Climate scientists said on Wednesday that they had 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.
The introduction of heat-absorbing greenhouse gases leads to a narrowing of the usual temperature difference between different layers of the atmosphere — thus a more stable atmosphere that is less conducive to rain, said the report.
"For the same increase in temperature, solar heating will induce an overall higher level of rainfall than greenhouse gases," said a Nature press statement.
"Less rainfall (under the greenhouse-gas warming scenario) means on average increasing chances for droughts," said co-author Bin Wang of the University of Hawaii's International Pacific Research Center.
Solar radiation can be affected by factors like volcanic activity, the level of aerosols in the atmosphere and changes in the Earth's orbit around the Sun.
The study says the estimates for future rainfall are a global average, and do not apply to what is expected to happen locally.
Previous research has pointed to risks of regional flooding or water stress as climate change alters traditional winds and the amount of moisture they pick up at sea.