The Atlantic Meridional Overturning Circulation (AMOC) is a system of currents which helps to regulate temperatures around the world.
It carries warm water north from the tropics in the Atlantic - and any change to the way it works could radically alter temperatures around the world.
AMOC hit the headlines this week as a study predicted that an abrupt ‘tipping point’ could happen relatively rapidly (which has not happened for 10,000 years). It could have devastating effects on the climate around the world, with a rapid drop in temperatures predicted in Europe in particular.
What is AMOC?
AMOC is a system of currents which works like a conveyor belt, carrying warm water from the tropics northwards.
There, colder, dense water sinks into the deep ocean and spreads slowly south.
The cycle is hugely important in maintaining global temperatures and also modulates the impact of man-made global warming.
In Britain, AMOC brings tropical heat via the sea, which makes our country warmer and wetter than it would otherwise be.
Scientists said the AMOC is one reason that average temperatures in Britain are warmer than those of many places at similar latitudes. For example, Moscow and the southern extremes of Alaska are further south than Edinburgh.
Why might it change or collapse?
Climate change is believed to be behind the changes to the AMOC, which are fuelled in part by melting ice sheets in Greenland.
The AMOC is already thought to be 15% weaker than it was in 1950.
The Met Office says, "Climate models suggest that the AMOC will weaken over the 21st Century as greenhouse gases increase.
"This is because as the atmosphere warms, the surface ocean beneath it retains more of its heat. Meanwhile, increases in rainfall and ice melt mean it gets fresher too. All these changes make the ocean water lighter and so reduce the sinking in the ‘conveyor belt’, leading to a weaker AMOC"
What would happen in Britain?
Britain would see sea level rise, temperatures would drop by around 3C and rainfall would drop sharply.
Some experts have suggested that it would make farming difficult across much of the UK.
If AMOC collapses, storms coming from the Atlantic would not pick up warm moisture over the ocean, and harsh wintry storms would become the norm.
Snow could lie for months at a time.
When could it happen?
The Met Office still says a collapse of the AMOC this century would be very unlikely - although previous research has suggested a range of dates from 2025 to 2095.
A new paper in Science Advances looked at warning signs in salinity levels in the Atlantic between Cape Town and Buenos Aires.
This week’s research did not suggest a date for AMOC collapse but suggested it could happen extremely quickly once the tipping point is reached.
The key point of the new research is that an AMOC collapse is a real possibility.
The researchers write: "This is bad news for the climate system and humanity as up till now one could think that Amoc tipping was only a theoretical concept and tipping would disappear as soon as the full climate system, with all its additional feedbacks, was considered.”
The researchers also warn that the changes could happen so rapidly it will be difficult to adapt.
Lead author René van Westen, of Utrecht University said: “What surprised us was the rate at which tipping occurs. It will be devastating.
“We are moving towards it. That is kind of scary. We need to take climate change much more seriously.”