The Nina could be here to stay if the Atlantic ocean current collapses, scientists warn

This story first appeared on The Conversation.

Climate change is slows down the transport belt of sea currents that brings warm water from the tropics to the North Atlantic.

Our researchpublished today in Nature Climate Changelooks at the profound consequences to a global climate if that Atlantic transport collapses completely.

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We have found that the collapse of this system – called the South Atlantic upside-down circulation – would change the Earth’s climate for the worse. La Nina-as a state.

This would mean more flood rains over eastern Australia and worse droughts and wildfire seasons over the southwestern United States.

East Coast Australians know how relentless La Nina feels. Climate change has charged our atmosphere with more humid air, while two summers of La Nina have warmed the ocean north of Australia.

Both contributed to some of the wettest conditions ever experienced, with record floods in New South Wales and Queensland.

Meanwhile, over southwestern North America, record droughts and severe forest fires have put enormous strain on emergency services and agriculture, and the 2021 fires estimated alone have cost at least $ 70 billion.

The Earth’s climate is dynamic, changing and ever-changing. But our current trajectory of incessant greenhouse gases is giving the whole system a giant kick that will have uncertain consequences – consequences that will rewrite our textbook a description of the planet’s ocean circulation and its impact.

What is the Atlantic reversing southern circulation?

The Atlantic reversal consists of a massive flow of warm tropical water to the North Atlantic that helps keep Europe’s climate mild, allowing the tropics a chance to lose excess heat.

An equivalent reversal of Antarctic waters can be found in the Southern Hemisphere.

Climate records dating back 120,000 years reveal that the Atlantic reversal turned off, or slowed dramatically, during ice ages.

It ignites and pacifies Europe’s climate during so-called “interglacial periods”, when the Earth’s climate is warmer.

Since human civilization began about 5,000 years ago, the Atlantic upheaval has been relatively stable. But in recent decades a slowdown has been detected, and this has worried scientists.

Why the slowdown? One unambiguous consequence of global warming is the melting of polar ice caps in Greenland and Antarctica.

North NSW was hit hard by devastating charging earlier in 2022. Credit: AAP
Record-breaking floods occurred in New South Wales and Queensland in 2022. Credit: AAP

When these ice sheets melt, they dump massive amounts of fresh water into the oceans, making water more buoyant and reducing the sinking of dense water at high latitudes.

Just around Greenland, massive five trillion tons of ice have melted in the past 20 years. That is equivalent to 10,000 Sydney Harbors worth of fresh water.

This melting rate is set to increase in the coming decades if global warming continues unabated.

A collapse of the North Atlantic and Antarctic upheavals would profoundly alter the anatomy of the world’s oceans.

It would make them cooler at depth, drain them of oxygen, and starve the upper ocean of the upstream flow of food provided when deep water would resurface from the ocean abyss.

The implications for marine ecosystems would be profound.

With Greenland melting already well underway, scientists estimate that the Atlantic upheaval is weakest for at least the last millennium, with predictions of a future collapse on the charts in the coming centuries if greenhouse gases are uncontrolled.

The consequences of slowing down

In our studywe used a comprehensive global model to examine what the Earth’s climate would look like under such a collapse.

We turned off the reversal of the Atlantic by applying a massive meltwater anomaly to the North Atlantic, and then compared that to an equivalent run with no meltwater applied.

Our focus was to look beyond the known regional impacts around Europe and North America, and to see how the Earth’s climate would change in distant places, as far south as Antarctica.

The first thing the model simulations revealed was that without an Atlantic overturn, a massive amount of heat is accumulating just south of the equator.

The system crash could result in more rain and flooding for Australia. Credit: AAP

This excess of tropical Atlantic heat pushes warmer humid air into the upper troposphere (about 10 kilometers into the atmosphere), causing dry air to descend over the eastern Pacific.

The descending air then reinforces wind gusts that push hot water into the Indonesian seas. And this helps put the tropical Pacific in the Nina-like state.

Australians may think of La Niña summers as cool and wet. But under the long-term warming trend of climate change, their worst effects will be flooding, especially over the east.

We also show that an Atlantic reversal would be felt as far south as Antarctica. Rising warm air over the western Pacific would trigger wind gusts that spread south to Antarctica.

This would deepen the atmospheric low pressure system over the Amundsen Sea, which sits off West Antarctica.

This low-pressure system can affect ice sheet and ice sheet melting, as well as ocean circulation and sea ice extent as far west as the Ross Sea.

A new world order

At no point in Earth’s history, aside from giant meteorites and supervolcanoes, has our climate system been shaken by changes in atmospheric gas composition like what we impose today by our relentless burning of fossil fuels.

The oceans are the inertia of the Earth’s climate, slowing the rate of change by absorbing heat and carbon in vast quantities. But there is a payback, with rising sea levels, melting ice and a significant slowdown in the Atlantic overturning circulation 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 by growing a new low-carbon economy.

Doing so will change, for the second time in less than a century, the course of Earth’s climate history – this time for the better.

Matthew England is a Professor of Science and Deputy Director of the ARC Australian Center for Excellence, Andréa S. Taschetto is an Assistant Professor at UNSW Sydney and Bryam Orihuela Pinto is a PhD Candidate at UNSW Sydney.

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