Scientists forecast the tropic belt would spread by about 2 degrees of latitude north and south of the Equator by the end of the 21st century. That amount of tropical expansion has already occurred, and was confirmed by five independent ways of measuring it, the report found.

For mapmakers and astronomers, there is no question about where the tropic zone ends: it is at 23.5 degrees north and south of the Equator at the Tropics of Capricorn and Cancer, boundaries determined by Earth's tilt on its axis. These tropical borders are the furthest point from the Equator where the sun shines directly overhead at the summer solstice.

But climate scientists define the tropic band by what happens on the land, in the water and in the air, and that is what is changing, the study said.

'SERIOUS HARDSHIPS'

Tropical temperatures are warm, and it rains a lot, with little seasonal or day-to-day change. The subtropics, by contrast, are generally dry. If the warm, wet tropical climate is spreading poleward, the dry subtropic climate may head for the poles too.

Those dry subtropical bands could include some of the most heavily populated places on Earth, the scientists said: the Mediterranean, the US Southwest, northern Mexico, southern Australia, southern Africa and parts of South America.

"Shifts in precipitation patterns would have obvious implications for agriculture and water resources and could present serious hardships in marginal areas," the authors wrote.

For those who live in the middle latitudes, like most US, European and Asian residents, the change could affect the storm tracks that largely determines weather in these areas, said co-author Dian Seidel of the US National Oceanic and Atmospheric Administration.

"Those storm tracks are linked with the position of the jet stream, which is one way we use to delineate the width of the tropics," Seidel said by telephone from NOAA's Air Resources Laboratory outside Washington. "The jet streams are moving poleward, and so, presumably, would the storm tracks."

This poleward migration of storm tracks is in line with predictions by the Intergovernmental Panel on Climate Change. This worldwide panel of scientists and policymakers is convening in Bali, Indonesia this week to determine how to deal with climate change.