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It would mean severe global climate repercussions, with Europe bearing the brunt of the consequences.
Scientists have successfully modelled a climate tipping point that would plunge large parts of Europe into a deep freeze - and it could be closer than previously thought.
Using a complex climate model, researchers from Utrecht University in the Netherlands simulated the collapse of an important Atlantic Ocean current which currently brings warm water north and keeps temperatures in Europe mild.
An abrupt shutdown of this current would change weather worldwide with Europe suffering the most severe consequences. Melting ice sheets could cause its collapse - but when or how likely is it to happen?
What is the Atlantic Ocean Circulation?
The Atlantic Meridional Overturning Circulation (AMOC) is an important system of ocean currents. It transports warm water, carbon and nutrients north via the Atlantic Ocean where the water cools and sinks into the deep.
The AMOC helps to distribute energy around the planet, moving heat through the ocean like a conveyor belt and regulating our climate.
Warm water - more salty due to evaporation - flows north on the surface of the ocean keeping Europe milder than it would otherwise be. When this water cools it sinks because its high salinity increases its density. It then flows back to the southern hemisphere along the bottom of the ocean.
But studies of past episodes of dramatic cooling in Europe over the last 100,000 years suggest melting ice sheets could weaken the AMOC due to changes in salinity and temperature.
Fresh water reduces the saltiness - and therefore the density of the water- on the surface of the ocean. This means less of the surface water sinks, potentially slowing the flow of the current.
Are we heading for a catastrophic tipping point?
Some research has suggested that climate change may be slowing the flow of the current. One study from 2023 based on sea surface temperatures suggested that a complete collapse could happen between 2025 and 2095.
There is huge uncertainty about how, when or even if this 'tipping point' could actually happen and modelling the scenario is tricky. Most previous computer simulations that showed a collapse involved adding huge, unrealistic quantities of fresh water all at once.
The Dutch team used a supercomputer to carry out the most sophisticated modelling so far to look for warning signs of this tipping point. They added water gradually, finding that a slow decline could eventually lead to a sudden collapse over less than 100 years.
Up until now, the paper says, an AMOC tipping point was only a "theoretical concept" and its authors say the rate at which the tipping of this vital current occurred in their modelling was "surprising".
But researchers had to run the simulation for more than 2,000 years to get this result and still added significantly more water than is currently entering the ocean as Greenland's ice sheet melts.
"The research makes a convincing case that the AMOC is approaching a tipping point based on a robust, physically-based early warning indicator," says University of Exeter climate scientist Tim Lenton, who wasn't involved in the research.
"What it cannot (and does not) say is how close the tipping point is because it shows that there is insufficient data to make a statistically reliable estimate of that."
Lead author of the study, René van Westen also says there isn't currently enough data to say anything definitive about a potential future AMOC collapse. More research is needed to work out a timeframe - including models that incorporate increasing levels of carbon dioxide and global warming.
"We can only say that we're heading towards the tipping point and that AMOC tipping is possible."
Some of the changes seen in the model before the collapse do, however, correspond with changes we've seen in the Atlantic Ocean in recent decades.
"When the AMOC loses stability, as we know from the available reconstructions, it is more likely that abrupt transitions may develop in the future," van Westen adds.
Lenton adds that we have to "hope for the best but prepare for the worst" by investing in more research to improve the estimate of how close a tipping point is, assess the potential impacts and work out how we can manage and adapt to those impacts.
What would a collapse of the ocean current mean for Europe?
If the AMOC collapses, previous research has shown the resulting climate impacts would be nearly irreversible in human timescales. It would mean severe global climate repercussions, with Europe bearing the brunt of the consequences.
Some parts of Europe could see temperatures plunge by up to 30C. On average, the model shows London cooling by 10C and Bergen by 15C.
The report's authors say that "no realistic adaptation measures can deal with such rapid temperature changes".
Temperatures in the southern hemisphere would rise with wet and dry seasons in the Amazon rainforest flipping.
Van Westen also says it could mean less rainfall and a sea level rise of up to one metre in coastal areas of Europe.
"The overall picture that AMOC collapse would be catastrophic fits with my own group's recent work showing that it would likely cause a widespread food and water security crisis," Lenton adds.
