Sea ice around Antarctica has started melting a month earlier than previous seasons, reversing recent trends and surprising scientists.
The ice has been growing over recent winters and hit a record high in September, 2014, exceeding 20 million square kilometres for the first time since records began in 1979.
Dr Jan Lieser, of Antarctic Climate and Ecosystems Cooperative Research Centre, says the ice has started its spring retreat very early after peaking at 18.5 million square kilometres in August. It is close to the lowest winter maximum on record.
"Within the space of just two years, we have gone from a record high winter sea ice extent to record daily lows for this point in the season," Dr Lieser says.
"This is a fascinating change, and a great reminder that we are dealing with an extremely variable component of the climate system."
The big change showed it was unwise to "leap to conclusions" about the link between Antarctic sea ice and climate change on the basis of one or two years of data, Dr Lieser says.
Sea ice cover in the Artic had been reducing steadily over the past few decades and climate models predicted the same thing for Antarctica.
"It is the long-term trends that are most important, as well as the regional variability, which is high around Antarctica.
"It's likely that last year's powerful El NiÃ±o event is playing a role in this year's sea ice distribution, but there is also a likely contribution by weather events at the local scale."
'It's one of the big mysteries'
While climate scientists have been predicting the melting of Antarctic sea ice for years, it was too early to tell whether this recent trend was due to climate change, a University of Otago scientist says.
Professor Pat Langhorne says the ice remains "one of the big mysteries" of climate change as it has defied expectations and continued to grow year on year. In contrast, the ice around the Artic has been melting for years and is forecast to continue decreasing.
"The predictions say it should be decreasing, so the question is what have we not got right," she says.
"We have to be careful about looking at one small point in time and saying things have changed and we need to worry about it. Year to year there is variability in sea ice around Antarctica."
The professor was not "prepared to gamble" on whether the ice would continue to decrease but said she "would not be surprised".
"Our models say it should have been decreasing already. There are many clever people thinking about why it's got bigger when we thought it would get smaller."
Sea ice has a major impact on climate around the world and affects up to 40 percent of the Southern Ocean during the winter.
Its annual cycle of retreat and advance can impact the temperature of the Earth and the movement of ocean water around the world.