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Warmer Earth will be drier model predicts

Fingerprinting climate A new study adds to evidence that future warming will produce lower average rainfall around the world, even though Earth's past warming episodes led to more precipitation.

A team of Chinese and US scientists modelled climate data over the last millennium. Their work may help resolve intense debate surrounding palaeoclimatic records and model projections of future greenhouse gas warming.

Writing in the journal Nature, the researchers say global warming caused by anthropogenic 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 say

"Less rainfall (under the greenhouse-gas warming scenario) means on average increasing chances for droughts," says study co-author Bin Wang of the University of Hawaii's International Pacific Research Center.

During the Medieval Warm Period from 100 to 1250 AD, the Earth received more solar radiation than it does today. But the late 20th century was not only warmer than the Medieval Warm Period, it was also drier.

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, say the researchers.

"For the same increase in temperature, solar heating will induce an overall higher level of rainfall than greenhouse gases," they say.

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.

Fingerprinting global warming

While the model doesn't give us a "concrete answer" about the climate's future, it does provide an explanation for why we see discrepancies between paleo-records and climate models, says Alex Sen Gupta, senior lecturer at the Climate Change Research Centre at the University of New South Wales.

For example, he says, paleo-records indicate the temperature gradient and the winds across the tropical pacific in pre-Industrial times were stronger, but climate models suggest that as temperatures increase the temperature difference across the Pacific and the winds should get weaker.

"This modelling argument explains why there's this discrepancy - it's because we have a different response of the climate system to solar compared to greenhouse gas forcing," says Sen Gupta, who was not involved in the research.

He says the research could also help "fingerprint" global warming in the future.

"You could say if rainfall is going up at this faster rate then we'd know it was due to solar forcing, if it's going up at this slower rate then we'd know it's going up because of greenhouse gases.

"The problem with that is that we just don't have enough observations to distinguish between the two," he says adding that we will need at least another 50 years of satellite data to back up this modelling.

"But what this study is showing is that there isn't really a discrepancy our records of the past and what our climate models are saying, they're just doing different things for different reasons."