climate change. The implication of this study is that the giant East Antarctic Ice sheet that sits mostly on rock above sea level and was thought to be more stable, will melt if CO2 levels of 600ppm or above are reached and sustained for centuries to millennia, contributing (to) … 60 metres of global sea level rise.”

He says carbon dioxide levels were expected to reach between 400ppm and 1000ppm by the end of this century, depending on which mitigation pathway we follow.