

Humans are "very close" to triggering irreversible change in Earth's polar ice sheets, a University of Otago professor says.

Antarctica and Greenland's ice sheets may collapse even if global warming caused by humans is limited before it hits 2 degrees Celsius above pre-industrial revolution levels, according to a new research review. They'll shrink at rates similar to the past decade, and possible faster.

And both have "tipping points" at or slightly above the 1.5 to 2C that will cause irreversible ice loss in Greenland and the collapse of major drainage basins in Antarctica.

It means both ice sheets are in big trouble, even if we can stick to the Paris Agreement goals of limiting global average temperature to "well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5C above pre-industrial levels".

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Professor Christina Hulbe, from the University of Otago School of Surveying, said the work had "one clear message: we are very close to triggering irreversible change in Earth's polar ice sheets".

JOE RAEDLE/GETTY IMAGES Antarctica and Greenland's ice sheets may shrink at rates similar to the past decade, and possible faster.

Melting ice caps means rising sea levels.

Even if we meet the Paris targets and keep the warming in check, we're still committed to continued ice loss over the 21st century, and with it continued sea level rise, Hulbe said.

"I would add to this caution that some of the tipping processes may have already been invoked, at least in some parts of Antarctica but understanding that in detail requires more work."

The review found that the ice sheets would continue to lose mass at a similar to those of the past decade.

However, it could not rule out that they might collapse even faster.

In June, analysis showed that the rate of melting in Antarctica had tripled since 2012.

Associate Professor Rob McKay, from Victoria University of Wellington's Antarctic Research Centre, said after the tipping points were reached at each polar ice sheet, "retreat potentially becomes unstoppable".

NASA/EARTH OBSERVATORY Ice and snow in Antarctica, captured by a Nasa plane as it flew overhead as part of Operation Icebridge, which observes changes in the ice.

Like Hulbe, he said even if the world met the Paris climate agreement targets, we would be extremely close to the point of no return for accelerated retreat.

It is serious, but McKay also pointed out it won't happen overnight.

"While this melting will play out over hundreds to thousands of years, it is apparent from this work that the more we overshoot the 1.5C target, the more rapid this accelerated ice sheet melt will be."

TIPPING POINTS

Hulbe said it had been known for some time that both ice sheets were vulnerable to climate change and that the physical processes governing them had tipping points - thresholds beyond which the ice sheets were guaranteed to shrink no matter what we do next.



"Which processes matter most are different in the north and the south and the different research groups use somewhat different approaches to represent them in computer models. But all the computer models all point in the same direction: the threshold for irreversible ice loss in both Greenland and Antarctica is somewhere between 1.5 and 2C global mean warming. We're already at a bit more than 1C warming.

"Where the models diverge is just how fast the ice sheets retreat once the threshold is crossed. The differences between models are helpful though because they show which processes and which regions need more study.

"New Zealand is an international leader in this kind of research. We conduct challenging fieldwork in Antarctica and the Southern Ocean and we're actively engaged in building better models to predict what will happen next and how it will play out for us here at home."

HUMAN ACTIVITY

Victoria University of Wellington Professor James Renwick: "What's happening with the ice and the climate, it's totally down to human activity. We are causing this. The power is in our hands to stop doing what we have been doing the last 100 years or so and move to renewable energy, stop emitting fossil fuels, that's what this is all about.

"This should be another call to action, that humanity has the power to change the way we operate."

"The latest modelling results suggest that some time in the next 50 years we could get to a threshold where at least the west Antarctic ice shelf starts to melt irreversibly. And once it starts, you can't stop it - and parts of the Greenland ice sheet, same story.

"So we might be locking in about 5m of sea level rise even if we limit global warming to 2C.

Every day people emitted greenhouse gases brought those tipping points closer, he said.

IAIN MCGREGOR/STUFF Sea ice breaking up in the Ross Sea, Antarctica.

But New Zealand had potential to lead the way on climate action.

"We can respond quickly we can show the world what can be done. That argument that 'we're tiny so we can't do anything' doesn't seem to apply to the America's Cup or the Rugby World Cup. We rule the world in some areas even though we're tiny.

"There's no reason we couldn't be doing that on climate change, on renewable energy, on smart technologies. It's just a mindset, it's just a matter of will."

The review paper, which is a summary of the current understanding of a subject based on research, was published in the journal Nature.

Professor Tim Naish, from the Antarctic Research Centre at the Victoria University of Wellington, said it was timely, given the recent release of the IPCC Special Report on "Global Warming of 1.5C" and given that the world was close to 1.5C warming.

"Without some degree of direct carbon extraction from the atmosphere, we are unlikely to avoid it."

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