Pollution-laden clouds may be partly to blame for India's dwindling rice harvests, according to research. A US team found brown clouds, which cloak much of South Asia, have a negative impact on rice output by reducing sunlight and rainfall. They discovered elevated levels of greenhouse gases also reduced yields.

The study, reported in Proceedings of the National Academy of Sciences, came a day after researchers said new crops adapted to a warmer climate are needed. Since the 1980s, India has faced ever-declining harvests of its staple food, raising concerns that shortages could occur.

To investigate the cause, researchers looked at the impact of the "brown clouds" or "Asian haze" which cover the region. South Asia has one of the most widespread atmospheric brown clouds on the planet.

Blocking the rays
These layers of air pollution, which contain soot and other fine particles, are primarily created from burning fossil fuels and other organic matter. The clouds interfere with the local climate by blocking the Sun's radiation from reaching the ground, leading to cooler and dimmer conditions. Recent research has revealed the polluted haze can also reduce rainfall.

Using climate models and historical data on Indian rice harvests, the team built up a picture of the brown clouds' effect on rice growth over the years. "We found if there had been no atmospheric brown clouds between 1985 and 1998, the annual rice harvest yield would have been 11% higher than it was," said Maximilian Auffhammer of the University of California at Berkeley.

The team concluded the clouds had a negative effect on rice yields. He said while the cooler night-time temperatures caused by the clouds were beneficial for the rice, the negative impact of the decreasing rainfall outweighed these benefits.

Useful levers
Yields would also have been higher under lower concentrations of greenhouse gases, the researchers found. Many researchers, Dr Auffhammer told the BBC News website, had been worried that reducing brown clouds could boost temperatures and so further diminish rice yields.

This was because previous research found the clouds' cooling properties can mask the heating effect of greenhouse gases. But, he said, the team's findings revealed that reducing clouds alone or with reductions in greenhouse gases would benefit rice output. He added the study only looked into farming regions which used rain to water their crops. He said the effect would be less pronounced on areas which relied on other irrigation methods.

On Monday, the Consultative Group on International Agricultural Research (CGIAR), the global network of research centres devoted to farming in developing countries, warned of an urgent need to develop new strains of staple crops, including rice, which are tailored to the changed conditions of the future. The researchers said they now plan to look at other countries with polluted atmospheres, such as China and Indonesia.

Dr Auffhammer said: "I think this research is crucial because it gives policymakers a lever to increase rice output. "This study shows that decreasing the levels of these particulates may boost agricultural output."
(BBC, 05/12/2006)
http://news.bbc.co.uk/go/pr/fr/-/2/hi/science/nature/6206766.stm