Missed Paris target cues 20m sea rise

The planet’s oceans could rise by 20m if the world fails to keep emissions below the threshold set by the Paris Agreement. A new study, led by Victoria University and published in Nature, showed up to one-third of Antarctica’s ice sheets melted during a time in Earth’s history when carbon dioxide levels were around the same as today.

As a response to that high CO2, temperatures in the Pliocene epoch, around three million years ago, were several degrees warmer.

“This study has important implications for the stability of the Antarctic ice sheet and its potential to contribute to future sea levels,” Victoria University glaciologist Professor Tim Naish said.

“If we do not keep our greenhouse gas emissions in line with the Paris Climate Agreement target of 2 degrees warming, then we may potentially lose not only the West Antarctic Ice Sheet but also the vulnerable margins of the East Antarctic Ice Sheet.”

Antarctica’s vast ice sheets store an equivalent 60m of potential sea level rise and scientists have been urgently trying to understand how the frozen continent might respond to a warming world.

In the new study, Dr Georgia Grant, a recent Victoria graduate now based at GNS Science, used a new method of analysing marine geological sediments to construct a global sea level record.

By analysing the size of particles moved by waves, Grant was able to then infer how much oceans had risen millions of years in the past.

The best place to search for these clues happened not to be in the icy environs of Antarctica, but in New Zealand, in the Whanganui Basin.

She was ultimately able to show that, during the past warm period of the Pliocene, global sea levels regularly fluctuated between 5m and 25m.

A crucial point was that more than 90 per cent of the heat from global warming to date has gone into the ocean, and much of it into the Southern Ocean which bathes the Antarctic ice sheet.

One-third of Antarctica’s ice sheet — equivalent to up to 20m sea level rise — sat below sea level and was vulnerable to widespread and catastrophic collapse from ocean heating.

It melted in the past when atmospheric carbon dioxide levels were 400 ppm, as they are today.

“Our new study supports the idea that a tipping point may be crossed, if global temperatures are allowed to rise more than 2 degrees, which could result in large parts of the Antarctic ice sheet being committed to meltdown over the coming centuries,” she said.

“It reinforces the importance of the Paris target.”