“Very extreme”: Climate change the most reasonable explanation for record Antarctic sea ice deficit

A never-before-seen reduction in sea ice around Antarctica smashed records last year, and there’s one reason that stands out more than others.

For those hoping the answer isn’t climate change, you’ll be disappointed.

According to the British Antarctic Survey (BAS), 2023’s unprecedented low sea ice saw an area of ice the size of the Northern Territory fail to form during the winter, compared to the year before.  

Using a large climate model called CMIP6, which includes data from about 100 models, the BAS found such a low sea ice extent should only happen about once every 2,650 years.

This severe sea ice volume becomes 4 times more likely when a strong climate change scenario is applied – about once every 580 years.

“This is the first time this large set of climate models has been used to find out how unlikely 2023’s low sea ice actually was,” says the BAS’ Rachel Diamond, the study’s lead author.

“We only have 45 years of satellite measurements of sea ice, which makes it extremely difficult to evaluate changes in sea ice extent. This is where climate models come into their own. According to the models, the record-breaking minimum sea ice extent would be a one-in-a-2,000-year event without climate change.

“This tells us that the event was very extreme – anything less than one-in-100 is considered exceptionally unlikely.”

The BAS analysis considers a “strong” climate scenario in its assessment. Such scenarios are worst-case, though, and based on current carbon reduction measures, a milder (but certainly not best-case) scenario is likely. When those midrange scenarios are applied, the climate-induced frequency of a 2023-like event lengthens from every 580 years to around every 1,330.

Concerningly, the modelling performed by the BAS doesn’t show a return to normal. Diamond and her colleagues report that “when these rare reductions are simulated, sea ice takes around 10 years to recover to a new, lower, area”.

Such a recovery takes about a decade.

“The impacts of Antarctic sea ice staying low for over 20 years would be profound,” says Louise Sime, who leads the Survey’s ice dynamics and paleoclimate team.

“[That’s] including on local and global weather and on unique Southern Ocean ecosystems – including whales and penguins.”

Sea ice decline was blamed for the wipeout of a penguin breeding colony in 2022 and other reductions in wildlife numbers in Antarctica.

Useful data, but care required with these findings: Australian researcher

Tasmania-based sea ice researcher Edward Doddridge has previously analysed causes for the recent record declines around Antarctica.

The BAS acknowledged his 2023 research published with fellow Australian researcher Ariaan Purich which also suggests Antarctic sea ice formation could be moving towards a new, permanent low state resulting from a ‘regime shift’ in atmospheric and ocean conditions.

Doddridge told Cosmos the BAS modelling raises important data, but there remains uncertainty as to whether the BAS assessment of 2023’s record severity is on the money.

“Exactly how rare [these events] are and exactly how much less rare they become is very hard to be precise about,” Doddridge says. “But they show a general change towards these events becoming less rare because of climate change.”

Doddridge’s chief concern about the BAS’ study is the CMIP6 models themselves.

CMIP6 is considered very accurate overall, but its constituent models are less precise at simulating sea ice– a case of the best product available.

That may mean the results aren’t quite as dramatic, but Doddridge says the key point – that 2023 was an unprecedented event squarely at the foot of climate change – remains strong.

“I’m a little nervous about using CMIP6 models to do this sort of analysis,” he says.

“They are absolutely the best tool that we have to use, so they are an extremely appropriate thing to look at, but we do know their simulations of Antarctic sea ice are not perfect.”

He backs the BAS’s point that climate change is the most probable explanation for the 2023 record lows: “that is the key takeaway message, we don’t know exactly what it is, but climate change is going to make that [frequency] number smaller.”

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