Iceberg in water with mountain range in the background.
Small boat in water with large iceberg in the background.
Recent ice loss may signal the start of a long period of ice-sheet retreat and global sea-level rise.

Nick Golledge is worried. He’s been crunching the numbers to work out the ‘tipping point’ for the Antarctic Ice Sheet. And the picture is grim.

Evidence suggests a new phase of ice-sheet melting may be under way—and it could be more dramatic than at any time in the past.

Nick, a professor of glaciology at Te Puna Pātiotio—the Antarctic Research Centre, is part of a team investigating the ebbs and flows of the Antarctic Ice Sheet.

The team’s research, published in Nature Communications, adds to growing data that recent ice loss may signal the start of a long period of ice-sheet retreat and global sea-level rise.

Flicking the switch

Analysis done by Nick and his colleagues reveals there have been past episodes of abrupt ice-sheet loss in Antarctica—they detected at least eight in the past 20,000 years.

These events were ‘switched on’ rapidly over a decade or two, and then ‘switched off’ again just as quickly, allowing for ice build-up, Nick says.

But this pattern occurred when the world’s average temperatures were much cooler. Today’s rapid global warming is changing the outlook.

“One thing that’s quite scary is this past ‘switching’ in the ice sheet was happening as we came out of the last ice age—between 10,000 and 20,000 years ago—when the climate was eight to 10 degrees colder at the Antarctic latitudes.

“With today’s much warmer climate, one of our concerns is the ice sheet will actually keep retreating, not just for a few hundred years as it did in the past but maybe several thousand. There won’t be any brake to slow it down.”

Getting to the core

Much of the task of figuring out what’s happening with the ice sheet relies on marine core samples from the Southern Ocean.

These samples are vertical slices of mud layers on the sea floor, collected about a decade ago in a project led by Nick’s co-researcher Dr Michael Weber from the Institute of Geosciences at the University of Bonn.

“The key thing we work with is something called iceberg-rafted debris, which is a fancy way of saying gravel,” Nick says.

“The key thing we work with is something called iceberg-rafted debris, which is a fancy way of saying gravel.”
– Professor Nick Golledge

“When you’re in the deep ocean, most of the sediment that’s settling on the sea floor is either very fine-grained mud, because that’s the only thing that really stays afloat in the water, or it’s biological material such as plankton that lives in the water and then settles on the sea floor when it dies.

“You don’t really get any coarse-grained material, because it’s just too heavy and tends to settle much closer to the coast. Once you’re out in the deep ocean, if you start seeing grains of gravel, it means the only way it could have got there is if it was deposited by an iceberg.

“As the iceberg is detached from the ice sheet, it floats out into the ocean and, as it melts, it releases the gravel. So, we can count the layers where this gravel is appearing, and by dating these layers, you get a time series of how the iceberg discharge has changed through time.”

This geological record tells researchers what happened and computer modelling shows the possibilities of what can happen, he says.

“One of our concerns is the ice sheet will actually keep retreating, not just for a few hundred years as it did in the past but maybe several thousand.”
– Professor Nick Golledge
Partially snow-covered mountains with blue sky above.
Global warming is already affecting Antarctica.

Modelling behaviour

In its latest research, the research team tuned its modelling to not only reproduce the amount of change in the ice sheet measured from satellites, but also to quantify other factors such as the amount of snowfall on the ice sheet, the amount that is melting from the bottom of the sheet, and the number of icebergs calving from it.

This modelling now indicates the ice sheet may have passed a “critical tipping point” leading to irreversible loss of parts of the ice sheet, Nick says.

The Antarctic Research Centre is recognised internationally for its work connecting field-based evidence to its cutting-edge computer modelling to help tackle urgent questions about climate change and its effects on the Antarctic.

Nick knows better than many the value of combining geological data with computer modelling.

He started his career as a geologist for the British Geological Survey, walking the Scottish Highlands and mapping glacial deposits.

“Every day, I’d go out and walk for eight to 10 hours, but every year I was getting asked to cover more ground but given less time and money to do it. Ultimately, that means you end up trying to look for ways that you can interpret the landscape and understand the geology more quickly.

“Then I moved to New Zealand and that was when I pretty much left the boots behind and started spending more time behind a desk. I still get in the field, so I can’t really complain, but most of what I do now is computer modelling.”

Other articles