Is the West Antarctic ice sheet already doomed?

An international science team is on an urgent mission to find out if the West Antarctic ice sheet is already doomed, or whether there is still time to stop burning fossil fuels to prevent 4-5 metres of sea level rise.

Drilling mission co-leader New Zealander Richard Levy had a major disappointment on the ice last year when he turned up to check on the drilling effort and heard only silence.

A bold plan to use a fibreglass drilling tube didn't work and the effort had to be abandoned.

This year, they're back to using a well-tested steel tube and professor Levy is quietly confident they'll return with a useable cylinder of ancient rock to study.

While other Antarctic ice shelves, such as Thwaites, are thought to be irreversibly melting, the fate of the Ross ice shelf is debated, Levy, of GNS Science and Wellington's Victoria University, said.

"It's pretty contentious and that's why we're doing this work, but we think maybe there's a little bit of room for us to move. Maybe we've warmed the planet to the point where we might be approaching a threshold, but we haven't quite crossed it in the Ross Sea region, maybe it's 1.6C, 1.7C maybe it's as much as 2 degrees Celsius [heating above pre-industrial levels] before we lose the Ross Ice Shelf."

His expedition co-leader Tina van de Flierdt, a professor of Isotope Geochemistry at Imperial College London, said ice sheet models give conflicting answers on what happened the last time temperatures were roughly the same as they are today.

The only way to settle it, is by drilling.

"What we don't have for these time periods is direct geological evidence from close to the centre of the West Antarctic sheet of whether West Antarctica lost its ice during those times," she said.

She said the rocky layers under the ice have different characteristics depending on whether they formed under ice, or under open ocean.

Those "fingerprints" in the sediment can be paired with rock dating techniques to determine whether the ice sheet survived through certain time periods.

The team wants to retrieve sediments formed at least a few hundred thousand, but ideally millions, of years ago.

First they'll melt through 580m of ice with hot water, then lower a drill through the ice into a cavity of sub-zero ocean water.

Incidentally, whether this cavity stays cold is what will determine whether the Ross Ice Shelf survives, or starts melting like the more vulnerable Thwaites, Levy said.

If the shelf goes, Levy said it probably means saying goodbye to the whole West Antarctic Ice Sheet and hello to 4 to 5 metres of sea level rise.

Although that scale of melting would take many decades to unfold, so would moving a major coastal city out of harms way, he said.

"If we can say with more confidence the Ross Ice Shelf is going to collapse when we reach 2C warming above preindustrial [levels]....if we can say that if you cross that or pass it, you simply cannot keep the Ross Ice Shelf intact...and then if that melts it will take maybe 50-100 years, then you've got maybe 50-100 years before the West Antarctic ice sheet collapses, we can provide that early warning to our planners..and cities."

Professor van de Flierdt said people needed to know what's coming.

She continues to hope that governments will respond quickly enough to avert the whole ice sheet melting.

"I think it's our duty as climate scientists to not give up, there's hundreds of million of people - like you in New Zealand - living very close to a low lying coast where every few centimetres of sea level rise more or less, actually makes a difference."

She said it was possible the melting could still be stopped, if heating stops at under 2C above pre industrial levels, as countries agreed to do under the Paris Agreement.

The planet is approaching 1.5C already, and carbon dioxide levels in the atmosphere are still rising.

While Earth has been hotter than this before, due to natural causes, Levy said this pace of heating is not like anything humans have lived through.

"Our human ancestors have obviously survived many of these natural climate cycles, but of course there were many fewer of us on the Earth, and the rate of change was relatively slow so you sort of had time to adjust," he said.

"What's of great concern to us right now is the rate at which change is occurring is unprecedented, and the rate at which Earth's systems will respond is unprecedented, certainly in human timescales."

He said the team feels pressure to come back with a crucial missing piece of the puzzle, revealing what the tipping point temperature might be.

"If we see evidence ice was present, we know Earth can get that warm and still keep the ice shelves.

"If we find evidence the ice was gone and there were marine organisms in the water column, we know the ice must have collapsed."

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