Climate scientists said they found evidence to back predictions for a future with lower average rainfall, even though Earth's past warming episodes had led to more precipitation, not less.
Writing in the journal Nature, researchers said they had found proof that global warming caused by Man's greenhouse-gas emissions has a different effect on rainfall than warming caused by increased solar radiation.
Warming induced by carbon emissions is expected to accompany a rise in droughts in the future, they said.
This runs contrary to experience during the so-called Medieval Warm Period, from 1000 to 1250 AD, when Earth was hotter than today as a result of solar heating - but also wetter.
Scientists have long battled to understand the apparent contradiction.
Now they have shown that the two causes induce warming in different regions of the atmosphere, with different outcomes for rainfall formation.