Extent of Arctic sea ice in September 2016 versus the the 1981-2010 average minimum extent (gold line) NASA

It’s a new low point. The area of the world’s oceans covered by floating sea ice is the smallest recorded since satellite monitoring began in the 1970s. That means it is also probably the lowest it has been for thousands of years.

The latest observations from the US National Snow & Ice Data Center in Boulder, Colorado, show how the ice extent has fallen to a new low this year (bright red trace in the graph below).

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In the Arctic, the low in sea ice coverage is a result of both global warming and unusual weather events probably influenced by global warming.

But in the Antarctic, the current low in seasonal sea ice could just be a result of natural variability.

The extent of Arctic sea ice should be growing rapidly during the northern hemisphere winter. But not only has the Arctic been warming rapidly, this winter repeated incursions of warm air have pushed temperatures even further above average.

It has been so warm that on occasions this winter the sea ice coverage has actually temporarily shrunk, as shown by dips in the blue line in the graph below.

US National Snow and Ice Data Centre, Boulder CO

Meanwhile in Antarctica, the area of sea ice has been falling faster than usual during the southern summer, as the blue line in the graph below shows.

US National Snow and Ice Data Centre, Boulder CO

The extent of seasonal sea ice around the Antarctic is expected to decline in the long term as global warming continues, but this decline has not started yet. In fact, the average area of Antarctic winter sea ice has grown somewhat since observations began in 1979.

Both this growth and the current unusual low could just be the result of natural variability, says climatologist John Turner of the British Antarctic Survey in Cambridge, UK. “The Antarctic climate is extremely variable,” he says.

The immediate cause of the current dip could be a weakening of the winds around Antarctica in November 2016. “It was a very exceptional month,” says Turner. “Relatively small changes in winds can have a huge effect on sea ice.”

In the Arctic, by contrast, there is a long-term decline in sea ice due to global warming. This warming seems to be weakening the winds that circle the pole, allowing warm air to intrude into the Arctic.

And when warm air intrudes, cold air spills south. This is why parts of Asia and Europe have experienced unusually cold weather at times this winter.

As a result of the simultaneous lows at both poles, the total area of sea ice on the planet has fallen to a record low. Reconstructions of past levels of sea ice in the Arctic suggest it is likely the lowest it has been for thousands of years, says meteorologist Eric Holthaus.

@EricHolthaus “probably the lowest in millennia” sources:https://t.co/ff9OzZEcbp

Current levels in context w/ 18,000yrs of reconstructions. pic.twitter.com/fqZan8ag74 — Eric Holthaus (@EricHolthaus) 14 January 2017



Sea ice levels in the Arctic might increase again in the next few years before falling to even lower levels. Climate scientist Ed Hawkins of the University of Reading, UK, likens the long-term decline in Arctic sea ice to a ball bouncing down a hill. The general trend is downwards, but big falls are often followed by apparent rebounds.