The conveyor belt of ocean currents that transports warm water from the tropics to the North Atlantic is slowing down as a result of climate change.
Our study, which was published in Nature Climatic Change today (June 6), looks at the global climate repercussions if the Atlantic conveyor completely fails.
We discovered that if this mechanism – known as the Atlantic meridional overturning circulation – were to collapse, the Earth's climate would move to a more La Nia-like condition.
More flooding rains would be expected in eastern Australia, as well as severe droughts and wildfire seasons in the southwest United States.
East-coast Australians are familiar with the debilitating effects of La Nia. Climate warming has added moisture to our atmosphere, while La Nia has warmed the ocean north of Australia for the past two summers.
With record-breaking floods in New South Wales and Queensland, both contributed to some of the wettest conditions ever seen.
Meanwhile, a record drought and major bushfires in the southwest of North America have put a significant burden on emergency services and agriculture, with the 2021 flames alone costing at least US$70 billion.
The climate on Earth is dynamic, varied, and always changing. However, our current course of unchecked greenhouse gas emissions is giving the entire system a massive push that will have unforeseeable implications — effects that will rewrite our textbook descriptions of the planet's ocean circulation and impact.
What is the Atlantic overturning meridional circulation?
The Atlantic overturning circulation is a large movement of warm tropical water into the North Atlantic that helps maintain the European temperature moderate while enabling the tropics to cool down. In the Southern Hemisphere, there occurs a similar overturning of Antarctic seas.
During ice ages, the Atlantic overturning circulation has shut down or slowed considerably, according to climate data dating back 120,000 years.
During so-called 'interglacial times,' when the Earth's climate is warmer, it turns on and cools the European climate.
The Atlantic overturning has been generally steady since human civilisation began roughly 5,000 years ago. However, scientists have seen a slowing during the last several decades, which has them concerned.
What is the cause of the halt? The melting of polar ice caps in Greenland and Antarctica is an unmistakable consequence of global warming.
When these icecaps melt, enormous volumes of freshwater are released into the oceans, making water more buoyant and minimizing thick water sinking at high latitudes.
In the last 20 years, a staggering 5 trillion tons of ice has evaporated around Greenland alone. That's enough freshwater to fill 10,000 Sydney Harbours.
If global warming continues unchecked, the melting rate is expected to grow in the next decades.
The architecture of the world's oceans would be significantly altered if the North Atlantic and Antarctic overturning circulations collapsed.
It would make them fresher at depth, deplete them of oxygen, and deprive the top ocean of the nutrient upwelling that occurs when deep waters return from the ocean's depths. The consequences for marine ecosystems would be huge.
With Greenland glacier melt now well started, experts believe the Atlantic overturning is at its lowest level in at least a millennium, with forecasts of a potential collapse if greenhouse gas emissions continue unchecked in coming generations.
The ramifications of a slowdown
In our research, we developed a global model to simulate what the Earth's climate might be like if it were to collapse.
We turned off the Atlantic overturning by injecting a huge meltwater anomaly into the North Atlantic, then comparing it to a run without meltwater.
Our goal was to explore beyond the well-known regional consequences in Europe and North America and see how the Earth's temperature might alter in far-flung places like Antarctica.
The model simulations initially demonstrated that without the Atlantic overturning, a significant buildup of heat occurs just south of the Equator.
More warm moist air is pushed into the high troposphere (about 10 kilometers into the atmosphere) as a result of the surplus tropical Atlantic heat, forcing dry air to drop over the east Pacific.
The trade winds intensify as the air descends, pushing warm water into the Indonesian seas. This aids in the formation of a La Nia-like condition in the tropical Pacific.
Summers at La Nia are often described as chilly and damp by Australians. However, as a result of climate change's long-term warming trend, its biggest effects will be flooding rain, particularly in the east.
We also show that a shutdown of the Atlantic overturning current would be sensed all the way down to Antarctica. Warm air rising over the West Pacific would cause wind variations that would spread south to Antarctica. The atmospheric low-pressure system over the Amundsen Sea, which lies off the coast of west Antarctica, would be deepened as a result.
As far west as the Ross Sea, this low-pressure system is known to impact ice sheet and ice shelf melt, as well as ocean circulation and sea-ice extent.
A new world order
With the exception of enormous meteorites and super-volcanoes, our climate system has never been jolted by changes in atmospheric gas composition like the ones we are imposing today via our unchecked combustion of fossil fuels.
The seas are the Earth's climate's flywheel, slowing change by absorbing massive amounts of heat and carbon. With sea level rise, glacier loss, and a major slowing of the Atlantic overturning circulation expected during this century, there will be retribution.
We now know that this slowdown will effect not only the North Atlantic area, but also Australia and Antarctica.
We can avert these changes by fostering the development of a new low-carbon economy. For the second time in less than a century, this will alter the path of Earth's climatic history, this time for the better.
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