The world's tropical zones are growing, threatening to drive the world's great deserts into southern Europe and other heavily populated areas, alarming new research suggests.
The study - based on satellite measurements over the past quarter of a century - shows that the tropics have widened by 140 miles since 1979. Scientists suspect that global warming is to blame.
Up to now the most startling evidence that the world is heating up has come from the poles where ice sheets have disintegrated, sea ice shrunk, and glaciers started racing towards the sea. But new research published in the journal Science suggests that equally dramatic changes are under way in the hottest parts of the planet.
"It's a big deal," says Professor Thomas Reicher of the University of Utah, one of the authors of the study. "The movement has taken place over both hemispheres, indicating that the tropics have been widening. This may be a totally new aspect of climate change."
Professor Reicher and colleagues at the University of Washington and Lanzhou University in China found that the giant jet streams 30,000-50,000 feet up in the atmosphere have shifted towards the poles, in the first direct satellite evidence that global warming is affecting the worldwide circulation of air.
These vast rivers of air - often hundreds of miles wide - meander from west toeast, pushing weather across the globe and marking the boundary between tropical and temperate regions to both the north and south of the Equator.
The research found that the air currents have moved about one degree latitude - equivalent to 70 miles - towards the North and South Poles, making a total widening of 140 miles.
"The jet streams mark the edge of the tropics. So, if they are moving poleward, that means that the tropics are getting wider," says Professor John Wallace, of the University of Washington.
The famous lines on the atlas marking the Tropics of Cancer and Capricorn will remain at 23.5 degrees north and south, because these mark the limits of where the sun is directly overhead at some point during the year - the official measurement of the tropics. But the study suggests that they will become irrelevant as boundaries of the tropical climatic zones.
It shows that the areas just outside the tropics, at around 30 degrees north and south - running through China, North India, the Middle East, North Africa, Florida and the US Gulf Coast, and through Australia, Southern Africa and Argentina - are warming particularly fast.
The zones immediately outside the tropics are often very dry - containing many of the world's great deserts - and these are also expected to move towards the poles as part of the tropical shift.
The scientists believe that this may explain the recent droughts in southern Europe and the south-western United States. They say that if the process continues it could move the deserts into heavily populated areas, with devastating results.
They are unable to prove that the shift is being caused by global warming, though they believe it is a likely explanation; another possible factor is the depletion of the world's ozone layer.
But the evidence that global warming is causing more severe hurricanes grew stronger last week as the annual season for them opened.
Forecasters are predicting another torrid year with some 16 named tropical storms, 10 of them hurricanes. Four are expected to hit the United States. There is estimated to be a one in three chance that New Orleans will be hit again, and insurers as far north as New York are reluctant to provide cover for the storm damage.
Two new studies last week confirmed research which indicated that rising sea temperatures, caused by global warming, are increasing the strength of hurricanes. On Wednesday Jeb Bush - the Governor of Florida and the brother of the President - met some of the scientists who had conducted the research, saying that he found their information "compelling".