Record high to record low—what on Earth is happening to Antarctica's sea ice?
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.
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.
Winds of change
The problem we face in Antarctica is that the climate varies hugely from year to year, as typified by the enormous swing in Antarctica sea ice over the past two years.
This means 37 years of Antarctic surface measurements are simply not enough to detect the signal of human-caused climate change. Climate models tell us we may need to monitor Antarctica closely until 2100 before we can confidently identify the expected long-term decline of Antarctica's sea ice.
In short, Antarctica's climate remains a puzzle, and we are currently trying to see the picture with most of the pieces still missing.
But one piece of the puzzle is clear. Across all lines of evidence a picture of dramatically changing Southern Ocean westerly winds has emerged. Rising greenhouse gases and ozone depletion are forcing the westerlies closer to Antarctica, and robbing southern parts of Australia of vital winter rain.
The changing westerlies may also help explain the seemingly unusual changes happening elsewhere in Antarctica.
The expansion of sea ice, particularly in the Ross Sea, may be due to the strengthened westerlies pushing colder Antarctic surface water northwards. And stronger westerlies may isolate Antarctica from the warmer subtropics, inhibiting continent-scale warming. These plausible explanations remain difficult to prove with the records currently available to scientists.
Australia's unique climate position
The combination of Antarctica's dynamic climate system, its short observational records, and its potential to cause costly heatwaves, drought and sea-level rise in Australia, mean that we can't afford to stifle fundamental research in our own backyard.
Our efforts to better understand, measure and predict Antarctic climate were threatened this year by funding cuts to Australia's iconic climate research facilities at the CSIRO. CSIRO has provided the backbone of Australia's Southern Ocean measurements. As our new paper shows, the job is far from done.
A recent move to close Macquarie Island research station to year-round personnel would also have seriously impacted the continuity of weather observations in a region where our records are still far too short. Thankfully, this decision has since been reversed.
But it isn't all bad news. In 2016, the federal government announced new long-term funding in Antarctic logistics, arresting the persistent decline in funding of Antarctic and Southern Ocean research.
The nearly A$2 billion in new investment includes a new Australian icebreaking ship to replace the ageing Aurora Australis. This will bring a greater capacity for Southern Ocean research and the capability to push further into Antarctica's sea ice zone.
Whatever the long-term trends in sea ice hold it is certain that the large year-to-year swings of Antarctica's climate will continue to make this a challenging but critical environment for research.