Atmospheric Brown Clouds Contribute to the Slowing of India’s "Green Revolution"

A pale band of haze covers northern India, just south of the Himalayas in this image, taken Feb. 5, 2006, by NASA's Aqua satellite. Haze also intrudes into the skies of southern Nepal and Bangladesh. A study by UC researchers suggests that reducing this type of air pollution could increase rice harvests in India.

(Image courtesy of Jeff Schmaltz, Moderate Resolution Imaging Spectroradiometer Land Rapid Response Team at the NASA Goddard Space Flight Center.)

Press release in pdf format.

New research shows that reductions of human-generated pollution would generate unexpected agricultural benefits in one of the world’s poorest regions. The new findings by Maximilian Auffhammer of the University of California, Berkeley’s College of Natural Resources, V. "Ram"Ramanathan, an atmospheric scientist at Scripps Institution of Oceanography, and Jeffrey Vincent, an economist and environmental research director at the UC Institute on Global Conflict and Cooperation (IGCC), in the Proceedings of the National Academy of Sciences (PNAS), December 26, 2006. Link to the paper.

Rice harvests in India, which grew dramatically during the "Green Revolution"of the 1960s and 1970s, have slowed since the mid-1980s, raising concerns that food shortages could recur in this densely populated and poor nation. Several explanations for the slowdown have been proposed, but until now, none took into account the complex interactions of two pollution-related sources of climate change: atmospheric brown clouds (ABCs), which form from soot and other fine particles in the air (collectively termed aerosols), and the better-known problem of global warming caused by greenhouse gases such as carbon dioxide.

Studies on the agricultural impacts of climate change have focused on projecting the future effects of greenhouse gases. In the PNAS paper, Auffhammer, Ramanathan, and Vincent analyze historical data on Indian rice harvests and examine the combined effects of atmospheric brown clouds and greenhouse gases. They find that the combined effects were negative and were greater after the mid-1980s than before, coinciding with the observed slowdown in harvest growth. They estimate that harvests would have been up to 20 to25 percent higher in some years during the 1990s if the negative climate impacts had not occurred.

Previous research by an international scientific team led by Ramanathan found that brown clouds have made the Indian subcontinent drier and cooler. Although this suggests the existence of a climatic tradeoff, with reductions in aerosols potentially unleashing a stronger warming trend, joint reductions in the two types of pollutants would in fact benefit Indian rice farmers. This is because reductions in aerosols would enhance rainfall, while reductions in greenhouse gases would reduce nighttime temperatures, which negatively affect growth of the rice plant.

"Greenhouse gases and aerosols in brown clouds are known to be competing factors in global warming,"said Ramanathan. "The major finding of this interdisciplinary study is that their effects on rice production are additive, which is clearly an unwelcome surprise."

According to Peter Timmer, senior fellow at the Center for Global Development, the study "links a sophisticated model of agricultural production in India to climate and pollution models, with the critical finding that ‘brown cloud’ pollution has already cost India millions of tons of food production."Vincent noted, "Air pollution control measures in India have been motivated mainly by concern about the health of residents of urban areas where most of the pollution is generated. Our study provides an additional motivation related to the economic health of poor rural areas.”

Auffhammer added that "while this study focuses on India's rain-fed states, ABCs exist throughout Asia’s main rice-producing countries, many of which have experienced decreasing growth rates in harvests too. Furthering our understanding of how air pollution affects agricultural yields is very important to ensure food security in the world’s most populous region.”

The paper is the result of a three-year collaboration among Auffhammer, Ramanathan, and Vincent. It was supported in part by the Giannini Foundation, the National Science Foundation, the National Oceanic and Atmospheric Administration, and IGCC.