Matthew Ward Agius
Matthew Agius is a science writer for Cosmos Magazine.
As scientists scramble to understand the forces driving sea ice decline in Antarctica, a new study is pointing to warming waters hundreds of metres under the ocean’s surface.
Today, Antarctica is missing enough sea ice to fill the land area of South Australia, relative to 2022.
And considering last year was a record low for sea ice at the Earth’s southern pole, another record year hot on its heels is troubling climatologists and Antarctic researchers.
Global warming seems an obvious culprit, with more and more greenhouse gas emissions from human activity spewing into the atmosphere and record temperatures being posted around the globe.
Could other factors be at play? Potentially.
Dr Ariaan Purich, a climate scientist at Monash University, told Cosmos earlier this year scientists didn’t yet understand the reasons behind sea ice change.
Now, she says a ‘regime shift’ that is influenced not just by the atmosphere is taking place in the ocean surrounding Antarctica.
“We know that sea ice is influenced by both the atmosphere and the ocean, but there’s really been a focus on understanding sea ice variability and change in terms of how the atmosphere is forcing these changes,” Purich says.
In a new study published in Nature Communications Earth & Environment, Purich and Dr Edward Doddridge from the University of Tasmania, find Antarctic sea ice has likely entered a new state of reduced levels, and subsurface ocean warming is playing a big part. They suggest a greater focus is required on the role the oceans are playing in preventing sea ice from forming.
A state change for Antarctica
Since records of sea ice extent began in 1978, it appeared that while sea ice in the northern hemisphere was on the wane, Antarctica’s ice was holding up or increasing.
But that changed in 2016 when sea ice growth was persistently lower than the years before, culminating in today’s record low. The potential environmental disaster was symbolised recently with news of a total breeding failure of emperor penguin colonies in West Antarctica.
Purich and Doddridge identify subsurface ocean – the part of the water column between 100-600m below sea level – as a likely driver of low sea ice formation.
“What we’re seeing is that the subsurface layer of the ocean is warming,” Purich says.
When this lower-level water warms and pushes up to the surface, it diminishes the conditions required for sea ice to form.
Purich emphasises how little is known about these processes – “it’s really hard to get observations of the ocean under the sea ice”, she says – but with data available, it seems that these waters are simply become too warm to freeze.
A complicating problem will be the potential for feedback processes to further drive ice change. Antarctica’s white ice acts as an effective sun shield – reflecting energy back to space and, in turn, keeping the planet cool. Ice has high ‘albedo’ – the term given for a surface’s ability to reflect light – but surrounding oceans are very dark, and therefore have extremely low albedo.
This poses a problem. If sea ice is failing to form, the ability of light to be reflected by Antarctica is diminished both by a loss of sea ice area, and an increase in dark ocean area absorbing heat. With oceans trapping more energy, this feedback loop will make it harder for sea ice to form as time goes on.
And while other natural cycles could be influencing sea ice growth in Antarctic winters, the constant in this dilemma appears to be the increasing concentration of carbon in the atmosphere, and the oceans.
“The key point here as well is that we’re increasing global temperatures by increasing greenhouse gas concentrations. That’s what’s forcing this increase this long-term trend in the Southern Ocean temperatures,” Purich says.
“They’re really confronting changes. The sea ice is crazy low, we’ve had the devastating news about the catastrophic breeding seasons of emperor penguins from last November.
“When think about what we expect in a future warming world, we know we’ll have warmer temperatures, intuitively, we can expect less sea ice and the climate models tell us there will be continued loss of sea ice.”