Why a mysterious ocean pattern could rapidly change our climate

Run AMOC

Meet the AMOC - the Atlantic Meridional Overturning Circulation. Like a giant conveyor belt, it carries water, salt and sediment across the oceans. Alongside these it brings heat from the south, across the equator and into the north, depositing that heat into the atmosphere.

"That's why, in Cambridge, it's relatively warm compared to other places at our latitude," says Starr. "It's much colder in landlocked places on our latitude, like Siberia."

Over the last 100 years, researchers have built a complex picture of how the AMOC functions. In 1961, oceanographer Henry Stommel realised that Earth's oceans collectively circulate according to certain rules, governed by equilibrium states. These states are holding patterns that the oceans stabilise in. They only jump between states when certain conditions arise. In the 1980s, Wally Broeker discovered rapid warming intervals in the geological record. During these times, Greenland would warm 10 or 15 degrees in a couple of decades.

These rapid fluctuations became known as Dansgaard-Oeschger events - the flick of the switch from one equilibrium to another. Broeker recognised this switch-flick mechanism in the AMOC, in a paper called 'Unpleasant surprises in the greenhouse.' He would later coin the phrase 'global warming'.

It turns out that the modern AMOC has two states - 'strong', which is stable and warmer, and 'weak', which is low energy and colder. Broeker realised that ocean fluctuations could totally change the climate in the North Atlantic: not gradually, but suddenly.

Starr has seen these sudden changes written in ice cores.

"In the ice, you can see glacial periods lasting around a hundred thousand years. But then if you zoom in, you see these jumps in the record. That's a flicking of the AMOC switch."

Currently, we're in a stable, warm AMOC state. We have been for all our lifetimes, and for many generations before. Our interglacial is called the Holocene, and has kept us cosy for at least 11,000 years. But there are some signs that we might be heading towards a tipping point.

"The thing about a tipping point is, you might not know it's crossed until it's too late. Once you're in the new equilibrium state, it takes a lot to throw the switch back."

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