Forget about the Gulf Stream: Britain is really kept warm in winter by the Rocky Mountains

Generations of schoolchildren have been raised on the belief that the mild British winters and cool summers are due to the moderating influence of the Gulf Stream, a warm ocean current flowing from the Gulf of Mexico to the shores of western Europe.

Without the Gulf Stream, our teachers told us, Britain's winters would be as cold and ice-bound as a frozen port in Newfoundland and its summers as hot and stuffy as a Moscow August.

But the text books have got it wrong, according to scientists who have just finished a study of what makes western Europe cool in summer and mild in winter.

The scientists found that Britain's moderate climate is due not to the Gulf Stream, but to the Rocky Mountains in the western US 4,000 miles away.

Using weather data gathered over the past 50 years and powerful computer models to describe how heat is shunted around the globe, they discovered that the contribution of the Gulf Stream was negligible compared with the influence of warm southerly winds originating in the Rockies.

These winds, they said, played a big role in explaining why winters in Britain could be anything up to 15C or 20C warmer than the same latitude in eastern North America. "Belief in the benign role of the Gulf Stream is so widespread that is has become folklore," said Richard Seager, the scientist who led the study from the Lamont Doherty Earth Observatory at Columbia University in New York.

"And I say folklore because the idea persists in the absence of any evidence, using modern data and scientific methods, to show that it is actually true.

"The textbooks are completely wrong because what the textbooks mainly do is to put the role of the Gulf Stream first of all. They leave the impression that it is the dominant impact and that's really wrong," Dr Seager said.

The belief that the Gulf Stream is responsible for Britain's mild, maritime climate appears to have originated with the publication in 1856 of a book by Maurice Fontaine Maury, a lieutenant in the American Navy.

"One of the benign offices of the Gulf Stream is to convey heat from the Gulf of Mexico, where otherwise it would become excessive, and to disperse it in regions beyond the Atlantic for the amelioration of the climates of the British Isles and of all Western Europe," Maury wrote.

To this day, the idea has persisted in texts ranging from the Hutchinson Dictionary of Science to the Oxford Encyclopaedia of Wine. Even scholarly scientific papers on climate change extol the importance of the Gulf Stream in keeping Britain and Western Europe from freezing in winter.

"This idea is one reason why so much climate research has been focused on the impact of changes in the circulation of the North Atlantic Ocean," Dr Seager said.

Several recent studies, for instance, have suggested that global warming might slow down or even stop the Gulf Stream – which carries energy equivalent to 27,000 times the total output of all of Britain's power stations – so bringing a far more variable continental climate to Western Europe.

Dr Seager's study, published in the current issue of the Quarterly Journal of the Royal Meteorological Society, suggests that the Gulf stream accounts for no more than 10 per cent of the winter temperature differences between Britain and Newfoundland, Canada.

The scientists found that the real reason for Britain's mild weather was twofold. First, there is a genuine maritime effect of being surrounded by a relatively warm body of water, but this has nothing to do with the Gulf Stream.

Second, this maritime influence is bolstered by south-westerly winds bringing a warm air mass from the south. These winds would not blow if the Rockies did not exist, the researchers found.

Even without the Gulf Stream, Britain would be bathed in prevailing westerly winds that bring in the warmth stored in the Atlantic ocean. Water retains summer heat far longer than land, which is why the winter-summer difference in temperature is about 5C over the North Atlantic and yet nearer 50C at the same latitude in Siberia.

Dr Seager said his study showed that this phenomenon – which was independent of the Gulf Stream – accounted for about half of the winter temperature difference between Britain and Newfoundland.

The other half, he said, was due to the prevailing winds over the maritime regions of western Europe not being due-westerlies, but from the south-west. Those south-westerlies brought additional heat to western Europe. Their origins could be traced to a massive "meander" in the north-south wind patterns over North America, which was generated by the presence of the Rockies.

"One such meander occurs east of the Rocky Mountains and brings cold air into eastern North America and warm air into Europe," Dr Seager said.

"This vast kink in the atmosphere circulation helps to explain the winter temperature contrast across the North Atlantic Ocean." When the scientists removed the Rockies from their computer models – in effect making the Western US flat – the temperature difference between Newfoundland and Britain was reduced by about 9C.

"About half of the total 15C temperature contrast between eastern North America and western Europe is caused by the Rocky Mountains while the other half is explained by the seasonal release of heat from the ocean," Dr Seager said. "While almost none is explained by the Gulf Stream."

If the Rockies did not exist, the winds over Britain would primarily be west to east but instead they tended to be even warmer south-westerlies.

"Thanks to the Rockies, they are not just coming from the west but they also have north-south components and wherever the winds come from the south they bring in warmer air.

"If people are worried about surprises in a greenhouse-warmed world then they should be more concerned about how atmospheric circulation patterns will be altered by greenhouse gases, not just ocean circulation patterns," Dr Seager said.

Hot air? 150 years of scientific 'fact'

The first documented account of the climatic influence of the Gulf Stream was published in 1856 in The Physical Geography of the Sea and its Meteorology. Its author, Maurice Fontaine Maury, right, an American naval lieutenant, wrote that without the Gulf Stream "the soft climates of both France and England would be as that of Labrador, severe in the extreme, and ice-bound". Even in recent scientific articles, the effect of the Gulf Stream is taken as fact. One such article on the influence of global warming on ocean circulation, published in Nature in 1997, said the north Atlantic received vast amounts of heat from the Gulf Stream and its extension, the North Atlantic Current. "This heat is released to the atmosphere and warms the winds that blow across Europe," it said.

The Gulf Stream originates in the Gulf of Mexico where it is a warm, vast, moving body of water, travelling north-east at a speed of about 3mph and heated to about 25C. The stream eventually widens and slows down to form the North Atlantic Current.