Researchers from Japan and Canada found that less than a quarter of harvested areas saw an increase in yield variability while up to a third saw a decrease, with roughly one-fifth to one-half of the changes attributable to climate change.

Toshichika Iizumi at Japan’s National Institute for Agro-Environmental Sciences and Navin Ramankutty from the University of British Columbia in Canada believe that the main downward trend in variability can be put down to investments in irrigation facilities and crop varieties that are more tolerant of extreme conditions. And they recommend that these investments continue.

"Although the decreased yield variability is a main trend worldwide, this largely owes to the agricultural investments in the past," said Iizumi. "Therefore, ongoing adaptation efforts and related investments to agriculture are important to aid more robust food production systems in the future."

Yield variability is of interest because of the potentially damaging effects of enhanced climate extremes. Droughts in 2010 in Russia and in 2012 in the US both led to significantly reduced crop harvests, with the former issuing an export ban on wheat for nearly a year.

But little is known about how much changes in climate have affected variations in crop yield. On the one hand, scientists know that crops in recent decades have generally been exposed to more critical high temperatures during their reproductive growth periods. On the other hand, only a few crops in specific countries have had changes in yield variability definitively linked to changes in climate variability over the growing season.

Iizumi and Ramankutty attempted to paint a clearer picture by exploring how recent changes in daily temperature and rainfall have affected yield variability. "Our study is the first global analysis detecting and attributing historical changes in yield variability to climate change, including daily weather extremes," said Iizumi.

The pair began by calculating changes in yield variability within each geographical cell from a new, high-resolution global yield dataset covering the period 1981 to 2010. For each cell the researchers also calculated the "agro-climatic index", a parameter that accounts for extremes of temperature and soil moisture (although in this study they focused only on temperature).

Comparing the yield variability with the agro-climatic index revealed that 9–22% of harvested areas saw an increase in variability, while 19–33% saw a decrease. Of all changes in variability, 21–45% could be linked to the agro-climatic index.

"I am not disappointed at this relatively limited explanatory power of the climate because it is well-known that the climate is important but not the sole determinant of yield variability," said Iizumi. "I have no data to explain the remaining portion … but changes in agronomic technology, management, agricultural and environmental policies and others may contribute."

Aside from farmers continuing to invest in adaptation measures, Iizumi believes there is more work for researchers to do. "This study only examined the last three decades," he said. "Scientists need to monitor if the trend towards an increase in yield variability lasts and becomes more widespread, so that policy makers can take appropriate measures when necessary."

The study is published in Environmental Research Letters (ERL).

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