A rapidly slowing Atlantic Ocean current could change Earth’s climate and cause perpetual conditions in La Niña for Australia if it collapses.
Climate change is slowing the conveyor belt of ocean currents that carry warm water from the tropics to the North Atlantic.
UNSW research, published today in Nature Climate Change, examines the profound consequences on the global climate if this Atlantic transporter completely collapses.
We found that the collapse of this system – called the southern Atlantic tilting circulation – would shift the Earth’s climate to a state more like La Niña. That would mean more flood rainfall over eastern Australia and worse droughts and bushfire seasons in the southwestern United States.
East Coast Australians know what La Niña inexorably feels like. Climate change has loaded our atmosphere with wetter air, while two summers in La Niña warmed the ocean north of Australia. Both contributed to some of the wettest conditions ever experienced, with record flooding in New South Wales and Queensland.
Meanwhile, in southwestern North America, a record drought and severe forest fires have put a strain on emergency services and agriculture, with the 2021 fires alone estimated to have cost at least $ 70 billion.
The Earth’s climate is dynamic, variable and ever-changing. But our current trajectory of uninterrupted greenhouse gas emissions is giving the entire system a gigantic kick that will have uncertain consequences, consequences that will rewrite our textbook description of the planet’s ocean circulation and its impact.
What is the Atlantic that overturns the southern circulation?
The Atlantic tipping circulation includes a massive flow of warm tropical water to the North Atlantic which helps keep the European climate mild, allowing the tropics a chance to lose excess heat. An equivalent reversal of Antarctic waters is found in the Southern Hemisphere.
Climate records dating back 120,000 years reveal that the inverted circulation of the Atlantic stopped, or dramatically slowed down, during the ice ages. It ignites and places 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 reversal has been relatively stable. But there has been a slowdown in recent decades and this has worried scientists.
Why the slowdown? An unequivocal consequence of global warming is the melting of the polar ice caps in Greenland and Antarctica. When these polar ice caps melt they dump huge amounts of fresh water into the oceans, making the water more dynamic and reducing the sinking of dense water in high latitudes.
Around Greenland alone, 5 trillion tons of ice have melted over the past 20 years. This equates to 10,000 freshwater Sydney Harbors. This melting rate is set to 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 make them cooler at depths, depleted them of oxygen and starve the upper ocean of the nutrient rise provided when the deep waters re-emerge from the ocean abyss. The implications for marine ecosystems would be profound.
With Greenland ice melting already well underway, scientists estimate that the Atlantic tilt is at its weakest point for at least the last millennium, with predictions of future collapse on the cards in the coming centuries if greenhouse gas emissions don’t will be checked.
The consequences of a slowdown
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 Atlantic tipping by applying a massive meltwater anomaly to the North Atlantic and then compared it to an equivalent run without meltwater application.
Our goal was to look beyond the well-known regional impacts in Europe and North America and see how Earth’s climate would change in remote locations, as far as Antarctica.
The first thing the model simulations revealed is that without the Atlantic overturning, a huge build-up of heat builds up just south of the equator.
This excess tropical heat from the Atlantic pushes warmer, wetter air into the upper troposphere (about 10 kilometers into the atmosphere), sending dry air down over the eastern Pacific.
The descending air then strengthens the trade winds, which push the warm water towards the Indonesian seas. And this helps bring the tropical Pacific into a La Niña-like state.
Australians may think of La Niña summers as cool and humid. But under the long-term warming trend of climate change, their worst impact will be pouring rain, especially in the east.
We also show that an Atlantic overturning arrest would be felt as far as Antarctica. Rising warm air over the western Pacific would trigger wind changes that spread south to Antarctica. This would deepen the low atmospheric pressure system on the Amundsen Sea, which lies off West Antarctica.
This low-pressure system is known to affect the melting of the ice sheet and ice shelf, as well as ocean circulation and the extent of sea ice to the Ross Sea.
A new world order
At no time in Earth’s history, giant meteorites and supervolcanoes aside, has our climate system been shaken by changes in atmospheric gas composition like the one we are forcing today due to our incessant burning of fossil fuels.
The oceans are the driver of Earth’s climate, slowing the pace of change by absorbing heat and carbon in large quantities. But there is a payback, with rising sea levels, melting ice and a significant slowdown in the Atlantic that reverses the circulation forecast for this century.
We now know that this slowdown will not only affect the North Atlantic region, but also Australia and Antarctica.
We can prevent these changes from happening by growing a new low-carbon economy. This will change the course of Earth’s climatic history for the second time in less than a century, this time for the better.
This article originally appeared on The conversation and has been reproduced with permission
Originally published as La Niña could become the norm for Australia if an Atlantic Ocean current collapses
