WASHINGTON - SOME of the world's most populous areas - southern Europe, northern Africa, the western US and much of Latin America - could face severe, even unprecedented drought by 2100, researchers said on Tuesday.

Increasing drought has long been forecast as a consequence of climate change, but a new study from the US National Center for Atmospheric Research projects serious impact by the 2030s. Impacts by century's end could go beyond anything in the historical record, the study suggests.

To get an idea of how severe the drought might get, scientists use a measure called the Palmer Drought Severity Index, or PDSI. A positive score is wet, a negative score is dry and a score of zero is neither overly wet nor dry.

As an example, the most severe drought in recent history, in the Sahel region of western Africa in the 1970s, had a PDSI of -3 or -4.

By contrast, the new study indicates some areas with high populations could see drought in the -15 or -20 range by the end of the century.

'Historical PDSI for the last 60 years show a drying trend over southern Europe but nothing like those values at the end of this century,' study author Aiguo Dai said in answer to e-mailed questions. 'Decadal mean values of PDSI have not reached -15 to -20 levels in the past in any records over the world.'

Drought may hit more areas