2016 continues to be a momentous year for Australia’s climate, on track to be the new hottest year on record.
To our south, Antarctica has also just broken a new climate record, with record low winter sea ice. After a peak of 18.5 million square kilometres in late August, sea ice began retreating about a month ahead of schedule and has been setting daily low records through most of September.
It may not seem unusual in a warming world to hear that Antarctica’s sea ice – the ice that forms each winter as the surface layer of the ocean freezes – is reducing. But this year’s record low comes hot on the heels of record high sea ice just two years ago. Overall, Antarctica’s sea ice has been growing, not shrinking.
So how should we interpret this apparent backflip? In our paper published today in Nature Climate Change we review the latest science on Antarctica’s climate, and why it seems so confusing.NASA, Author provided
First up, Antarctic climate records are seriously short.
The International Geophysical Year in 1957/58 marked the start of many sustained scientific efforts in Antarctica, including regular weather readings at research bases. These bases are mostly found on the more accessible parts of Antarctica’s coast, and so the network – while incredibly valuable – leaves vast areas of the continent and surrounding oceans without any data.
In the end, it took the arrival of satellite monitoring in the 1979 to deliver surface climate information covering all of Antarctica and the Southern Ocean. What scientists have observed since has been surprising.
Overall, Antarctica’s sea ice zone has expanded. This is most notable in the Ross Sea, and has brought increasing challenges for ship-based access to Antarctica’s coastal research stations. Even with the record low in Antarctic sea ice this year, the overall trend since 1979 is still towards sea ice expansion.
The surface ocean around Antarctica has also mostly been cooling. This cooling masks a much more ominous change deeper down in the ocean, particularly near the West Antarctic Ice Sheet and the Totten glacier in East Antarctica. In these regions, worrying rates of subsurface ocean warming have been detected up against the base of ice sheets. There are real fears that subsurface melting could destabilise ice sheets, accelerating future global sea level rise.
In the atmosphere we see that some parts of the Antarctic Peninsula and West Antarctica are experiencing rapid warming, despite average Antarctic temperatures not changing that much yet.
In a rapidly warming world these Antarctic climate trends are – at face value – counterintuitive. They also go against many of our climate model simulations, which, for example, predict that Antarctica’s sea ice should be in decline.
Authors: Nerilie Abram, Senior Research Fellow, Research School of Earth Sciences; Associate Investigator for the ARC Centre of Excellence for Climate System Science, Australian National University