Farmers at a wheat field in Bengal’s Burdwan

New Delhi, Nov. 3: Air pollution is a far bigger threat than climate change to wheat and rice and a key factor holding back India from achieving optimum yields of these crops, a new study has suggested.

The study released today by scientists at the University of California, San Diego, has found that India’s wheat yields during 2010 were on an average about 36 per cent lower than they would have been in the absence of air pollution and climate change.

Researchers Veerabhadran Ramanathan and Jennifer Burney at the UCSD have also shown that up to 90 per cent of the reduction in the wheat yields could be attributed to air pollutants, mainly soot particles, or black carbon and ozone.

Vehicular exhaust and the burning of firewood are among the primary sources of soot and ozone. Their findings appeared today in the journal Proceedings of the National Academy of Sciences.

Earlier studies have indicated that yields of several crops could drop in response to rising temperatures and accompanying rainfall changes and rising ozone levels.

But the new study is the first to segregate the two impacts to determine which is more significant for India’s wheat and rice production.

“The large magnitude of the impact of air pollution (on these crops) is a surprise,” said Jayaraman Srinivasan, professor of atmospheric and oceanic sciences at the Indian Institute of Science, Bangalore, who was not associated with the study.

“We’ve been largely focused on how climate change may influence crop yields, these results suggest that air pollution is a larger threat,” Srinivasan told The Telegraph.

Black carbon absorbs direct and diffuse sunlight and reduces light available to plants.

About a decade ago, botanist Madhoolika Agrawal at the Banaras Hindu University, Varanasi, had measured the concentrations of certain air pollutants in rural areas and found that the higher their levels, the lower the yields of at least three crops: beans, spinach and wheat.

In the new study, Ramanathan and Burney used a set of computer simulations and crop yield models to calculate the combined effects of climate change and air pollution on wheat and rice from 1980 to 2010.

India is the world’s second largest producer of rice and wheat and yields of both crops have steadily increased over the years. The modelling study shows that the yields could have been even higher in the absence of air pollution and climate change.

The study’s findings suggest that in Uttar Pradesh — India’s most populous state and the largest producer of wheat and rice, providing about 30 per cent of the country’s wheat and 14 per cent of rice — wheat yields were 50 per cent lower in 2010 than they would have been without air pollution and climate change.

Similar negative impacts with wheat yield losses ranging from 15 to 56 per cent also showed up in Bihar, Jharkhand, Madhya Pradesh, Rajasthan and Chhattisgarh.

Air pollution also appears to have negatively impacted rice production by more than 15 per cent in Uttar Pradesh, Bihar, Jharkhand and Bengal along the heavily polluted Gangetic plains, the researchers said.

“Crop yield increases could be a huge potential additional benefit from cleaning up air pollution that does not typically get taken into account in cost-benefit analyses,” Burney, assistant professor at the UCSD, said.

“And the impact of air pollution mitigation is almost immediate, so this kind of action could help mitigate some of the longer-term impacts of climate change on crops,” she said.

The study, however, observed no significant impacts on wheat yields in Haryana and Punjab from either air pollution or climate change. In Andhra Pradesh and Tamil Nadu, temperature and rainfall changes are likely to have a greater impact on rice yields than air pollution.

The scientists said the larger impact on wheat could be attributed to two factors.

The greatest build-up of air pollution over the Indian subcontinent coincides with the main growing season for wheat.

Wheat may be more sensitive than rice to ozone, another air pollutant.

Srinivasan cautioned that the results are based on models of crop yields and emissions. “These models must be validated against actual Indian data on both air pollution and crop yields for its results to be relevant.”

But Ramanathan and Burney say their findings suggest that reducing air pollution could be important for food security. The rise in yield from any reduction of air pollution, they said in their research report, could help offset the anticipated future expected yield losses from changes in temperature and rainfall.