Iceberg and large fragments of drifting ice floating in front of the Antarctic Peninsula. Photo: AFP / Claudius Thiriet
Researchers have found evidence of a major shift in the circulation of the Southern Ocean, which could accelerate climate change for the whole planet.
They say the finding caught them off guard, and the sea ice around Antarctica could be in "terminal decline".
The study, led by the University of Southampton in the United Kingdom and helped by Spanish and other European researchers, found evidence of a change which has shocked other researchers, and which could explain the region's rapid loss of sea ice.
The authors say the circulation of the Southern Ocean has reversed.
Typically, as global heating melts the ice around Antarctica, climate scientists would expect saltier water at the surface of the ocean to be replaced by fresh water, because melting ice typically makes the ocean fresher.
But new satellite data shows the opposite is happening.
Because salt water draws up heat from the deep ocean and makes it harder for sea ice to regrow - as well as bringing up carbon dioxide from the depths - a reversal could accelerate ice loss and global heating.
The authors - who published their findings in the peer-reviewed journal PNAS - say their salt content measurements "can now provide a coherent explanation for the rapid Antarctic sea ice loss that had puzzled the scientific community".
However the consequences are potentially disruptive for the planet.
They say the Southern Ocean plays an essential role in regulating the planet's heat and carbon and its disruption could trigger cascading effects on other ocean circulation systems, with potential consequences as far away as Europe.
"We are witnessing a true change in ocean properties in the Southern Hemisphere - something we've never seen before," said Antonio Turiel, co-author of the study, published by the Spanish marine research institute Institut de Ciències del Mar.
"While the world is debating the potential collapse of the AMOC (Atlantic Meridional Overturning Circulation) in the North Atlantic, we're seeing that the Southern Ocean is drastically changing, as sea ice coverage declines and the upper ocean is becoming saltier. This could have unprecedented global climate impacts."
"What we found was astonishing," said co-author Alessandro Silvano, an oceanographer at University of Southampton in a piece for The Conversation.
"By combining satellite observations with data from underwater robots, we built a 15-year picture of changes in ocean salinity, temperature and sea ice," he said.
"Around 2015, surface salinity in the Southern Ocean began rising sharply - just as sea ice extent started to crash. This reversal was completely unexpected."
The fleet of underwater robots used in the study are Argo floats, many of them deployed by New Zealand scientists, which drift with ocean currents and return data that any scientist can use.
New Zealand researchers react
The new paper does not explain why the reversal is happening, and New Zealand scientists say more work is urgently needed to complete the puzzle.
For decades, the surface of the Southern Ocean was getting fresher and colder, helping sea ice expand - seemingly in defiance of global heating.
At the same time, sea ice in the Arctic at the opposite pole was in freefall.
Now Antarctic sea ice is also shrinking, contributing to a feedback loop where larger areas of darker open ocean reflect less of the sun's heat back than ice would have - further increasing the heating and melting.
Professor Wolfgang Rack, a glaciologist at University of Canterbury and Director of Gateway Antarctica, said the growth of sea ice in the Southern Ocean was seen as climate paradox until it stopped around 2015.
"Many scientists expected the trend to reverse at some point, but the rate of the current retreat is completely unexpected and mind boggling," he said.
He said the ocean south of Wellington was the "most under-surveyed region globally" and more and urgent work was needed.
Associate Professor Inga Smith, a sea ice physicist at the University of Otago, said the new results were "shocking" for those researching Antarctic sea ice.
"In a warming world, fresher water from melting of land-based ice sheets and floating ice shelves would be expected to dominate at the ocean surface."
Instead, the ocean surface was getting warmer and more salty right at the time the extent of the sea ice shrank.
She said, although the paper didn't explore why, "the authors will no doubt explore this in a longer paper sometime soon".
Dr Ken Hughes, a senior lecturer in coastal processes at the University of Waikato, says when he first began research in 2012 scientists assumed that the ocean and big ice shelves buffered the Antarctic system in some way so that the warming climate was not wrecking havoc on the ice extent in the same way it was for the Arctic.
That security could no longer be taken for granted, he said.
He said the most difficult question was whether the decline will continue.
Professor James Renwick, a climate scientist at Victoria University of Wellington said other research had shown how upper-ocean heat content and westerly winds were changing in the Southern Ocean.
He said the new paper provided more evidence that climate change was causing a "major change" in the way the ocean was working.
"The implications are very worrying," he said.
"Antarctic sea ice extent will likely continue to trend downwards from here (with ups and downs from year to year). That will accelerate the melting of ice shelves and land-based ice, increasing the rate of sea level rise and pushing us closer to the irreversible loss of the West Antarctic ice sheet. It will also reduce the reflectivity of the planet, bringing more warming," he said.
Natalie Robinson, a marine physicist at Earth Sciences New Zealand (formerly NIWA) and associate professor at Victoria University despite covering only 17 percent of the planet, the Southern Ocean was a critical player in the global climate - and the annual cycle of sea ice forming and shrinking was a major player in stabilising the climate, especially for New Zealand.
"We are moving into uncharted climate territory," she said.
"What happens in Antarctica has implications for the entire globe, but here in New Zealand we are impacted by changes to Antarctic sea ice more directly than most.
"Early indications are that a warmer Southern Ocean, exacerbated by retreat of the sea ice edge, contributes to increased storminess for New Zealand.
"When combined with the steadily warming waters of New Zealand's Exclusive Economic Zone, we should expect to see more of the intense rainfall events we've experienced recently, since warmer air can hold more moisture," she said.
"In order to secure a liveable future for ourselves and our children, it is imperative that we drastically reduce our reliance on fossil fuels."
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