The Gulf Stream is showing signs of instability and is now the weakest it has been in the past 1,000 years.
The Atlantic Ocean current takes warm water from the tropics and delivers it to the northern hemisphere - giving Europe, including the UK, its temperate climate.
Essential, thanks to the Gulf Stream, also known as the Atlantic Meridional Overturning Circulation (AMOC), the UK is a lot warmer than other areas of similar latitude around the globe.
But now researchers from the Potsdam Institute for Climate Impact Research say the AMOC has reached a 'critical threshold'
The new study - published in Nature Climate Change - shows the AMOC may have been losing stability over the course of the last century due to climate change.
“The Atlantic Meridional Overturning really is one of our planet’s key circulation systems,” says the author of the study, Niklas Boers.
A potential collapse of the ocean current system would have severe consequences, given the spotlight by Hollywood disaster epic The Day After Tomorrow.
If the AMOC stopped, it is thought temperatures in Europe could fall by as much as eight degrees centigrade.
Mr Boers said: "The loss of dynamical stability would imply that the AMOC has approached its critical threshold, beyond which a substantial and in practice likely irreversible transition to the weak mode could occur.”
Long-term observational data of the strength of the AMOC does not exist, but the current leaves so-called fingerprints in sea-surface temperature and salinity patterns of the Atlantic ocean.
“A detailed analysis of these fingerprints in eight independent indices now suggests that the AMOC weakening during the last century is indeed likely to be associated with a loss of stability,” said Mr Boers.
”The findings support the assessment that the AMOC decline is not just a fluctuation or a linear response to increasing temperatures but likely means the approaching of a critical threshold beyond which the circulation system could collapse.”
A number of factors are likely important for the phenomenon – factors that add to the direct effect that the warming of the Atlantic ocean has on its circulation.
These include freshwater inflow from the melting of the Greenland ice sheet, melting sea-ice, increasing precipitation and river run-off. Freshwater is lighter than saltwater and reduces the tendency of the water to sink from the surface to greater depths, which is one of the drivers of the overturning.
“I wouldn’t have expected that the excessive amounts of freshwater added in the course of the last century would already produce such a response in the overturning circulation,” said Mr Boers.
“We urgently need to reconcile our models with the presented observational evidence to assess how far from or how close to its critical threshold the AMOC really is.”
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