CLIMATE change has contributed far more to high crop yields in the US over the past two decades than expected. This indicates that improvements in farming practices have had less effect than thought, so target yields could be harder to meet in the future.
Crop scientists know that cooler temperatures often lead to bumper crop yields and hotter weather can slash them. But constant improvements in fertilisers and crop varieties have improved yields even in regions where the local temperature has risen. This has largely masked the sensitivity of crops to temperature change. Previous studies have not even tried to separate out the role of temperature. 鈥淭hey assumed all the increases in yields were due to technology and management,鈥� says David Lobell, an ecologist at Stanford University in California.
To tease out the effect of climate on crop yields, Lobell and his colleague Gregory Asner analysed 17 years鈥� worth of climate and yield data from hundreds of corn (maize) and soybean-producing counties across the US.
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The regions had all benefited from similar advances in agricultural practices. But because they had experienced different changes in local climate, it was easy to identify the impact of climate on yields.
Lobell and Asner found that varying amounts of sunlight and rainfall had no noticeable effect on yields, but temperature had a far more significant influence than previously thought. They attributed around 20 per cent of the yield increases seen during the past two decades to cool temperatures in farming regions, rather than improved agricultural techniques.
This may spell bad news for the future. Many scientists expect crop-growing regions of the US which have remained cool in recent years to warm in the future. Lobell and Asner鈥檚 study shows that for every 1 掳C rise in temperature, yields can be expected to drop by 17 per cent. 鈥淲e used very simple statistics, but no one had done it before because until recently, it was difficult to get good climate data,鈥� says Asner.
鈥淚t鈥檚 a good wake-up call,鈥� says Cynthia Rosenzweig, who leads NASA鈥檚 Climate Impacts Group at Columbia University in New York. 鈥淭he common interpretation is that US agriculture is invulnerable.鈥�