It's a real polar puzzle — why sea ice is on a general decline in the Arctic when it hit a record high in the Antarctic this year.
In the face of all the attention on climate change and global warming, the 2014 sea ice experience in Antarctica seems particularly counterintuitive.
But not so fast, say the scientists who study the fluctuating levels of sea ice in the world's polar regions.
"The broader issue here is we need to look at the big picture of what's happening on the planet," says Mark Serreze, director of the U.S. National Snow and Ice Data Centre and a professor in the department of geography at the University of Colorado Boulder
The centre reported that the extent of sea ice in the Arctic this year fell to the sixth-lowest level on satellite record since 1979, covering just over five million square kilometres on Sept. 17.
"We seem to be, overall, headed down, and we're probably looking at an Arctic Ocean that in summer, late summer, is free of ice by middle of the century, maybe as early as 2030," says Serreze.
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Meanwhile, the sea ice surrounding the Antarctic continent reached its maximum extent ever on Sept. 22, coming in at 20.1 million square kilometres. That figure was 1.54 million square kilometres above the 1981-2010 average.
"Climate change is playing a big role in both the Arctic and Antarctic sea ice conditions," says David Barber, Canada Research Chair in Arctic System Science at the University of Manitoba in Winnipeg.
"The main difference is that the Arctic is an ocean surrounded by land and the Antarctic is land surrounded by an ocean."
When Canadian searchers found one of the two lost ships of the ill-fated Franklin expedition in the Canadian North this summer, there was an icy irony in the discovery in the frigid waters of Queen Maud Gulf.
Tough ice conditions further north in the Victoria Strait, where this year's search was intending to focus, forced the Parks Canada-led search to spend more time in a southerly area, where HMS Erebus was ultimately discovered.
"It's interesting now, in terms of finding the ship and doing all the work up there, to find you're dealing with the same thing, the vagaries of ice, the vagaries of weather in the Arctic," says Mark Serreze.
"In many ways, we're dealing with just the same things that Franklin was back in his time."
When some sea ice in the Arctic disappears, that changes the surface from a bright white to a very dark ocean.
"This allows more energy from the sun to penetrate into the surface mixed layer of the ocean and thus warms things up even more," Barber says.
In the Antarctic, however, the ice scenario is quite different.
"Yes, what's happening in the Antarctic is a bit of a surprise, but let's look at all the rest of the evidence put together," says Serreze.
He quickly rattles off a list: Air temperatures warming up. Ocean temperature rising, too. Sea levels rising. Changing vegetation patterns in the Arctic and elsewhere.
"We see again and again and again a consistent set of change that is telling us that the climate is changing and the globe is warming, and that humans are the primary cause for this," says Serreze.
"Having said that, there are still aspects of the change that we are seeing, which we still don't quite have a handle on, and what is happening in the Antarctic is one of those changes."
Lots of theories
Science may not have a complete handle yet on what's happening in the Antarctic, but there are plenty of theories — and research — trying to figure things out.
"In the Antarctic, climate change is having two noticeable effects on sea ice," says Barber.
One is increasing the amount of snow deposited on the Antarctic ice sheet.
"This causes the sheet to grow in the middle, but to lose glacial ice mass at the margins. This means more icebergs surrounding the Antarctic continent.
"These icebergs tend to slow down the motion of the sea ice, thereby increasing the growth of sea ice," Barber says.
The second effect he sees is an increasing summer melt of the ice sheet, which in turn increases the amount of freshwater that comes off it and flows into the surrounding ocean.
"This freshwater is easier to freeze in the fall because it has lower salinity than the ocean water," says Barber. "This enhances growth of sea ice in the fall."
Serreze says scientists have to get a better understanding of how weather patterns could be influencing ice cover in the Antarctic.
"We know that there have been changes in the weather patterns around the Antarctic, and these are consistent with this increase in the sea ice coverage," he says.
"The deeper issue is why are we seeing these changes in weather patterns.
"When I'm talking about weather patterns, I'm talking about patterns of winds because winds can blow the ice from place to place, and winds can be either warm winds or cold winds and that will all influence the ice cover."
In the Arctic, Serreze says, scientists need to be focusing on coming up with better seasonal forecasts of what the sea ice will look like.
"We've got that downward trend, but we've got all those ups and downs linked to the natural variability in the system. We need to get a better handle on those natural variations, the reason being that the Arctic is becoming a busier place."
The extent of this year's Arctic sea ice comes two years after the record low coverage of 2012.
And with what happened last year and this year, someone might be tempted to say there's a recovery underway. But Serreze rejects that outlook.
"You can't just look at a few years like that. You have a very different story when you look at the overall trend," he says.