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 Change, looks at the profound consequences for a global climate if this Atlantic transport collapses completely.
We found that the collapse of this system – called the South Atlantic reverse cycle – would change the Earth’s climate to a more La Niña-like state. This would mean more flood rains over eastern Australia and worse droughts and forest fire seasons over the southwestern United States.
East Coast Australians know how relentless La Niña feels. Climate change has charged our atmosphere with drier air, meanwhile two summers of the Girl 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 drought and severe forest fires put huge strain on emergency services and agriculture, only with the 2021 fires estimated to 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 reverse cycle consists of a massive flow of warm tropical water to the North Atlantic that helps keep European climates mild, while allowing the tropics a chance to lose excess heat. An equivalent reversal of Antarctic waters can be found in the Southern Hemisphere.
Climate records reaching 120,000 years ago reveals that the Atlantic upheaval has been turned off, or dramatically slowed down, during ice ages. It ignites and calms the European 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 over the over the past few decades a slowdown has been detected and this worried scientists.
Why the slowdown? One unambiguous consequence of global warming is the melting of polar ice caps in Greenland and Antarctica. 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.
Around Greenland alone, crowd 5 trillion tons of ice has 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 ice melting already well underway, scientists are estimating the Atlantic upheaval is weakest for at least the last millenniumwith predictions of a future collapse on the cards in coming centuries if greenhouse gas emissions are uncontrolled.
The consequences of slowing down
In our study, we 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 upheaval, a massive amount of heat is accumulating just south of the equator.
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 a La Niña-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 is known to affect ice sheet and ice sheet meltas 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 an Australian Research Council Laureate Fellow, Deputy Director of the Climate Change Resource Center, and Chief Investigator at the ARC Center of Excellence in Climate System Science at UNSW Sydney. Andréa S. Taschetto is an Assistant Professor at UNSW Sydney and Bryam Orihuela Pinto is a PhD candidate at UNSW Sydney. This piece first appeared on The Conversation.