City Growth is Reducing Rainfall PDF Print E-mail
Contributed by mark eakle   
Monday, 11 June 2007

By Jim Giles – NewScientist.com news service

The extraordinary growth of China's cities is changing regional climate and reducing rainfall, say researchers.

The region around Hong Kong, known as the Pearl River Delta, is experiencing extraordinary urbanisation: in the nine years from 1988 to 1996, urban areas expand by 300%. And that growth is leaving its mark on the region's rainfall patterns.

 

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Pearl River Delta

Using a statistical technique adapted from economics, Robert Kaufmann at Boston University, US, and colleagues compared satellite imagery of urban growth with data from 16 local weather stations. After controlling for year-to-year fluctuations in weather, they found that urbanisation was having a statistically significant impact on rainfall around the region's cities.

Kaufmann is now running further tests to confirm that size and magnitude of the link, but says that it appears that urbanisation reduces rainfall during the dry season.

Speedy runoff

It is not the first time that cities have been seen to impact on weather. Temperatures are known to be higher in urban areas, for example.

But Kaufmann's work is different in that is suggests that cities may be cutting rainfall, says Marshall Shepherd, who researches urban climatology at the University of Georgia in Athens, US, and was not involved in the study. Most previous work has suggested that higher temperatures and other urban effects help force warm air upwards, generating clouds and extra rain.

Kaufmann says he is the first to look at how urbanisation and climate change together over time, a situation made possible by China's recent rapid growth. Other studies tended to compare rural and urban areas and may have missed this trend.

The decrease in rainfall may be because the loss of vegetation, and the fast rate at which water runs off city streets, reduces the transfer of water to the atmosphere, he adds.

Although urban areas currently take up just a few per cent of the world's surface, it is worth studying them now because they are growing so rapidly, says Kaufmann. By 2030, he notes, it is predicted that 60% of the world's people will live in cities.

Journal reference: Journal of Climate (vol 20, p. 2299)


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