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Global warming may worsen droughts in water-strapped parts of the world and lead to more rainfall elsewhere, said NASA.

Researchers at the US space agency compared historical records on how changes in the sun's output affected climate on earth with forecasts of how a warmer climate, driven by heat-trapping gases, will change rainfall patterns.

Rainfall could decrease further in areas such as the south-western US, Mexico, parts of North Africa, the Middle East and Australia, and it may increase across the western Pacific, along the equator and in Southeast Asia, the study said.

Much of the Mediterranean area, North Africa and the Middle East are rapidly becoming drier and "if the trend continues as expected, the consequences may be severe in only a couple of decades," lead author Drew Shindell said in a summary of the study on Monday.

The original research was published in the December 27 issue of the American Geophysical Union journal.

Shindell, a NASA climatologist, has publicly accused officials in President George W Bush's administration of trying to suppress global-warming findings by US government scientists.

After critics accused him for years of dragging his feet on the issue, Bush in January acknowledged climate change as a "serious challenge." He proposed steps to slash US petrol consumption and the country's dependence on imported oil, but has not endorsed mandatory caps on greenhouse-gas emissions.