The study by Jennifer Burney and V. Ramanathan, scientists at the University of California, project that a one degree centigrade rise in temperature could lead to a crop decline of four per cent for wheat and five per cent for rice. But losses from pollution could be greater.
“For context, the yield loss for wheat attributable to pollutants alone in 2010 corresponds to over 24 million tons of wheat: around four times India’s wheat imports before the 2007—2008 food price crisis and a value greater than $5 billion,” the authors write in a paper on the study published November in Proceedings of the National Academy of Sciences.
Most pollutants impact temperature by absorbing incoming radiation from the sun and reflected heat from the earth. Black carbon aerosols and ozone are of special concern as they affect crops directly — black carbon changes the amount of radiation reaching the surface while ozone is toxic to plants.
In 2010, wheat yields were 36 per cent lower and the models show that 90 per cent of that change was due to the pollutants. The impact was most drastic in the state of Uttaranchal and Uttar Pradesh. Wheat yields in Uttar Pradesh were 50 per cent lower than they would have been without the current climate and pollutant trends with two-thirds of the decrease attributable to pollutant levels.
In the case of rice, 15 per cent of yield decrease in the Gangetic plains could be attributed to pollutants. The Gangetic plains seem to accumulate surface level ozone and aerosols before the monsoons.
“Previous studies have shown that wheat is more sensitive to ozone than rice,” Burney tells SciDev.Net. “Also, the dry season has more pollutants.”
“I am pretty sure, based on other evidence, that yield declines due to pollution and warming are real, but I think that they are unlikely to be as large as the headline results in this paper,” says E Somanathan, professor at the Indian Statistical Institute, New Delhi. “Whether we believe the estimates of yield losses depends on whether we believe the regression model. Here, I am sceptical.”
The authors acknowledge limitations in the study, but insist that ozone and black carbon have had “significant impact on crop yields in India in recent decades”.
> Link to article in Proceedings of the National Academy of Sciences
This article has been produced by SciDev.Net's South Asia desk.