There is about 10 per cent less sea ice in Antarctica this year than the previous record minimum - a stunning reversal after new highs were set in 2014.

The sea ice extent around the southern continent has shrunk to 2.1091 million square kilometres on Tuesday, Jan Lieser, a sea ice scientist at the Hobart-based Antarctic Climate and Ecosystems Co-operative Research Centre, said.

The area covered by sea ice has been tracking below the previous record low of 2.32 million square kilometres set in February 2011 for most of the past three weeks, and is now about 10 per cent lower. (See chart compiled by Japan Aerospace Exploration Agency for ice coverage.)

"One would probably say that the old record was obliterated," Dr Lieser said.

An increase in sea-ice extent earlier in the week that appeared to have signalled an end to the melting phase now looks premature, with even smaller ice coverage still possible.

The switch from a sea-ice maximum around Antarctica to its annual low is "one of the biggest natural cycles we see in the world", with as much as 90 per cent of the ice only a year old at the most, he said.

Reliable satellite records only go back to 1979, and it's harder to access ice thickness compared with the North Pole, with Arctic ice mostly accessible from above or via submarine below.

In the southern winter of 2014, sea ice around Antarctica reached a record large extent. At the time, climate change sceptics were keen to highlight the increase in the south as a counterpoint to the more steadily decreasing Arctic ice.

Last winter, though, ice around Antarctica began thawing about a month earlier than normal. Minimum air temperatures have been breaking records daily since about early November in a region of the planet where global warming has been amongst the most rapid, Dr Lieser said.

'Along came 2016'

"[Sea-ice] variability was typical of what we'd seen for the whole period [since 1979], but then along came 2016," said Ian Simmonds, from the  School of Earth Sciences at the University of Melbourne. "It's remarkable."

The average ice coverage around Antarctica last year shrank 1.2 million square kilometres – or about the size of NSW, Tasmania and Victoria – compared with 2015, he said.

(See chart below from the US National Snow and Ice Data Center, showing how rapidly ice coverage anomalies shifted.)

Arctic too

Sea ice is now at record lows at both ends of the planet, exposing more of the dark seas to solar radiation, rather it being reflected back to space.

The lack of ice will likely add to the build-up in heat in the oceans that could hinder ice recovery in the south and accelerate the melt in the north as seasons shift towards winter and summer, respectively.

​

The Ross Sea is virtually ice free and half the Weddell Sea ice has gone, Dr Lieser said.

While the loss of floating ice doesn't affect sea levels, its absence leaves shore-based ice shelves exposed to faster melting and accelerated glacier movement. "It opens up the vulnerability of the ice shelves around there," Dr Lieser said.

Professor Simmonds said several factors were at play in the Antarctic, such as the strengthening of westerly winds that tend to push sea-ice northwards.

Countering that, though, was the long-anticipated thermodynamic effect that warming ocean temperatures - with the Southern Ocean a major heat sink globally - would limit sea ice growth by melting the bergs from below.

While it's too early to tell whether the second effect is becoming a dominant factor during the current ice retreat, long-term climate models suggest that it will play the major role at some point, Professor Simmonds said..

Antarctic temperatures - along with those in the high Arctic - have been among the fastest rising anywhere, as rising greenhouse gases drive climate change. 

Gwen Fenton, chief scientist of the Australian Antarctic Division, told Senate estimates on Monday that air over the Antarctic Peninsula had warmed about 2.8 degrees over the past 50 years alone.