Around one-fifth of the world's cereal crops – rice, in particular – are grown in India. Farmers depend on the monsoon to grow these crops. So any abnormalities in the monsoon can have drastic consequences for Indian agriculture, with significant knock-on effects for global food grain prices. Climate change is predicted to increase the total monsoon rainfall, but it is also expected to alter the daily variability of rainfall. So how will climate change affect Indian agriculture?

To answer this question, Ram Fishman from George Washington University, US, analysed daily rainfall and crop yield data from across India, gathered between 1970 and 2003. He used an illustrative climate change simulation to investigate the impacts of future rainfall variability on crop yield up to 2099.

The historical data clearly showed that crop yield is strongly linked to the number of rainy days. When heat exposure and total rainfall were kept fixed, Fishman found that every additional rainy day increased rice yields by around 0.83%. By contrast, when the number of rainy days was fixed but overall precipitation increased, he found that an addition 12 mm of precipitation (equivalent to one average rainy day) increased yields by just 0.13%. For crops, regular steady rain is better than a deluge interspersed with dry spells.

"Every farmer in India will tell you that variability, in the form of long dry spells, for example, means that plants don't have access to a steady supply of soil moisture and that yields can suffer as a result," said Fishman, "but until now, we didn’t have statistical proof or quantification of the extent of the damages from large scale data."

Across all crops, including cotton, groundnuts, maize, millet, pulse, sorghum, sugarcane and rice, the impact of an extra rainy day had a more significant impact on crop yield than an increase in overall rainfall. The findings are published in Environmental Research Letters (ERL).

Looking ahead, Fishman used the IPCC's AR4 projection for South Asia, which includes a 100 mm increase in rainfall, and as an illustration, a published projected decrease of 15 rainy days by 2050, to investigate the impact on crop yields. He found that these changes alone are likely to lead to an 11% loss in rice yields by 2050. Add to this the increased heat stress from rising temperatures and yields could really plummet. Similar results were obtained for most rainy-season crops.

Most of this climate change over the coming decades is now inevitable – a consequence of greenhouse gas emissions to date – and so farmers will have to look for ways of adapting to the new weather patterns. Irrigation is likely to become an important solution, along with collecting rainwater and storing it for use during dry periods. Breeding new crop varieties that are more tolerant to variability is also a possibility.

"In theory, one could overcome a lot of variability in this way, but this may not be easy to implement in practice, particularly since India is running short of water supplies for irrigation," said Fishman.

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