Dark waters as Antarctic researchers dive into grim climate picture

Today, sea ice surrounding Antarctica was more than 1.68 million square kilometres short of last year’s previous record low point. That’s almost enough sea ice – by area – to fill Queensland, or Alaska.

Even more stark is the comparison of 2023 ice formation compared to Antarctica’s longer-term average – about 2.337 million km² short.

Meteorologists remind us extreme events are nothing new. Even in natural climate cycles, there are very hot days, very cold days, unusual marine heatwaves and, sometimes, ice doesn’t form.

What’s alarming is the sheer extent of the change in 1 year. The drop-off in sea ice decline is at such a magnitude it should happen once on scales of millions of years.

Scientists have been searching for answers to explain the event within natural contexts.

“Scientifically, that’s absolutely the correct thing to do,” says Professor Martin Siegert, a renowned glaciologist and Deputy-Vice Chancellor at the University of Exeter, UK.

Siegert led a review, published today in Frontiers in Environmental Science, investigating the risk of extreme events in Antarctica. In it, he and his collaborators studied existing models and projections across more than 120 research papers.

They conclude extreme events may continue to grow in frequency and size.

Siegert has spent decades studying Antarctica. While he agrees searching for scientific explanations for such freak events is important, he’s plain about where the catalyst for such an event probably lies.

“We can look at whether there is evidence that these events are linked with global warming, and of course, the evidence is that we predicted this sort of thing would happen, and now they’re happening” Siegert says.

“We can ask ourselves the next question: with continued global warming, and continued burning of fossil fuels and planetary heating, would we expect more of this to happen with more regularity and more intensity?

“The answer, unfortunately, is yes we would.”

Antarctica could be on the precipice

Climate action advocates sometimes describe the changing of Earth’s systems as a climate meltdown, and that language certainly invokes imagery of an iceless Antarctica.

Antarctic ice melt and repeated failure for it to form at sea over the cold winter months is certainly a major problem for humans.

Even a fraction of change to polar land ice composition can contribute tens of centimetres of sea level rise, particularly when the same thing is being witnessed at the opposite end of the planet and glaciers are melting in the Himalayas.

Once land ice disappears – particularly if the world’s temperature is high enough to prevent it reforming – it’s very hard to get back.

But retaining water is not the only valuable role Antarctic ice plays for Earth.

Thanks to its snow-white complexion, Antarctica’s 14 million km² surface is like a massive mirror – reflecting our central star’s light back to space and reducing the amount of energy retained as heat by the planet in a mechanism known as the Albedo Effect. Reduction in sea ice eats away at the capacity of Antarctica to do this.

With ocean temperatures going up, sometimes by 40 degrees above normal, as happened during a 2022 marine heatwave, dark ocean water will struggle to freeze.

That presents another problem. Where sea ice’s reflective Albedo Effect bounces about 90% of solar radiation back to space, the dark ocean water from which it forms absorbs about the same amount. If water doesn’t freeze, we’re left with a sink that captures 90% of the sun’s heat.

Until recently, Antarctica has been resistant to the changes witnessed in Greenland and the North Atlantic over many years. South Pole sea ice grew to record extents just a decade ago.

But 4 of the past 7 years have been marked by record ice lows, including the current confronting minimum.

“Models have been predicting for years that sea ice should be declining,” says Professor Matt King, a polar geodesist at the University of Tasmania.

Geodesists like King study the changing shape in response to fluctuations in water and ice movement. While models have predicted Antarctica should be losing ice, the southern continent has defied predictions, until recently.

That they’re changing so suddenly – by millions of square kilometres – is challenging scientific understandings of a still poorly-understood part of the world.

“In some ways, the surprise was that sea ice extent had been growing slowly over the last decades, but now it’s actually declined very rapidly,” King says. “Very, very large reductions in sea ice extent is not a gradual retreat at all.

“We can’t explain – yet – what’s caused those events, and so until we do, we won’t really be able to say confidently what’s driving them.

“It is entirely possible that this is a very, very, very rare event that has occurred and the sea ice will recover and then later in a few decades the sea ice starts to decline because of climate warming.

“Or, you could say, actually, something’s changed in the system, and such a change obviously is consistent with global warming. I think it’s entirely reasonable to expect, or have the hypothesis, that this is associated with warming climate.”

Antarctica is also home to other complex systems. It’s surrounded by the Southern Ocean and supports massive, slow-moving deepwater circulations. University of New South Wales scientists have projected the circulation will slow by around 30% by the midpoint of the century.

The concern, highlighted by Siegert, is echoed by scientists working close to the situation. Dr Natalie Robinson, an Antarctic oceanographer based at New Zealand’s National Institute of Water and Atmospheric Research, says even subtle changes to these systems can have major knock-on effects.

“The Southern Ocean is a complex system with changes cascading through multiple parts of the system and potentially feeding back on itself through multiple mechanisms,” Robinson says.” This makes it very difficult to predict exactly how things will play out.”

Robinson says the Albedo effect, deepwater circulations, and delicate ecosystems both on water and land are all vulnerable to sudden changes.

“The vast surface of the sea ice provides a surface for algae to grow on – these perform a similar function to grass meadows in terrestrial ecosystems. So, sea ice provides the foundational support for the entire marine food web,” she says.

“Emperor Penguins need the sea ice to breed on, so as the sea ice declines, we expect to see them in similar decline. If we stay on track with current rates of emissions, Emperors are predicted to decline by more than 80% by 2100.”

It’s not a tipping point, yet

For years, teams at the University of Exeter have been studying tipping points – moments in time where major systems on the Earth pass a ‘point of no return’ and permanently change, at least in ways that can’t be reversed for generations.

These systems include the massive AMOC circulation running through the Atlantic Ocean, changes to Amazon forests, loss of permafrost in the northern hemisphere and the collapse of ice sheets in Greenland and Antarctica.

“What we’re seeing in Antarctica isn’t necessarily that,” Siegert says. But it’s not a good situation in any case.

“The Antarctic ice sheet starts to retreat, the sea ice starts to retreat, and further warming happens because of the loss of a reflective surface,” Siegert says.

“That isn’t necessarily a tipping point, but it is an acceleration of the warming effect and it is a feedback process that goes on.”

The default question is therefore: how can humans prevent this?

The answer – at least from the scientific community – is the clarion call to cut carbon emissions.

But when it comes to Antarctica, there is now a new urgency, because while specialists aren’t rubber-stamping climate change as the cause of this winter’s sea ice decline, they sure don’t want to see it repeated next year, or every year, either.

Clarification (10/08/23): We have updated this story to distinguish land and sea ice.