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 prestigious scientific journal 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 two 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.
A lesson from the past
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 right here 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 5 to 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 two degrees, which could result in large parts of the Antarctic ice sheet being committed to melt-down over the coming centuries," she said.
"It reinforces the importance of the Paris target."
She said the study also has implications for computer-based ice sheet modelling.
"Our new sea-level estimates provide a target for testing the results from computer models and improving their ability to make accurate projections of the Antarctic contribution to global sea-level rise."
An emerging threat
Scientists have long been building up a picture of what happened to the highly-sensitive, marine-based parts of Antarctica's ice sheets when CO2 levels were last sitting at 400 parts per million.
The new study fleshed that picture out more by showing how such parts of both East and West Antarctica were regularly lost during warm inter-glacial periods of the Pliocene.
Naish pointed out how that, in the warmest times, the entire marine-based Antarctic ice sheets collapsed, raising global sea levels by as much as 23m.
The new findings, he said, didn't just constrain the magnitude of global sea level change, but also told us that melting of the Antarctic ice sheet was the dominant source.
Greenland may have also contributed up to five metres to the mix.
But Naish said the study didn't make things any more alarming than they were now.
"We already knew that if global warming is not restricted to the Paris target of 2C, then we potentially commit the planet to multi-metre sea-level rise over the coming centuries," he said.
"What is new is that, that commitment may involve the entire loss of the marine-based parts of the Antarctic ice sheet and Greenland.
"It reinforces just how sensitive the polar regions are to small changes in global atmospheric CO2 and average global temperature, and this is because of powerful amplifying feedbacks in the high latitudes.
"It reinforces the importance of making the Paris target if we are to save the Antarctic ice sheet."
A big outstanding question facing scientists was just how quickly these processes would play out.
"Paleoclimate records can inform on possible rates and show us that under past natural climate change sea-levels have risen as fast as 3m per century," he said.
"However, we really rely on improved computer model predictions of future ice sheet behaviour to reduce uncertainties in the near-term.
"These models are improving all the time, as we understand more about the critical processes and incorporate these into the physics of the models."
The United Nations' latest major climate change report implied Antarctica could contribute up to 20cm more than what was estimated in its landmark Fifth Assessment Report, published in 2014, if emissions kept rising at current rates.
But it didn't factor in the results of a major international ice sheet modelling initiative which will be used to make sea level rise projections in the next assessment report.
Naish said the current likely estimate for sea level rise by the end of the century, under a business-as-usual scenario, was about 1.2m.
"However, though unlikely, there is a 5 per cent chance it might be as high as 2m."