One of New Zealand's top scientists says a new study highlighting the plight of the world's glaciers is "sobering stuff" and shows we have little time left to stop filling the atmosphere with greenhouse gases.
Glaciers worldwide are now losing 298 billion tonnes of ice every year, the study published in Nature found, up from 227 billion tonnes 20 years ago.
New Zealand glaciers are losing about 1.5m a year, shrinking seven times faster than they were in the early 2000s.
"Some of them are hundreds of metres thick so you might think 1.5 metres doesn't' sound like much, but that's on top of another 1.5m and another 1.5m," James Renwick, a climate scientist at the National Institute of Water and Atmospheric Research, told The AM Show on Thursday.
"We can see in photographs of things like the Tasman Glacier, we can see how much thinner it is now... it used to be up the side of the valley and now it's much below that."
In February, Wellington geologist Christoph Kraus sent Newshub two photos he took of the Franz Josef Glacier - one taken in 2007, the other this year - which starkly illustrated the problem. In 2007 the glacier reached right down into the valley, but in 2020 it could hardly be seen.
"I knew it had retreated over that time, but it was still surreal to see it in person," he said.
The new study used high-resolution imagery from NASA's Terra satellite to track the size of glaciers from around the world.
Glacier melting accounts for about a fifth of sea level rise, scientists estimate - the rest coming from thermal expansion - warm water expands - and the melting of permafrost and ice sheets. Glaciers respond more quickly to climate change, so are something of a canary in the coalmine.
"The biggest pieces of ice, the big ice sheets in Antarctica and Greenland, they're going to keep melting for hundreds of years into the future even if we turn off the tap now, but at a very slow rate," said Dr Renwick. "It's all a question of how quickly we can stop increasing the warming and eventually the ice melt will stop."
There are various estimates of how much sea levels will rise in the coming decades and centuries. The current best estimate from the Intergovernmental Panel on Climate Change is it'll rise nearly a metre by 2100; other studies have suggested three-metre rises are possible by then, and up to 25m over the next few hundred years, depending on what measures the world takes to stop it.
Dr Renwick says there's only one real solution.
"Stop emitting greenhouse gases, especially carbon dioxide. Do what the Zero Carbon Act is calling for, do what the Paris Agreement is calling for. If we can stop the increase in greenhouse gases in the air, that will stop the warming and that will stop the ice melting... that's the way to go."
Some scientists are sceptical that will bring back what we've already lost.
"I have no expectation, in all honesty, that even substantial action to reduce our emissions and control the Earth's temperature rise is going to grow our glaciers," Twila Moon, a glaciologist at the US National Snow and Ice Data Center, told Reuters.
New Zealand made no new commitments to cutting emissions at the latest climate summit, hosted by US President Joe Biden, who committed the US to cutting emissions in half by 2030.
