(ORDO NEWS) — Climate change is slowing down the conveyor belt of ocean currents that brings warm water from the tropics to the North Atlantic.
Our study, published in the journal Nature Climate Change, looks at the profound consequences for the global climate if this Atlantic conveyor belt were to collapse completely.
We have found that the disruption of this system, called the Atlantic meridional circulation, will cause the Earth’s climate to shift into a more La Niña-like state.
This will mean more severe flooding in the east of Australia and more severe droughts and bushfire seasons in the southwestern US.
Residents of the east coast of Australia know what the relentless La Niña is. Climate change has saturated our atmosphere with wetter air, and two La Niña summers have warmed the ocean north of Australia.
Both events contributed to the wettest conditions on record, resulting in record flooding in New South Wales and Queensland.
Meanwhile, in southwestern North America, a record drought and massive bushfires have placed a huge strain on emergency services and agriculture. The 2021 fires alone are estimated to have cost at least US$70 billion.
Earth’s climate is dynamic, changeable and constantly changing. But our current trajectory of relentless greenhouse gas emissions is giving a giant boost to the entire system that will have uncertain consequences – consequences that will rewrite our description of the planet’s ocean circulation and its impact.
What is the Atlantic Inverted Meridional Circulation?
The Atlantic Overflow Circulation is a powerful flow of warm tropical water into the North Atlantic that helps maintain the mild European climate by allowing the tropics to lose excess heat. An equivalent circulation of Antarctic waters is observed in the Southern Hemisphere.
Climate records of 120,000 years show that during ice ages, the Atlantic circulation shuts down or slows down dramatically.
It turns on and calms the European climate during the so-called “interglacial periods” when the Earth’s climate is warmer.
Ever since human civilization began about 5,000 years ago, the Atlantic circulation has been relatively stable. But a slowdown has been found over the past few decades, and this has worried scientists.
Why slowdown? One of the unambiguous consequences of global warming is the melting of the polar ice caps in Greenland and Antarctica.
When these ice caps melt, they release massive amounts of fresh water into the oceans, making the water more buoyant and reducing the sinking of dense waters at high latitudes.
Around Greenland alone, 5 trillion tons of ice has melted in the last 20 years. This is equivalent to 10,000 freshwater Sydney harbours.
If global warming continues, the rate of ice melt will increase in the coming decades.
The collapse of the North Atlantic and Antarctic circulations will profoundly change the anatomy of the oceans.
It will make them fresher at depth, deplete them of oxygen and deprive the top layer of the ocean of nutrients that enter the upper ocean when the deep waters emerge from the ocean abyss. The implications for marine ecosystems will be profound.
With the melting of the Greenland ice caps already in full swing, scientists estimate that the Atlantic tipping is at its weakest level in at least the last millennium, and collapse is expected in the coming centuries if greenhouse gas emissions are not controlled.
Consequences of the slowdown
In our study, we used a comprehensive global model to explore what the Earth’s climate would look like under such a collapse.
We turned off Atlantic rollover by applying the North Atlantic meltwater anomaly and then compared it to a similar model without meltwater.
We wanted to go beyond the well-known regional impacts in Europe and North America, and test how the Earth’s climate will change in remote areas, as far as Antarctica.
The first thing the simulations showed was that, in the absence of the Atlantic circulation, a huge amount of heat builds up 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), causing dry air to sink over the eastern Pacific.
The sinking air then reinforces the trade winds, which push warm water towards the Indonesian seas. This helps bring the tropical Pacific into a La Niña-like state.
Australians may think of the La Niña summer as cool and wet. But with a long-term warming trend, their worst consequence will be heavy rains, especially in the east of the country.
We also show that the Atlantic tipping closure will be felt as far south as Antarctica. The rise of warm air over the western Pacific will cause wind changes that will spread as far south as Antarctica. This will deepen the low pressure atmospheric system over the Amundsen Sea off western Antarctica.
This low pressure system is known to influence the melting of ice sheets and ice shelves, as well as ocean circulation and sea ice extent all the way to the Ross Sea.
New world order
At no time in Earth’s history, apart from giant meteorites and supervolcanoes, has our climate system been subjected to such changes in the composition of atmospheric gases as we are seeing today as a result of the constant burning of fossil fuels.
The oceans are the flywheel of the Earth’s climate, slowing down the rate of change by absorbing heat and carbon in huge amounts. But there is a price to pay: this century is forecast to see sea levels rise, ice melt, and the Atlantic Inverted Circulation slow significantly.
We now know that this slowdown will affect not only the North Atlantic region, but also as far away as Australia and Antarctica.
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