Study finds early warning signals of global ocean conveyor belt collapse

The collapse of the Atlantic Meridional Overturning Circulation would trigger cooling and sea level rises in the North Atlantic region and encourage drought in central Africa (Photo: Shutterstock)

We could see early warning signs of the collapse of a key component of the global climate up to 250 years in advance, a new study has shown – ample time to either prevent or prepare for the consequences of abrupt climate change. The University of Exeter study analyzed the Atlantic Meridional Overturning Circulation (AMOC), sometimes referred to as the global ocean conveyor belt, in a highly-complex and realistic simulation model, and identified the likely mechanisms that would drive such a collapse.

The AMOC is crucial to Earth's climate. It transports heat from the tropics to the cooler North Atlantic and then up into the atmosphere, driven by differences in density in ocean layers that are caused by salinity and temperature variations. Without it, the surface air temperature of the North Atlantic region would cool by around 1-3 degrees Celsius, with isolated pockets cooling by up to 8°C.

It would drive the intertropical convergence zone southward, encouraging drought in the Sahel region (south of the Sahara desert). And it would result in dynamic sea level changes on the coasts of Europe and North America of up to 80 cm (31 in). (Dynamic sea level relates to the sea level deviation from the geoid, which is the level the ocean would be at if affected only by Earth's rotation and gravitation – not wind, tides, and other circulatory forces.)

As freshwater glacial ice melts or surface temperature increases, the density of surface waters in the North Atlantic shrinks. This appears to be happening now, and it's triggering a positive feedback loop that accelerates the process, though there's little evidence as yet of any impending collapse.

In the long run, though, the system could cross a threshold known as "critical slowing down," which would make it unstable and likely lead to relatively sudden collapse – on the scale of months. The researchers sought to identify the early warning signals that precede this phenomenon.

They ran simulations with the FAMOUS climate model, which predicts AMOC collapse in around 800 years under a scenario that has the rate of freshwater flows into the system in line with recent history. Their analysis found that telltale signs of critical slowing down appeared around 250 years before the collapse: the natural fluctuations in the circulation got longer and longer, and the warning signals were latitude dependent.

Moreover, notes co-author Tim Lenton, "The best early warning signals in the model world are in places where major efforts are going into monitoring the circulation in the real world – so these efforts could have unexpected added value."

So is forecasting AMOC collapse as simple as monitoring the real-world circulation and comparing the data to simulations? Not quite. The study may have used a more realistic simulation of the climate system to search for early warning signals than any that came before, but it still relied on assumptions that may not pan out. Freshwater forcing of the system, for instance, could increase at a greater rate than the simulations considered, and the scientists are unsure if the early warning signals would still appear if that happened.

Regardless, the researchers advocate caution and increased monitoring. "We don’t know how close we are to a collapse of the circulation, but a real-world early warning could help us prevent it, or at least prepare for the consequences," says Lenton.

A paper describing the research was published in the journal Nature Communications.

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