鈥淪ea ice around Antarctica has shrunk to the smallest annual extent on record after years of resisting a trend of man-made global warming,鈥� is how Reuters put it on 14 February in a story reproduced around the world.
It might seem obvious that this record low is due to global warming聽鈥� but we don鈥檛 yet know if it is.
Since satellite observations of Antarctica began in 1979, the maximum and minimum area of sea ice has varied each year, but the average area has grown slightly. Climate change deniers have seized on this increase as evidence that the world is not warming. They鈥檙e wrong. However, to suggest that this year鈥檚 trend-bucking low is certainly a sign of climate change would also be wrong.
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鈥淭here is little chance this is a signal of global warming,鈥� says Mark Brandon of the Open University in the UK, who studies the oceans around Antarctica.
How does this tally with the effect of climate change in the Arctic? Look at the differences between the two poles. The Arctic is an ocean surrounded by land. Much of the sea ice that forms in winter used to survive for several summers, getting thicker each year. Over the past 30 years, not only has the area of summer sea ice in the Arctic Ocean shrunk dramatically, its age and thickness has plummeted too. There is no question that this long-term change is due to global warming.
Antarctica, by contrast, consists of land surrounded by oceans. Most of the land is covered by ice sheets several kilometres thick. Where this ice slides into the sea, it forms floating ice shelves hundreds of metres thick. Some are hundreds of kilometres wide. Sea ice can only form in the waters north of the land and ice shelves, far from the pole. So hardly any sea ice survives the summer in the Antarctic.
That means the extent of Antarctic sea ice can vary greatly from year to year depending on many weather conditions聽鈥� not just the temperature. Winds blowing offshore can spread sea ice over a vast area, while onshore聽winds compact it.
The salinity of the water also matters. Sea water usually freezes at around 鈥�2 掳C, but if it is diluted by fresh water from melting ice, it can freeze at higher temperatures. This would explain sea ice staying in Antarctica, even with increased sea temperatures.
Exactly why the average area of seasonal sea ice around Antarctica has risen over the decades isn鈥檛 clear, but it could be due to changing winds around the continent. The reason researchers can鈥檛 say for sure is that there are few observations of what鈥檚 happening. For instance, we have no idea how thick the sea ice is.
What is clear is that this past year bucked the trend: the sea ice started melting earlier than usual in the spring and kept melting. It鈥檚 not yet official, though the US National Snow and Ice Data Center is expected to confirm within days that the extent of sea ice has shrunk to its lowest minimum since satellite observations began.
However, because Antarctic sea ice is so variable, it is possible that the extent of summer sea ice was even lower before records began 38 years ago. By contrast, we are sure there is less Arctic sea ice now than there has been for thousands of years.
There are innumerable effects globally due to climate change. But the record Antarctic sea ice low does not seem to be one of them聽鈥� and in 鈥減ost-truth鈥� times, sticking to the evidence is more important than ever.
