The AMOC is a system of ocean currents which plays an important role in the climate system by transporting warm water northwards in the Atlantic and is critical for moving heat around the planet. 

The circulation process begin as warm water near the surface moves towards the poles (such as the Gulf Stream in the North Atlantic), where it cools and forms sea ice. As this sea ice forms, salt is left behind in the ocean water. Due to the large amount of salt in the water, it becomes denser, sinks down, and is carried southwards in the depths below. Eventually this water gets pulled back up towards the surface and warms up in a process called upwelling, completing the cycle.

Even though the whole process is slow on its own (it takes around 1,000 years for any given cubic metre of water to complete its journey along the global ocean conveyor belt), evidence from recent years suggests the AMOC is now slowing further.

Should it slow to an eventual halt, the consequences will have far-reaching climate impacts.

“Many impacts are likely to be felt globally, including a shift in tropical rainfall belts, reduced oceanic carbon dioxide uptake (and thus faster atmospheric increase) as well as major additional sea-level rise particularly along the American Atlantic coast, and an upheaval of marine ecosystems and fisheries,” warn scientists.

Among the letter’s signatories is Professor David Thornalley, an expert in paleoclimate research at University College London where he specialises in historic shifts in ocean circulation patterns and their impacts on the climate. Professor Thornalley contributed to evidence indicating the AMOC is now vulnerable to disruption due to human-induced climate change.

“Our records place recent Atlantic circulation change in a longer-term context,” he said. “Although the details of exactly how things are changing are still being researched and debated, it’s clear that unprecedented changes in Atlantic circulation are occurring. This raises warning signs about the future.”

Such warnings have been raised before, however. Research from the University of Copenhagen published earlier this year found that, with 95% certainty, a collapse could occur between 2025 and 2095. Published in February this year, the university’s study suggests that most likely the collapse will occur in 34 years’ time, in 2057.

“Shutting down the AMOC can have very serious consequences for Earth’s climate, for example, by changing how heat and precipitation are distributed globally,” said the University of Copenhagen’s Professor Peter Ditlevsen, one of the 44 scientists to have signed this week’s open letter.

“While a cooling Europe may seem less severe as the globe as a whole becomes much warmer and heat waves occur more frequently, this shutdown will contribute to an increased warming of the tropics, where rising temperatures have already given rise to challenging living conditions,” he added.

Acting as an initial platform for concern within the oceans and climate science community, the letter was first presented at the Arctic Circle conference in Iceland this week. Its signatories have since called for urgent international cooperation to reduce carbon emissions and protect vital marine environments while taking action to mitigate the fallout from the slowing AMOC, emphasising the pivotal role it plays in maintaining global climate stability.

For more from the Oceanographic Newsroom follow here.