September saw records for sea ice set at both ends of the Earth. Just a couple of weeks after Arctic ice reached a new low, a record high for sea ice extent was recorded around Antarctica. New Scientist puts these records in context.
Why are we seeing record highs and lows for sea ice at the same time?
When the Arctic experiences summer, it is winter in Antarctica. So when sea ice reaches its annual low in the north, every September, it is at its maximum extent in the south. Different processes determine the extent of the ice in different seasons. In summer, melting is the driving factor. But the extent of winter ice is determined mainly by winds, which can either push ice out over a larger area or compress it into a smaller one.
Long-term trends, extrapolated from satellite observations that began in the late 1970s, show that the September ice extent has grown around Antarctica and shrunk in the Arctic.
Explore the data in our interactive graphic: "Ice records fall at both poles"
So why are these trends going in opposite directions?
The opposite ends of the Earth are very different places. While the Arctic consists of ocean largely enclosed by land, Antarctica is a mountainous continent surrounded by the vast Southern Ocean. This key difference is crucial to understanding what is going on in each hemisphere.
What's happening in the Arctic?
Put simply, global warming is exerting its strongest effect at the North Pole. Climate models have long predicted that the Arctic would warm faster than the rest of the globe. This "Arctic amplification" is largely due to a positive feedback effect caused by melting snow and ice. As darker sea and land is exposed, more of the sun's energy is absorbed at the Earth's surface rather than being reflected back.
Still, the loss of sea ice in the Arctic summer is happening faster than climate models predicted , and looks set to change the region fundamentally ? leading to increased extraction of minerals and other commercial activities and an altered ecology.
What about the Antarctic?
The huge ice-covered land mass of Antarctica was always predicted to warm more slowly than the Arctic ? there's a long way to go before large amounts of bare ground are exposed in summer and begin to exert similar positive feedback. Also, because ozone acts as a greenhouse gas, the ozone hole that forms above Antarctica each spring is exerting a cooling effect.
As temperate latitudes warm, the resulting temperature difference is causing the jet stream that circles the continent clockwise to strengthen and move closer to the pole. This strengthens surface winds that are spreading the winter sea ice over a larger area.
Is anything happening with the jet stream in the northern hemisphere?
As the Arctic warms, reducing the temperature difference with lower latitudes, the jet stream is getting weaker. Rather like a slow-moving river on a flood plain, it then meanders more from north to south. These waves can get stuck in place for months at a time, sometimes causing extended spells of extreme weather at temperate latitudes.
"The weather those waves are generating is going to stay in the same place for longer," says Jennifer Francis of Rutgers University in New Jersey, who described the link between extreme weather and Arctic warming in a paper published in March this year (Geophysical Research Letters, DOI: 10.1029/2012GL051000.
This mechanism may help explain this year's extended drought in the US and the persistent rainfall that soaked the UK back in April.
Given that winter sea ice in Antarctica is growing, is the alarm about the Arctic overblown?
That's how it is being spun on blogs promoting the climate sceptic narrative, with one post accusing the mainstream media of being deaf and dumb to the changes down south.
This not only glosses over the different mechanisms in play, but also ignores their different magnitudes. According to the US National Snow & Ice Data Center, the Arctic summer ice minimum is declining at a rate of about 91,600 square kilometres per year, while the upwards trend for the Antarctic winter maximum is just 16,000 square kilometres per year.
We know relatively little about the thickness of Antarctic sea ice, but the differences for ice volume are probably even more dramatic. The Arctic is switching from a system dominated by thick, multi-year ice to one in which a thin crust of ice forms and melts each year. By contrast, the exposed conditions in the Southern Ocean have always meant that most of the sea ice around Antarctica breaks up each summer.
Can we at least be fairly relaxed about the changes in the southern hemisphere?
That would be unwise. The main concern is the long-term future of the much larger volume of ice on the continent of Antarctica itself, and in the ice shelves that form where its glaciers meet the sea ? especially when the ozone hole heals and its cooling influence is diminished. The evidence points to continuing loss of Antarctic ice, which will lead to continuing sea level rise, says Sharon Stammerjohn of the University of Colorado at Boulder.
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