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 to east, 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. …