Polar heatwaves have ice in retreat at both ends of the planet

To be sure, the tale of disappearing Arctic ice has been told before.

But the big melt has lately not been the story of Antarctic sea ice. And it's certainly unusual for record lows to be plumbed for this time of the year at both of the world's extremities simultaneously.

This year, US agencies registered the second-smallest area covered by Arctic sea ice on record, with abnormal air and ocean temperatures to blame.

Zachary Labe, a PhD student tracking Arctic conditions from the University of California, Irvine, said the typical recovery of the sea ice in the region as it plunges into wintry darkness has been markedly slow, especially in October.

The area covered by ice is now more than 500,000 square kilometres less than its previous record low for this time of year, as he noted on Twitter:

"The thin [young] ice from earlier in the season - and previous years - has pre-conditioned the ice to be more susceptible to low extent and thickness," Mr Labe told Fairfax Media.

Predicting what happens next, weatherwise, can be difficult - not least because of the limited observation sites in the Arctic.

Still, "given the current computer model predictions of more above normal temperatures over the next 1-2 weeks in addition to the unusually warm ocean temperatures, it is likely sea ice extent will remain well below normal for the foreseeable future," Mr Labe said.

Indeed, according to the University of Maine's Climate Reanalyzer website, the Arctic region as a whole continues to be very warm - as much as 6.1 degrees above average - with the anomalous warmth covering almost all of the Arctic Circle:

As Mark Serreze, director of the National Snow and Ice Data Centre, told the Washington Post's Capital Weather Gang recently, a time when there is no Arctic sea ice for at least part of the year may not be that far off.

"The overall trajectory is clear - sometime in the next few decades, maybe as early as 2030, we'll wake up to a September with no Arctic sea ice," Dr Serreze said.

Mr Labe said it was "definitely possible that by 2030 we see an ice-free summer" but there remains high uncertainty, not least whether nations act to curb greenhouse gas emissions levels.

"[It] is more important to focus on the overall trends, and they are the loss of extent, thinning ice, and loss of multi-year old ice," he said. "The effects of these changes in Arctic climate and sea ice are already being felt."

A recent paper in Nature found that the so-called polar vortex - a persistent, deep low-pressure zone - has shifted towards Europe and Asia, bringing more days of bitter cold temperatures in February and March, with the loss in Arctic sea ice partly to blame.

Antarctic oddity

If the Arctic is experiencing relative warmth, so too is much of Antarctica.

According to the US National Snow & Ice Data Centre, the area of sea ice extent reached its maximum on August 31, at 18.44 million square kilometres, the earliest in the year since satellite records began in 1979.

The maximum area occurs on average more than three weeks later, on September 23-24.

While the largest extent was the 10th lowest in record, the region - as with the Arctic - hasn't been behaving like previous years. In fact, Antarctic sea ice is also at a record low, the centre said. (See chart below. Red line is 2016, green line is 2014, black line is average.)

The record low Antarctic sea ice comes just two years after it reached the opposite outcome - record large coverage.

Scientists attributed that record size to strengthening winds around the continent as well as accelerating glaciers, pushing out more ice into nearby waters.

In fact, the tale has always been more nuanced than in the Arctic.

As the Bureau of Meteorology and the CSIRO noted in their State of the Climate report for 2016, released last week, some regions - particularly the Ross Sea area - have seen increases in extent and duration of sea ice.

By contrast, the west of the Antarctic Peninsula, such as the Bellingshausen Sea, has tended to have less.

While scientists ponder the significance of the latest twist in the Antarctic, the near-term forecast points to more unusual warmth in the region. Temperatures are currently about 4 degrees above average, the University of Maine estimates. (See chart below).

Having less sea ice means less solar radiation is reflected directly back to space, and the oceans absorb more of the heat.

As the bureau and CSIRO report notes, sea-ice changes have little effect on sea level. That comes mostly from melting land-based ice, and the story isn't a promising one there, either.

Melting of the Greenland ice sheet has increased "dramatically over the last 25 years", their report notes, accelerating from 34 billion tonnes a year in 1992-2001 to 215 billion tonnes from 2002-11.

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