Scientists looking at a melting Antarctic glacier say they've worked out its point of no return - and it's not good news.
Using state-of-the-art climate modelling, they say Pine Island Glacier will suffer a "rapid and irreversible retreat" if ocean temperatures in the south increase another 1.2C - at the lower end of the Intergovernmental Panel on Climate Change's forecasts of 1-4C of global ocean warming by the year 2100, most of it concentrated in the southern hemisphere.
"The potential for this region to cross a tipping point has been raised in the past, but our study is the first to confirm that Pine Island Glacier does indeed cross these critical thresholds," said study leader Sebastian Rosier, a glaciologist at Northumbria University.
"Many different computer simulations around the world are attempting to quantify how a changing climate could affect the West Antarctic Ice Sheet but identifying whether a period of retreat in these models is a tipping point is challenging. However, it is a crucial question and the methodology we use in this new study makes it much easier to identify potential future tipping points."
A retreat is when a glacier shrinks, appearing to 'retreat' inland. In reality it's just melting faster than it can be replaced.
The fear is once Pine Island Glacier and its neighbour Thwaites Glacier - also known as 'Doomsday Glacier' - start to irreversibly melt, it could trigger the collapse of the entire West Antarctic Ice Sheet, which contains enough water to raise sea levels by more than three metres.
For comparison, sea levels have risen about a quarter of a metre on average in the past 120 years - already enough to cause problems for coastal communities.
"Should the glacier enter unstable irreversible retreat, the impact on sea level could be measured in metres, and as this study shows, once the retreat starts it might be impossible to halt it," said study contributor Hilmar Gudmundsson.
If all of the Antarctic ice melted, the seas would rise between 60 and 70 metres - wiping out most of Auckland and Wellington, and washing across the Hauraki Plains to totally submerge Hamilton.
The latest study was published in journal The Cryosphere.