Washington, Aug 10 - A changing rainfall pattern may deprive a billion people of fresh water in the tropics and subtropics in the coming decades, according to the latest research.

The rain band near the equator that determines the supply of such fresh water has been creeping northwards for more than 300 years.

If the band continues to migrate at just less than a mile a year -- the average for all the years it has been moving north -- then some Pacific islands near the equator, even those that currently enjoy abundant rainfall, may be drier within decades and starved of fresh water.

The prospect of additional warming because of greenhouse gases means that the situation could happen even sooner.

The findings suggest 'that increasing greenhouse gases could potentially shift the primary band of precipitation in the tropics with profound implications for the societies and economies that depend on it.'

'We're talking about the most prominent rainfall feature on the planet, one that many people depend on as the source of their freshwater because there is no groundwater to speak of where they live,' said Julian Sachs, professor of oceanography at the University of Washington (UW) and lead author of the paper.