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

Climate change is slowing down the conveyor belt of ocean currents that brings warm water from the tropics up to the North Atlantic. Our research, published today in Nature Climate Change, looks at the profound consequences to global climate if this Atlantic conveyor collapses entirely.

Collapse of this system—called the Atlantic meridional overturning circulation—would shift the Earth’s climate to a more La Niña-like state. This would mean more flooding rains over eastern Australia and worse droughts and bushfire seasons over southwest United States.

The Atlantic overturning circulation comprises a massive flow of warm tropical water to the North Atlantic that helps keep European climate mild, while allowing the tropics a chance to lose excess heat.

Climate records reaching back 120,000 years reveal the Atlantic overturning circulation has switched off, or dramatically slowed, during ice ages.

Since human civilization began around 5,000 years ago, the Atlantic overturning has been relatively stable. But over the past few decades a slowdown has been detected, and this has scientists worried.

Why the slowdown? One unambiguous consequence of global warming is the melting of polar ice caps in Greenland and Antarctica. When these icecaps melt, they dump massive amounts of freshwater into the oceans, making water more buoyant and reducing the sinking of dense water at high latitudes.

Around Greenland alone, a massive 5 trillion tonnes of ice has melted in the past 20 years. That’s equivalent to 10,000 Sydney Harbours worth of freshwater. This melt rate is set to increase over the coming decades if global warming continues unabated.

With Greenland ice melt already well underway, scientists estimate the Atlantic overturning is at its weakest for at least the last millennium, with predictions of a future collapse on the cards in coming centuries if greenhouse gas emissions go unchecked.

In our study, we used a comprehensive global model to examine what Earth’s climate would look like under such a collapse. We switched the Atlantic overturning off by applying a massive meltwater anomaly to the North Atlantic, and then compared this to an equivalent run with no meltwater applied.

The first thing the model simulations revealed was that without the Atlantic overturning, a massive pile up of heat builds up just south of the Equator.

This excess of tropical Atlantic heat pushes more warm moist air into the upper troposphere (around 10 kilometres into the atmosphere), causing dry air to descend over the east Pacific.

The descending air then strengthens trade winds, which pushes warm water towards the Indonesian seas. And this helps put the tropical Pacific into 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 impacts will be flooding rain, especially over the east.

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