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A huge Atlantic ocean current is slowing down—if it collapses, La Niña could become the norm for Australia

by Matthew England, Andréa S. Tachetto and Bryam Orihuela-Pinto,

Massive Atlantic current slows – if it collapses, La Niña could become the norm for Australia

Credit: Shutterstock

Climate change is to slow down the conveyor belt of ocean currents that carries warm water from the tropics to the North Atlantic. Our research, published today in Nature Climate changelooks at the drastic consequences for the global climate if this Atlantic conveyor belt completely collapses.

We found that the collapse of this system — the so-called Atlantic meridional tilting circulation — would shift Earth’s climate to a more La Niña-like state. This would lead to more flooding in eastern Australia and worse droughts and wildfires in the southwestern United States.

Australians on the east coast know how unrelenting La Niña feels. Climate change has loaded our atmosphere with moister air, while two summers of La Niña have heated the ocean north of Australia. Both contributed to some of the wettest ever conditions, with record-breaking flooding in New South Wales and Queensland.

Meanwhile, in southwestern North America, a record drought and severe forest fires have put a lot of pressure on emergency services and agriculture, just because of the fires in 2021 estimated to have cost at least $70 billion

The Earth’s climate is dynamic, variable and constantly changing. But our current trajectory of unabated greenhouse gas emissions gives the entire system a huge kick that will have uncertain consequences — consequences that will rewrite our textbook description of ocean circulation and the impact of the planet.

What is the Atlantic Tilting Meridional Circulation?

The Atlantic Ocean’s overturning circulation involves a massive flow of warm tropical water into the North Atlantic that helps to keep Europe’s climate mild, while giving the tropics a chance to lose excess heat. An equivalent tilt of the Antarctic waters found in the southern hemisphere.

Climate records going back 120,000 years reveals the Atlantic overturning circulation has shut down, or slowed down dramatically, during ice ages. It switches on 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 tilt of the Atlantic Ocean has been relatively stable. But about the a slowdown has been observed in recent decadesand this worries scientists.

Why the delay? An unequivocal consequence of global warming is the melting of the polar caps in Greenland and Antarctica. When these ice sheets melt, they dump huge amounts of freshwater into the oceans, making the water more buoyant and reducing the sinking of dense water at high latitudes.

Just around Greenland, a huge 5 trillion tons ice has melted over the past 20 years. That’s equivalent to 10,000 freshwater in Sydney’s harbours. This melting rate will increase in the coming decades if global warming continues unabated.

A collapse of the North Atlantic and Antarctic waters that would overturn the circulation would profoundly alter the anatomy of the world’s seas. It would freshen them at depth, deplete oxygen and starve the upper ocean from the upwelling of nutrients supplied when deep waters emerge from the ocean’s abyss. The implications for marine ecosystems would be profound.

With Greenland’s ice already in full swing, scientists estimate the Atlantic is tipping over has been at its weakest for at least the last millenniumwith predictions of a future collapse in the coming centuries if greenhouse gas emissions remain unchecked.

Een enorme stroming in de Atlantische Oceaan vertraagt ​​- als deze instort, kan La Niña de norm worden voor AustraliëStefano Crivellari, University of São Paulo/Research Gate†

The major components of the Atlantic meridional tilting circulation. The northward flowing upper branch (red arrow) transports warm, salty waters to the North Atlantic Ocean, forming the North Atlantic Deep Waters (NADW) at high latitudes. The southward flowing NADW lies above the Antarctic Bottom Water (AABW). Credit: Stefano Crivellari, University of São Paulo/Research Gate

The consequences of a delay

In our study, we used a comprehensive global model to explore what the Earth’s climate would look like in such a collapse. We eliminated the Atlantic tilt by applying a massive meltwater drift to the North Atlantic and then comparing it to an equivalent run without meltwater.

Our focus was to look beyond the known regional effects in Europe and North America to see how Earth’s climate would change in remote locations as far south as Antarctica.

The first thing the model simulations showed was that without the Atlantic overturning, a huge amount of heat builds up just south of the equator.

This excess of tropical Atlantic heat pushes more warm moist air into the upper troposphere (about 10 kilometers into the atmosphere), pushing dry air down over the eastern Pacific Ocean.

The descending air then amplifies the trade winds, which push warm water toward the Indonesian seas. And this helps bring the tropical Pacific into a La Niña-like state.

Australians may consider La Niña’s summers cool and wet. But under the long-term warming trend of climate change, their biggest impacts will be rain showers, especially in the east.

We also show that a reversal closure of the Atlantic Ocean would be felt as far south as Antarctica. Rising warm air over the western Pacific would cause wind changes that propagate south to Antarctica. This would deepen the low-pressure atmospheric system over the Amundsen Sea, which lies off western Antarctica.

This low pressure system is known to melting ice cap and ice shelfas well as ocean circulation and sea ice as far west as the Ross Sea.

A new world order

At no point in Earth’s history, barring giant meteorites and supervolcanoes, has our climate system been shaken by changes in atmospheric gas composition, as imposed today by our unrelenting burning of fossil fuels.

The oceans are the flywheel of Earth’s climate, slowing the rate of change by absorbing heat and carbon in large quantities. But there is a payback, with sea level rise, ice melt and a significant slowdown in the Atlantic, reversing the circulation projected for this century.

Now we know that this slowdown will affect not only the North Atlantic region, but as far away as Australia and Antarctica.

We can prevent these changes from happening by developing a new low-carbon economy. This will change the course of Earth’s climate history for the second time in less than a century — this time for the better.

Deep ocean warming due to climate change

More information:
Bryam Orihuela-Pinto et al, Interbasin and Interhemispheric Effects of a Collapsed Atlantic Overturning Circulation, Nature Climate change (2022). DOI: 10.1038/s41558-022-01380-y

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Quote: A huge current in the Atlantic slows – if it collapses, La Niña could become the norm for Australia (2022, June 7) Retrieved June 7, 2022 from https://phys.org/news/2022-06- huge-atlantic -ocean-flow-downif.html

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