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Last Updated: Monday, 23 July 2007, 19:49 GMT 20:49 UK
Humans 'affect global rainfall'
Changes in rainfall over land areas (1925-1999)

Human-induced climate change has affected global rainfall patterns over the 20th Century, a study suggests.

Researchers said changes to the climate had led to an increase in annual average rainfall in the mid-latitudes of the Northern Hemisphere.

But while Canada, Russia and northern Europe had become wetter, India and parts of Africa had become drier, the team of scientists added.

The findings will be published in the scientific journal Nature on Thursday.

Climate models have, for a number of years, suggested that human activity has led to changes to the distribution of rain and snow across the globe.

However, the computer models have been unable to pinpoint the extent of our influence, partly because drying in some regions has cancelled out moistening in others.

Making the link

The scientists from Canada, Japan, the UK and US used the patterns of the changes in different latitude bands instead of the global average.

A lorry driving through floodwater (Getty Images)
While our study shows a human influence on rainfall at the global scale, the role of human influence in the UK flooding remains uncertain
Dr Nathan Gillett,
Research co-author
They compared monthly precipitation observations from 1925-1999 to those generated by complex computer models to see if they could identify if human activity was affecting rainfall patterns.

"We show that anthropogenic forcing has had a detectable influence on observed changes in average precipitation within latitudinal bands," the researchers wrote in their paper.

"These changes cannot be explained by internal climate variability or natural forcing."

The team estimated that human activity, such as burning fossil fuels, was likely to have led to a 62mm increase in the annual precipitation trend over the past century over land areas located 40-70 degrees North, which includes Canada, northern Europe and Russia.

They also suggested the increase of greenhouse gases and sulphate aerosols in the atmosphere had contributed a 82mm increase in the southern tropics and subtropics (0-30 degrees South), and a 98mm decrease in precipitation in the northern tropics (0-30 degrees North).

They added that natural factors, such as volcanic eruptions, had contributed to shifts in the global rainfall patterns but to a much lesser extent.

One of the paper's co-authors, Nathan Gillett of the Climatic Research Unit at the University of East Anglia, UK, said the team's findings helped clear up any uncertainty.

"This study shows that there has been a significant human effect on global rainfall patterns, with human influence causing a decrease in rainfall in some regions, and an increase in rainfall in others."

However, Dr Gillett said it was not possible to make a direct link between the recent floods in the UK and human-induced climate change.

"While our study shows a human influence on rainfall at the global scale, the role of human influence in the UK flooding remains uncertain.

"Climate models generally predict that the UK will become wetter in winter and drier in summer," he explained.

"In the UK we have seen a trend towards more extreme rainfall in the winter but no clear trend in summer extreme rainfall."

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