It’s the big white elephant in climate change’s room: a vast Antarctic ice sheet that could collapse and lift oceans by metres - if only we could prevent it.

The irreversible collapse of Antarctica’s ice sheets remains the largest uncertainty in projections of future sea level rise, with the potential to displace hundreds of millions of people.

West Antarctica is especially vulnerable, as much of its ice sheet – locking up an equivalent five metres of sea level rise - overlies a warming ocean.

Scientists estimate that it’s been shedding nearly 150 billion of tonnes of mass each year since 2005.

As nations work to reach the Paris Agreement’s goal of halting climate change at 2C, scientists have been racing to understand what’s happening to the frozen continent – and more importantly, what the tipping points are.

The answers to those critical questions about our future may be found between 115,000 and 130,000 years in the past, when global temperatures were more than 1C warmer than today.

The first ever record retrieved from this period, called the last interglacial, are now to be analysed as part of a New Zealand-led, international study.

At this point in the Earth’s history, the temperature in Antarctica might have been as much as 6C warmer than now, while the world’s oceans were between 6m and 9m higher.

Studies have suggested that as much as 2m came from a melted Greenland, and another metre came through the expansion of a warming ocean.

That left Antarctica to provide the rest – and up to 3.3m could have come the western part of the continent alone.

HOW THE WEST IS LOST

Associate Professor Nancy Bertler, of GNS Science and Victoria University, said the West Antarctic ice sheet’s underbelly had some clear features that made it vulnerable.

“If you look at the bedrock beneath the ice sheet, two things are striking – firstly, a large part of the ice sheet sits on bedrock that lies up to 2000m below sea level,” she said.

“Secondly, the basin is deeper in the middle than at its edges. Together, this setting creates a runaway effect of ever faster ice sheet retreat once initiated.”

Not only had the trigger now been pulled, but the process was being compounded by other effects.

The most important of them was wind-driven warmer currents, called Circumpolar Deep Water, reaching under the ice shelves and thinning them out.

Bertler said these shelves were crucial for holding back the ice sitting on bedrock – but in just two decades, they’d been thinned by nearly 20 per cent.

“In recent years, we observed a 10-fold increase in the Antarctic contribution to sea level rise,” she said.

“In addition, it has now become also apparent that warmer surface temperature, changes in sea ice, and the gravitational adjustment of the bedrock as ice mass is lost, also significantly influence ice dynamics and with rates of sea level rise.”

At Roosevelt Island, at the northern edge of the sprawling Ross Ice Shelf, a previous Kiwi-led expedition recovered an ice core from 764m below the surface.

From that record, scientists had now poured through tens of thousands of samples – enough to fill gigabytes of data, and build a thorough understanding of the climate history held within them.

The new study, just funded with a $960,000 grant from the Marsden Fund, focused on the part of it capturing the last interglacial, with precious information about changes in local ocean temperatures.

Just as importantly, this 10m-long section enabled scientists to compare shifts in ice mass then with what could be expected if the Paris Agreement succeeded at holding the 2C line.

But first, the team would need to develop entirely new technology to do their analysis work – and use a new generation of models to apply what they found to future projections.

Study leader Bertler said the project would also involve running models of the last interglacial against their actual data to see if they stacked up correctly.

“This in return allows us to identify shortcomings in the model, quantify and reduce uncertainty, investigate important mechanisms and feedbacks, and identify tipping points.

“Ultimately, this leads to improved projections and reduced and quantified uncertainties.”

TROUBLING SCENARIOS

As they stood now, what pledges had been made under the Paris Agreement wouldn’t limit warming to 2C, but more than 3.2C – and the United Nations has warned that nations would need to triple their climate efforts.

Unrealistic or not, Bertler still saw an opportunity to rise to the challenge – the pay-off being that, by the century’s end, we might have only contributed another 43cm of sea level rise.

But if the world kept sending as much carbon dioxide into the atmosphere as it does now, oceans might have risen by as much as 2m by the century’s end.

That mattered greatly to the 680 million people – a tenth of the global population - currently living within range of projected sea level rise. By 2050, that proportion could look more like one billion.

But even assuming no emissions after 2100, projections indicated that, over the next several hundred years, seas could climb by metres and metres – the result of ice sheets having crossed the point of no return.

It begged a startling question: had that already happened?

Bertler said this was a topic of much debate.

Research by some respected scientists that the tipping point had been reached at the important Thwaites and Pine Island glaciers, essentially precipitating the collapse of the West Antarctic Ice Sheet.

Yet there was also a group of equally respected researchers whose models suggested the Paris Agreement could still save it.

“Either way, there is strong agreement that we don’t have time to lose,” she said.

“Action is needed now.”