Beneath the ocean, a giant conveyor belt is slowing down. If it stops, the deadly consequences could be global

There’s a huge conveyor belt in the ocean and it’s largely responsible for our European climate. If you look at the Spilhaus World Ocean Map, you’ll see that everything on Earth is connected.

Although there are five named oceans – the Arctic, Atlantic, Indian, Pacific and Southern Oceans – they are all joined together, making up one huge body of water. What happens in one part can impact things many miles away.

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One of the things connecting them is a huge, interconnected network of currents, both on the surface and in the deep.

One loop of such currents in the Atlantic is known as the Atlantic Meridional Overturning Circulation (or AMOC). The whole process is powered by wind as well as the differences in temperature and salinity between the waters in the Atlantic Ocean.

It acts a little like a conveyor belt: warm water travels north where it releases heat and cools. As it cools down via evaporation, it loses water and becomes saltier, and therefore heavier. This causes it to sink to deeper waters. There, the colder current returns south.

The loop continues like this: warm water heads north, cools, sinks and returns to the south where it warms and rises to the surface again, and so on.

The AMOC’s circulation is also influenced by the Denmark Strait Cataract (the world’s biggest waterfall, which is thousands of metres underwater).

It might be far from most people’s thoughts but it’s very important. According to the National Oceanography Centre(NOC), “this process delivers vital heat and nutrients to colder latitudes and transfers carbon down to the ocean depths.”

Although there are other factors too, the climate we know is partly because of this conveyor belt. “The mild winters we experience in the UK and north-western Europe happen partly because the AMOC brings heat up from southern latitudes,” adds NOC.

This cycle could be changing. “Climate models suggest that the AMOC will weaken over the 21^st century as greenhouse gases increase,” explains the Met Office on its website.

Increasing global temperatures means warmer oceans, more rainfall and more rapidly melting sea ice, which adds more freshwater to the oceans. “All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC,” they add.

People are concerned about the possible impacts of a weaker AMOC, which results in less warm water being transported from the tropics to the poles.

One reason is because warmer, less salty water can’t sink as deep because it isn't as heavy. “If this sinking process slows down, the ocean cannot transfer carbon dioxide from the atmosphere to the deep ocean as quickly,” says NOC. “This leaves larger amounts of greenhouse gases in the atmosphere, which accelerates climate change.”

Research isn’t yet clear on what might happen to the circulation. “While research shows it is weakening over the past century, whether or not it will continue to slow or stop circulating completely remains uncertain,” says NOAA’s National Ocean Service on its website. “If the AMOC does continue to slow down, however, it could have far-reaching climate impacts.”

This could include droughts in South Africa and sea level rise along the East coast of the USA.