The tropics have moved hundreds of miles towards the poles in less than 30 years because of global warming, researchers have concluded.

The expansion of tropics northwards and southwards threatens to have profound repercussions on the world’s weather systems as jet streams and storm tracks are bumped out of position. Research into the impact of global warming on the tropics suggests that they may have moved 500 miles (805km) or more in each direction and much faster than anticipated.

The findings were published on the eve of the United Nations climate-change conference starting in Bali today. Scientists, government officials and politicians from 192 countries are meeting to debate how global warming should be tackled.

Maps usually show the tropics as the regions between the Tropic of Cancer and the Tropic of Capricorn. Climatologists and geographers have less rigid definitions which take into account factors such as rainfall, temperature, wind direction and ozone.

The rate at which the tropics are expanding has taken scientists by surprise as it was found that they have already increased in size as much since 1979 as they had been forecast to shift over the next century.

“Some of the earliest unequivocal signs of climate change have been the warming of the air and ocean, thawing of land and melting of ice in the Arctic,” the research team from America said. “But recent studies are showing that the tropics are also changing.

“Several lines of evidence show that over the past few decades the tropical belt has expanded. This expansion has potentially important implications for subtropical societies and may lead to profound changes in the global climate system. Most importantly, pole-ward movement of large-scale atmospheric circulation systems, such as jet streams and storm tracks, could result in shifts in precipitation patterns affecting natural ecosystems, agriculture and water resources.”

Changes in the extent of the hot and wet tropics, measured as having moved up to 8 degrees northwards and southwards, mean dry subtropical conditions are shifted farther towards the poles. Arid areas of the Mediterranean are expected to dry out further as a result of the shift. Parts of southern America, southern Australia, southern Africa, Mexico and South America will suffer drier conditions.

Crop yields and the types of plants cultivated for food are likely to be affected, and human settlements and ecosystems face severe tests, researchers said in a report published in the journal Nature Geoscience.The research was led by Dian Seidel, of the National Oceanic and Atmospheric Administration in Maryland, with colleagues from the National Centre for Atmospheric Research in Colorado, and the Universities of Washington and Utah.

Professor Barry Brook, of the University of Adelaide, said: “The global implication is yet another signal that climate change is occurring sooner than expected.”