Why Antarctica is not getting any warmer
Strengthening winds are responsible for trapping cold air over the Antarctic in apparent defiance of global warming. And that's bad news for Australia. James Mitchell Crow explains.
The Arctic may be melting, but Antarctica remains icy cold. It’s the only part of the planet where temperatures aren’t rising, seemingly at odds with the global warming trend.
Australian researchers have figured out why – and for Australians, the news is not good. Global warming has strengthened southern winds, trapping cold air over Antarctica and robbing southern Australia of vital winter rainfall. Those winds are also powering ocean currents that may tip glaciers on Antarctica’s fringe into unstoppable meltdown.
Nerilie Abram at the Australian National University leads an international team that has built up a 1,000-year record of the westerly winds that whip around Antarctica. Publishing their findings in Nature Climate Change in May, they show that these winds now blow more strongly, and circle the continent more tightly, than at any time during the past millennium.
These strengthening winds keep more cold air bottled up over Antarctica, which is why the continent isn’t warming like the rest of the world, says Abram. And as the winds contract southwards to circle Antarctica more tightly, they are taking with them the storms that give southern Australia its winter rain. “We know that in southern Western Australia since the 1960s there has been a 20% decrease in winter rainfall, and that’s a continuing trend,” says Abram.
Over the last few decades, meteorologists have been aware that Antarctica’s winds were strengthening. But was this just a blip within the bounds of long-term weather variation or was it a sign of a changing climate? Accurate Antarctic record-keeping only went back to the 1950s. Establishing a long timeline required Abram and her colleagues to collect and analyse an ice core in the powerful westerly’s path. To get it they braved the punishing gales of James Ross Island off the northern Antarctic Peninsula.
Analysis of the isotope ratios of water molecules in the core, providing an indirect measure of past weather, revealed a past pattern of fluctuating winds. But starting from the 1940s the core showed an unprecedented strengthening – right about the time that human greenhouse gas emissions really began to take off. More recently, another major man-made factor has also been at work – the ozone hole created by chlorofluorocarbons.
Antarctica is surrounded by open ocean across which
the winds can rage unimpeded.
“Greenhouses gases over the whole of the 20th century are the likely major driver, but since the 1980s when the Antarctic ozone hole formed, that was an important driver too,” says David Karoly, a climatologist at the University of Melbourne.
Both factors influence the winds by increasing the temperature difference between the equator and the South Pole. The greater the gradient, the wilder the winds. Greenhouse gases have made the tropics hotter, while the ozone hole makes the stratosphere over Antarctica colder (heat is a by-product of ozone capturing UV light). And the stronger the winds, the more cold air is trapped over Antarctica, maintaining the temperature gradient in a positive feedback loop.
In the Arctic, the opposite has happened. Part of the difference is explained by the fact the Arctic is an ocean surrounded by mountainous lands, which act as a brake on the westerlies that could keep cold air trapped over the North Pole. But Antarctica is land surrounded by open ocean across which the westerlies rage unimpeded.
In the coming decades, researchers expect a temporary slowdown in the summer increase in wind speed around Antarctica. That’s because the ozone hole that opens up each spring and summer is recovering, so the kick it currently gives to Antarctic winds will gradually lessen. “But in the winter, when greenhouse gases are the only factor at play, all the models predict we will see a strengthening of the westerly winds,” Abram says. Australia, in other words, will get no respite – it will just go on getting drier.
Glacial woes from changing winds
Changing wind patterns are also powering ocean currents that may tip glaciers on Antarctica’s fringe into unstoppable meltdown.
Winds drive ocean currents. “As the winds get stronger they pump more of the relatively warm deep water up to the surface,” Nerilie Abram of the Australian National University says. This warm water is melting the glaciers at Antarctica’s fringe, attacking them from beneath. A vast tract of Antarctic ice called the Thwaites glacier is particularly vulnerable, and it may already be too late to save it, research published in Science in May shows.
The tongue of the Thwaites glacier currently rests on a shallow section of the seabed. But closer to the shore, the sea floor drops. The pocket of water trapped in this deep basin is currently sealed off from the open ocean by the glacier’s tongue. But in as little as 200 years, the tongue will have melted.
At that point, the glacier’s vast underbelly will suddenly be exposed to warm water, triggering catastrophic collapse. From there, the entire West Antarctic ice sheet, which contains enough water to raise sea levels 3-4 metres, is vulnerable. “It does take time for the ice to be lost, we are talking 1,000-year time scales,” says Abram. “But it now does look like part of that ice loss is a done deal.”