Climate models ‘underestimating’ tropical rainfall

Incidences of heavy rainfall are increasing with the warming climate — and at a rate higher than current climate models have predicted, researchers warn.

A study published today (8 August) in Science finds that the increase in rainfall is much larger than previously predicted, suggesting that current models may be inadequate in predicting the impact of climate change on rainfall patterns.

A team of UK and US researchers used satellite data from 1988–2004 to look at how changes in temperature and atmospheric moisture affect rainfall in the tropics.

They found that the frequency of heavy rainfall was strongly associated with increasing temperatures — results that agree with current climate predictions.

"We saw a distinct signal of the increased frequency of increased rainfall as the tropics warmed up and then a reduction as the tropics cooled down," says Richard Allan, a researcher at the Environmental Systems Science Centre in the University of Reading, United Kingdom, and co-author of the research.

Current climate models predict increases in heavy rainfall events, such as monsoons, in equatorial regions such as India and countries affected by the West African monsoon. This is matched by decreased rainfall in dry subtropical regions such as the Sahel region of Africa.

Increased rainfall occurs because the atmosphere can hold more water vapour as it warms, says Allan.

Some regions remain dry because the overall amount of rainfall doesn’t increase. "The average rainfall can’t keep up with the heavy rainfall so, to compensate, the regions away from the heavy rainfall will have to dry out slightly," Allan says.

He says the larger rainfall increases found in the new study might reflect limitations in current models. These may not pick up localised heavy rainfall events because their resolution isn’t high enough.

"It would be useful to develop high resolution versions of the models to see if they can capture some of these rainfall events."

Allan adds that the large-scale circulation of the atmosphere also shifts the regions of high and low rainfall around the globe.

"One of the key things is to be able to understand how the atmosphere is going to respond to warming — not just how much the wet regions will get wetter, but how those regions are going to move around."

A separate study published in the Proceedings of the National Academy of Sciences (5 August), found a link between warming in the Indian Ocean and reduced rainfall in eastern and southern Africa, predicting a 15 per cent decrease in rainfall every 20–25 years. 

The researchers suggest that human-caused warming has increased rainfall over the ocean, adding energy to the atmosphere and reducing the flow of moisture onshore. This brings dry air over the African continent, reducing rainfall.

Link to paper in Science [1193kB]

Link to paper in Proceedings of the National Academy of Sciences [920kB]