A rapidly slowing Atlantic Ocean current could change the Earth’s climate and cause perpetual conditions of La Niña in Australia if it collapses.
Climate change is slowing the conveyor belt of ocean currents carrying hot water from the tropics to the North Atlantic.
UNSW research, published today in Nature Climate Change, looks at the profound consequences for the global climate if this Atlantic conveyor belt sinks completely.
We have found that the collapse of this system, called the South Atlantic Circulation Circulation, would change the Earth’s climate to a more La Niña-like state. This would mean more flooding in eastern Australia and worse seasons of droughts and wildfires in the southwestern United States.
Australians on the East Coast know what a relentless La Niña is like. Climate change has charged our atmosphere with more humid air, while two summers in La Niña warmed the ocean in northern Australia. Both contributed to some of the wettest conditions ever experienced, with record flooding in New South Wales and Queensland.
Meanwhile, in the Southwest of North America, a record drought and severe forest fires have put great pressure on emergency services and agriculture, and only 2021 fires are estimated to have cost at least 70 billion. dollars.
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The Earth’s climate is dynamic, variable and constantly changing. But our current trajectory of unabated greenhouse gas emissions is giving the whole system a giant kick that will have uncertain consequences, consequences that will rewrite our description of the textbook of the planet’s ocean circulation and its impact. .
What is the Atlantic overturning in the southern circulation?
The upside-down circulation of the Atlantic includes a massive flow of warm tropical water into the North Atlantic that helps keep the European climate mild, while allowing the tropics to lose excess heat. An equivalent dump of Antarctic waters can be found in the southern hemisphere.
Climate records dating back 120,000 years reveal that the upside-down circulation of the Atlantic has been turned off, or drastically slowed, during the ice ages. It lights up and calms the European climate during the so-called “interglacial periods”, when the Earth’s climate is warmer.
Since human civilization began about 5,000 years ago, the Atlantic enclosure has been relatively stable. But a slowdown has been detected in recent decades, and this has worried scientists.
Why the slowdown? An unequivocal consequence of global warming is the melting of polar caps in Greenland and Antarctica. When these ice caps melt, they pour massive amounts of fresh water into the oceans, making the water more floating and reducing the sinking of dense water at high latitudes.
Around Greenland alone, 5 trillion tonnes of massive ice has melted over the past 20 years. That equates to Sydney’s 10,000 freshwater ports. This melting rate will increase over the next few decades if global warming continues unabated.
A collapse of the North Atlantic and Antarctic circulations would profoundly alter the anatomy of the world’s oceans. It would cool them to the depths, deplete them of oxygen, and starve the upper ocean of the supply of nutrients provided when deep waters rise from the ocean abyss. The implications for marine ecosystems would be profound.
With the melting of the Greenland iceberg already underway, scientists estimate that the reversal of the Atlantic is the weakest for at least the last millennium, with predictions of a future collapse in the charts in the coming centuries if greenhouse gas emissions are not controlled.
The ramifications of a slowdown
In our study, we used a comprehensive global model to examine what the Earth’s climate would be like under such a collapse. We turned off the Atlantic surge by applying a massive melting water anomaly in the North Atlantic, and then compared it to an equivalent route with no meltwater applied.
Our goal was to look beyond the known regional impacts of Europe and North America, and see how the Earth’s climate would change in remote places, as far south as Antarctica.
The first thing the model simulations revealed was that without the Atlantic overturning, a massive pile of heat is accumulating just south of the equator.
This excess heat from the tropical Atlantic pushes more warm, humid air into the upper troposphere (about 10 kilometers into the atmosphere), causing dry air to descend over the eastern Pacific.
Then, the descending air reinforces the trade winds, which push the warm water towards the Indonesian seas. And that helps put the tropical Pacific in a La Niña-like state.
Australians may think La Niña summers are cool and humid. But under the long-term warming trend of climate change, its worst impacts will be flooding, especially in the east.
We also show that a closure to the Atlantic will be felt as far south as Antarctica. Rising warm air over the western Pacific would cause wind changes to spread south to Antarctica. This would deepen the low atmospheric pressure system over the Amundsen Sea, which lies west of Antarctica.
This low-pressure system is known to influence the melting of the ice sheet and the ice shelf, as well as ocean circulation and the extension of sea ice west to the Ross Sea.
A new world order
At no point in Earth’s history, apart from giant meteorites and supervolcanoes, has our climate system been shaken by changes in the composition of atmospheric gas like the one we are imposing today by our uninterrupted combustion of fossil fuels. .
The oceans are the driving force behind the Earth’s climate, slowing the rate of change by absorbing heat and carbon in large quantities. But there is a recovery, with rising sea levels, melting ice and a significant slowdown in the upheaval circulation of the Atlantic projected for this century.
We now know that this slowdown will not only affect the North Atlantic region, but as far away as Australia and Antarctica.
We can prevent these changes from happening by growing a new low-carbon economy. If you do, it will change, for the second time in less than a century, the course of Earth’s climate history, this time for the better.
This article originally appeared in The Conversation and was reproduced with permission
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