Ice in Greenland and Antarctica is melting so fast, it's overtaking scientists' worst-case scenarios, a new study has found.
When climate scientists use computer modelling to predict what the effects of greenhouse gases will be in the future, they usually provide a range of possibilities. Such was the case in the Intergovernmental Panel on Climate Change's last big wrap of the scientific data - the Fifth Assessment Report, published in 2014.
"Although we anticipated the ice sheets would lose increasing amounts of ice in response to the warming of the oceans and atmosphere, the rate at which they are melting has accelerated faster than we could have imagined," said Tom Slater, climate researcher at the Centre for Polar Observation and Modelling at the University of Leeds, who led the new research.
"The melting is overtaking the climate models we use to guide us, and we are in danger of being unprepared for the risks posed by sea level rise."
The new study looked at the IPCC's predictions from 2014 and what is actually happening on the ground at the poles, where most of the world's ice is located. The loss matched the worst-case scenarios laid out by the world's top climate scientists.
"We need to come up with a new worst-case scenario for the ice sheets because they are already melting at a rate in line with our current one," Dr Slater told AFP.
The problem is while the forecasts generally look at climate over decades, short-term fluctuations in weather - itself influenced by climate change - have driven an increase in melting at the poles, which are warming faster than the rest of the planet.
If all of Greenland's ice melted, it would drive sea levels up about six metres, and Antarctica 60m.
Interactive sea level maps show a 6m rise would submerge entire low-lying Auckland suburbs like Rosebank and Favona, as well as most of the city's airport.
If both Antarctica and Greenland's ice melted, Auckland would be an archipelago. The CBD, Mt Wellington, Otahuhu, Papatoetoe, Manurewa, Blockhouse Bay, Botany, Henderson and Devonport would all be under the sea. The ocean would even bury inland cities like Hamilton, coming down through the Hauraki Plains, and Palmerston North. Most of downtown Wellington would be gone too, and Christchurch would be completely wiped out.
Ice melting isn't the only way the sea rises. Most of it so far has been due to thermal expansion - warm water simply takes up more space than cold. It's believed ice melt has moved into first place in the last few years.
In August, scientists studying Greenland's glaciers said the melting had gone past the point of no return - there's no longer enough snow to replenish what's being lost to the ocean. Another group found it lost more ice last year than ever before - 482 billion tonnes, 60 tonnes for every person on Earth.
"It is not only Antarctica and Greenland that are causing the water to rise," said Ruth Mottram, climate researcher at the Danish Meteorological Institute who contributed to the new research.
"In recent years, thousands of smaller glaciers have begun to melt or disappear altogether, as we saw with the glacier Ok in Iceland, which was declared 'dead' in 2014.
The research was carried out at the University of Leeds and published in journal Nature Climate Change.
The IPCC is still working on its next big work, the Sixth Assessment Report, due for release in 2022 "in time for the first global stocktake under the Paris Agreement".