It’s a paradox that has scientists stumped – why, in a warming world with warming oceans, is Antarctica’s sea ice spreading and thickening each year?
Climate scientists from the US and Australia found expanding sea ice in recent years was mostly caused by natural variations in sea surface temperatures, which swamped the relatively small amount of warming due to greenhouse gases.
But this could be about to change, they say.
Sea ice – unlike the slow moving ice shelves or glacial ice – is seasonal. It forms when the ocean’s surface temperature drops to around -2 oC and chilly winds off Antarctica whip over the water, expanding and retreating each year.
Climate modelling has shown that rising sea temperatures, thanks to global warming, should stem this growth. Instead, Antarctic sea ice extends as far and wide as ever. What’s going on?
Gerald Meehr from the National Centre for Atmospheric Research in Boulder, US and colleagues examined 262 climate models – not looking for human effects, but natural variation.
Indeed, they found rising sea temperatures due to global warming were swamped by the interdecadal pacific oscillation, or IPO – naturally fluctuating atmospheric pressure that causes warms or cools sea surfaces over the entire Pacific Basin.
Each IPO cycle can last from 20 to 30 years. And since 1999, the IPO has been in a “negative phase”. Sea surface temperatures have plunged below average – and in Antarctica, this caused sea ice to spread far and wide. The rate of sea ice expansion increased five-fold in 2000.
William Hobbs, an oceanographer at the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, Australia, who was not involved with the study, says the IPO is a plausible explanation of Antarctic sea ice expansion but doesn’t explain Antarctica’s western Ross Sea where sea ice has been spreading fastest.
“This is a really complex interaction of land, atmosphere and ocean. It’s not well observed because it’s expensive – you can only look at the top of the ocean from a satellite and there are no trade routes down there,” he says.
Scientists think that in 2014, the IPO began to switch to a positive phase. If this is the case, and sea surface temperatures around Antarctica start to warm again, we should see less Antarctic sea ice, says Julie Arblaster from Monash University in Melbourne, Australia and study co-author.
So what’s going on up north in the Arctic? It’s a polar opposite in more ways than one: each year the winter sea ice extent is shrinking.
Unfortunately for the Arctic, there are no natural, IPO-like climate variations in the area that could negate warming from climate change, Arblaster says: “The Arctic is a very different region because it’s ocean enclosed by land, where the Antarctic is a big land mass surrounded by ocean.
“They have very different sea ice characteristics. The climate change signal is much stronger there and the sea ice is clearly declining.”
The modelling was presented in Nature Geoscience.