Melting ice around Antarctica will cause a rapid slowdown of a major global current in the deep ocean by 2050 that could alter the world’s climate for centuries and accelerate sea level rise, according to the scientists behind the new research.
The research suggests that if greenhouse gas emissions continue at today’s levels, the current in the deepest parts of the ocean could slow by 40% in just three decades.
Scientists said this could set off a cascade of effects that could raise sea levels, alter weather patterns and starve marine life of a vital source of nutrients.
A team of Australian scientists has examined the less than 4,000m deep ocean current that originates in the cold, fresh, dense waters that sink off the Antarctic continental shelf and spreads to ocean basins around the world.
Professor Matt England of the UNSW Climate Change Research Center and co-author of the research Published in NatureHe said that the entire Deep Ocean Current was about to collapse on its current path.
“In the past, these flows took more than 1,000 years or so to change, but that’s only happening in a few decades. It’s so much faster than we thought that these flows can slow down.
“We are talking about the possible long-term extinction of a distinctive body of water.”
The research looked at what might happen in the ocean depths around Antarctica if fresh water melted from ice sheets were added to climate modeling.
Modeling for the study assumed global greenhouse gas emissions remain on their current trajectory, but England said lower emissions could reduce the amount of ice melt which – in turn – could slow the decline.
The slowdown in the deep ocean current is related to the amount of water that sinks to the bottom and then flows north.
Dr Kian Lee, formerly of UNSW and now of MIT, was the lead author of the research, which was coordinated by England.
The study did not attempt to explain or quantify indirect effects, but the authors wrote that the slowdown would “profoundly alter the ocean’s upturn of heat, freshwater, oxygen, carbon, and nutrients, with effects felt throughout the global ocean for centuries to come.”
In summary, the authors said that deep-ocean currents have affected climate worldwide, with the potential for a dramatic shift in precipitation.
England said the slowing of the deep ocean current caused the deep waters to warm.
But as these deeper waters become isolated, they can then cause the upper ocean around the continent to heat up, starting a feedback loop where more melting slows the accelerating current, which then leads to more heating and more mantle melting. glacier.
England said the deep waters warming fastest in the study were in the same areas — particularly in West Antarctica — where ice sheets were already vulnerable and melting.
“We don’t want to trigger a self-reinforcing mechanism in those places,” he said, adding that the slowdown actually causes the ocean depths to stagnate, depriving it of oxygen.
When ocean organisms die, they add nutrients to ocean water that sinks to the bottom and circulates around the world’s ocean. These nutrients are returned in the boilers that feed the phytoplankton – the basis of the marine food chain.
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As deep ocean circulation slows, fewer nutrients will be returned to the upper layers of the ocean – affecting phytoplankton production, said Dr Steve Rintoul, oceanographer and Southern Ocean expert at the Australian government’s Commonwealth Scientific and Industrial Research Organization. This process would continue over a century.
He said: “Once this gyration slows down, we can only get it back up again by not releasing meltwater around Antarctica, which means we need a cooler climate and then we have to wait for it to turn back on.
“The longer we continue to achieve higher rates of greenhouse gas emissions, the more changes we commit to.”
In a briefing, the scientists said the modeling is consistent with recent observations of changes in deep ocean circulation that indicate a slowdown is indeed occurring.
Rintoul added, “Going back 20 years, we thought the ocean depths hadn’t changed much.
Scientists also think that another major ocean circulation in the shallow waters that stretch across the entire Atlantic Ocean — known as the Atlantic meridian overturning circulation — is also slowing.
Professor Stefan Ramstorff, an oceanographer and head of Earth system analysis at the Potsdam Institute for Climate Impact Research, said the new study – which he was not involved in – showed “the potential for more dramatic weakening around Antarctica in the coming decades”.
He said the models presented in major UN climate reports had a “long-term and significant shortcoming” because they did not capture how meltwater affected the ocean depths.
Ocean depths have been refreshed in only a few places on the planet near the major ice sheets of Greenland and Antarctica.
“Unfortunately, these locations are all close to the Greenland and Antarctic ice sheets, which are melting as a result of global warming caused by fossil fuels.
“The melt water dilutes the salt content in these areas of the ocean, making the ocean water less dense and therefore not heavy enough to sink and push out the water already there.”
The slowdown in the deep ocean current can also affect the amount of carbon dioxide2 Ramstorff said the deep oceans could be closing.
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