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Air pollution and climate change both reduce Indian rice harvests

An ever-changing mix of anthropogenic pollutants alters the chemical and physical properties of the atmosphere and thereby causes potentially negative impacts on human society. To establish a robust cause-and-effect chain, all the way from a particular kind of emission to its economic and/or social impacts, remains a transdisciplinary tour de force with several risks of failure along the way. The first major link along such a chain, that between increased aerosol loads (“atmospheric brown clouds,” or ABC) over the Indian subcontinent, globally increasing greenhouse gas (GHG) concentrations, and regional changes in temperature, rainfall, and surface-near radiation, requires consideration of chemical and physical processes, ranging in scale from microscopic particles to atmospheric flows across the entire continent and its surrounding oceans. However, even if a changing environment near the ground can be attributed to anthropogenic emissions, it is still another matter to prove and quantify the second crucial link: to the existence of attributable impacts on society. For example, it may be shown that the changing atmosphere affects crop growth in some way, but does it also impact the farmer's livelihood in some way? Ever since it had been shown that aerosols block some incoming radiation, and therefore might reduce direct warming effects from GHG (1), it had been a common notion that ABC and GHG could have locally counteracting effects. For rice yields in India, an innovative study by Auffhammer et al. (2) in this issue of PNAS provides compelling evidence that ABC and GHG both have reduced historical rice harvests well below the levels to be expected otherwise. The study has profound implications for ongoing and future efforts to improve both climate and air quality.