The poleward expansion of the tropics will have far-reaching impacts, notably in intensifying water scarcity in the Mediterranean and the US "Sun belt" as well as southern Africa and southern Australia, it warns.

The paper, appearing in a new journal, Nature Geoscience, is an overview of the latest published research into atmospheric systems at the tropics.

For cartographers, the tropical belt is defined quite simply by the tropics of Cancer and Capricorn, at latitudes 23.5 degrees north and south of the equator.

For climatologists, though, where the tropics end is fuzzier because of the complexity of a powerful high-altitude wind pattern, known as the Hadley circulation.

These powerful jet streams are what determine precipitation patterns of the tropics, which is characterised by lots of rain in the central part of the belt near the Equator and by dryness at its fringes.

Some years ago, the first credible computer simulations predicted that, as the Earth warmed, the Hadley jet streams and their associated wind and rainfall patterns would move poleward.

Under the most extreme scenario, the tropics were on average predicted to expand by about two degrees latitude, equivalent to around 200 kilometres (120 miles) over the 21st century.

The new paper looks at a batch of recent studies based on five different types of measurement from 1979-2005.

It concludes that this change in the tropical jet streams has already happened -- and the worst-case scenario has already been surpassed.

"Remarkably, the tropics appear to have already expanded -- during only the last few decades of the 20th century -- by at least the same margin as models predict for this century," it says.

"The observed widening appears to have occurred faster than climate models predict in their projections of anthropogenic [man-made] climate change."

The five datasets variously find expansion ranging from two degrees to 4.8 degrees latitude over the 25 years, or 200-480 kms (120-300 miles).

Many worrisome questions arise from these findings, says the paper.

First and foremost is about the accuracy of climate models that drive scientific conclusions about the pace of global warming and, in turn, inform policymakers about how to address the problem.

Also unclear are the mechanisms that have caused the widening of the tropics, says the paper. Possible factors include the ozone hole, warming of the sea's surface and an increase in the tropopause, a boundary layer between the troposphere and the stratosphere.

In any case, a widening of the tropics carries "worldwide implications," according to the study.

"The edges of the tropical belt are the outer boundaries of the sub-tropical dry zones, and their poleward shift could lead to fundamental shifts in ecosystems and in human settlements.

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

It voices particular concern for semi-arid regions that are at the fringes of the sub-tropical dry zone, including the Mediterranean, the southwestern United States and northern Mexico, southern Australia, southern Africa and parts of South America.

"A poleward expansion of the tropics is likely to bring even drier conditions to these heavily populated regions, but may bring increase moisture to others," it warns.

The paper is lead-authored by Dian Seidel of the US National Center for Atmospheric Research (NOAA) Air Resources Laboratory at Silver Spring, Maryland.

Publication on Sunday came on the eve of a UN conference in Bali, Indonesia aimed at revamping efforts to tackle global warming.

Around 10,000 delegates are expected to attend the 11-day gathering, which is tasked with agreeing on a blueprint for two years of negotiations for a pact that will slash greenhouse-gas emissions from 2012 and beef up support for poor countries that will bear the brunt of climate change.

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