LONDON - The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that transports warm water from the tropics northwards into the North Atlantic.

In recent years it has emerged that the AMOC - and other "overturning circulation" systems like it - have been weakening due to climate change, and could even collapse completely. This could have huge knock-on effects to our climate, including temperatures in Europe plunging by up to 10 degrees.

Now, some scientists have warned that these weakening circulations could also have an alarming effect on the way the ocean absorbs carbon.

It might mean, according to researchers at the Massachusetts Institute of Technology (MIT), that more carbon will be released into the atmosphere having a negative "feedback" effect that makes the impact of global warming even more pronounced.
What could happen to the oceans?

At present, the world’s oceans absorb 30% of the carbon from human activities. The new MIT study has warned that, as circulation weakens, it could cut the amount the ocean absorbs and also release carbon from the deep ocean into the atmosphere.

It's due to a change in chemicals in the ocean that feed tiny, CO2-guzzling organisms called phytoplankton.

Phytoplankton are microscopic, plant-like organisms that live on the ocean surface and consume a diet of carbon and nutrients - which come up from the deep ocean - and iron.

As a result, despite only making up around 1-2% of the total global plant carbon, they play a critical role in absorbing carbon. By some accounts, they "fix" about 40% of global carbon capture and storage.

The more phytoplankton can grow, the more carbon dioxide they can absorb from the atmosphere via photosynthesis.

Scientists fear a weakening ocean circulation will mean less carbon and nutrients are pulled up from the depths of the ocean, meaning a reduced amount of food for phytoplankton at the surface.

This would reduce the amount of phytoplankton, therefore decreasing the amount of carbon dioxide they can absorb from the atmosphere.

Study author Jonathan Lauderdale, a research scientist in MIT's Department of Earth, Atmospheric, and Planetary Sciences, says he ran computer models to simulate the effects of a weakening circulation - and found that the weaker the ocean's circulation, the more CO2 built up in the atmosphere.

"I thought there was some mistake," Lauderdale says. "Why were atmospheric carbon levels trending the wrong way?"

What could this mean?

The study shows it’s possible that not only will oceans absorb less CO2 from human activities due to changes in circulation - they might also actually release more carbon.

That, in turn, could cause a ‘feedback’ effect which could cause even faster climate change, the researchers warn.

Lauderdale says: "Some climate models predict a 30% slowdown in the ocean circulation due to melting ice sheets, particularly around Antarctica.

"This huge slowdown in overturning circulation could actually be a big problem: In addition to a host of other climate issues, not only would the ocean take up less anthropogenic CO2 from the atmosphere, but that could be amplified by a net outgassing of deep ocean carbon, leading to an unanticipated increase in atmospheric CO2 and unexpected further climate warming."

As a result, the MIT researchers warn that it may not always be possible to count on the oceans as a ‘sink’ which will absorb carbon - and that unexpected carbon releases from the deep ocean may lead to additional warming.

Lauderdale adds: "What we thought is going on in the ocean is completely overturned. We can't count on the ocean to store carbon in the deep ocean in response to future changes in circulation. We must be proactive in cutting emissions now, rather than relying on these natural processes to buy us time to mitigate climate change."