Climate change might actually cool down parts of Europe, scientists believe, due to a weakening of the Gulf Stream.
The warm tropical current from North America to Europe is one reason why Britain has milder winters than freezing Russia or Canada.
But the blast of warmth that has come for thousands of years could be weakened by changes in ocean winds, a new study warns.
In an extreme case, the whole system of Atlantic currents could collapse, causing temperatures in Europe to nosedive by 10 to 15°C.
Such an event would severely disrupt weather patterns and agriculture in the British Isles and western Europe.
The warnings emerge in a study by University College London researchers, who found the Gulf Stream was stronger in the past due to higher winds.
“If you see the reverse effect, so a weakening of winds in the future as a result of anthropogenic climate change, then that will likely trigger a weakening of the Gulf Stream,” lead author Jack Wharton said.
That might at least slow the trajectory of global warming, the UCL geographer told The National, or “if the Gulf Stream were to weaken very severely, then you would get much more significant cooling”.
Scientists have observed a decline in wind speeds over recent decades, known as 'global stilling', which could be caused by humans.
One theory is that cities act as speed bumps for the wind. Another is linked to the warming Arctic closing temperature gaps. A drop could affect our ability to generate clean energy from wind turbines.
A further concern is that the melting of Greenland's ice sheet messes with the warm Gulf Stream by diluting it with fresh water.
But the new study “is highlighting that there's an additional sensitivity of the Gulf Stream, and it's the wind-driven component”, Dr Wharton said.
“If winds were to weaken in the future, then the Gulf Stream would also weaken proportionately.”
Although much of Europe is usually cold or temperate, melting Alpine glaciers and extreme summer heatwaves have underlined the threat of climate change.
Europe has already warmed by 2.3°C compared to the pre-industrial age, faster than any other continent, meteorologists said last year.
Temperatures have for thousands of years been kept higher than they otherwise would due to the warm currents originating in the Gulf of Mexico.
That is one reason why Britain, for example, has more temperate weather than countries on the same latitude such as Russia.
According to the study, those currents were even stronger during the last Ice Age, about 20,000 years ago, due to more intense subtropical winds.
Oceanographers know this because Ice Age fossils on the seabed reveal how strong the current must have been back then.
The stream is part of a wider system of currents called the Amoc (Atlantic Meridional Overturning Circulation) that researchers say could collapse in an “unlikely but possible future scenario”.
Mark Maslin, a UCL geography professor and co-author of the study, said it was a paradox that a warmer climate could in fact “cool down much of Europe”.
“It’s not always recognised how much ocean currents are responsible for transferring heat around the planet and shaping our climate,” he said.
“Our new research adds to this understanding, and shows that the weakening of the winds which drive the Gulf Stream could reduce the circulation of heat, further affecting the continent.”
The study, Deeper and Stronger North Atlantic Gyre During the Last Glacial Maximum, is published in the journal Nature on Wednesday.
