Climate change: Southern Ocean’s shrunken sea ice has entered ‘new state’

Warming waters have pushed Antarctica’s shrunken sea ice into a “new state”, scientists report, following a freakish season that left the Southern Ocean missing more than a million square kilometres of its normal cover.

Removed as it is from our daily lives, the natural swelling and shrinking of Southern Hemisphere sea ice – the single largest change to occur on Earth each year – couldn’t be more important for the health of our planet.

It helps regulate our climate by reflecting sunlight back into space, driving ocean circulation and shielding the frozen continent’s vast ice shelves while supporting a plethora of life, from the iconic emperor penguin to tiny krill.

Yet this year, scientists have been alarmed to observe a baffling drop in sea ice, which, for the first time, has failed to recover to its usual maximum extent over the cold, dark months of winter.

Sea ice near Antarctica's McMurdo Sound, as Mt Erebus looms in the background. Photo / Jamie Morton

Since the lowest coverage ever was seen in summer earlier this year, sea ice extent has remained exceptionally low.

For most of winter, there was about 2.5 million square kilometres of sea ice missing - an area the size of Western Australia.

With the seasonal maximum approaching, satellite observations show there’s some 1.5m sq km less than normal.

Such was the staggering scale of this anomaly – sitting six sigma outside its usual range of variability – that experts have equated the odds of it occurring to about one in 7.5m.

But rather than being a one-off event, a new study suggests we’ve shifted to a new era of lower coverage – with our own meddling with the climate to blame.

“Our study suggests that since 2016, ocean warming due to global heating has pushed Antarctic sea ice into a new state of diminished coverage that it will struggle to recover from,” said its lead author, Dr Ariaan Purich of Australia’s Monash University.

“The characteristics of this new state suggest that the underlying processes governing Antarctic sea ice have fundamentally changed.

“It appears we’re seeing the decline of sea ice long predicted by climate models.”

Co-author Dr Edward Doddridge, of the University of Tasmania, say the new research showed ocean warming was changing the processes that drive sea ice formation and melt.

“Our results demonstrate that a lot of what we thought we knew about sea ice has changed,” Doddridge said.

“Previously, the ocean’s memory would be wiped every winter – there was no discernible relationship between the summer minimum and the following winter maximum.

“But, since 2016, this annual reset appears to have broken down.

“This raises the possibility that sea ice has entered a new state in which previously important relationships no longer dominate sea ice evolution.”

The findings, published in the scientific journal Communications Earth and Environment, follow a first-of-its-kind UK review that catalogued a series of recent extreme events across Antarctica.

They included the world’s largest recorded heatwave - when temperatures in East Antarctica reached 38.5C above the mean last year – along with retreating ice shelves and major iceberg calving events.

Earlier this year, scientists warned the gradual melting of Antarctica’s ice sheets – storing an equivalent 58 metres of global sea level rise – could be putting the continent’s surrounding deep currents on a path to collapse, with dramatic implications for our planet’s ocean and climate.

And last month, it was reported a major drop-off in sea ice led to an unprecedented breeding failure among emperor penguins in one of Antarctica’s fastest-melting regions – adding to fears climate change could leave nine in 10 colonies quasi-extinct this century.

Purich and Doddridge said dramatic changes in a seasonal cycle as reliable and critical as Antarctic sea ice underscored the urgency to reduce fossil fuel emissions.

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald reporting team in 2011 and has spent the last decade writing about everything from conservation and cosmology to climate change and Covid-19.