Economists Michael Roberts of North Carolina State University and Wolfram Schlenker of Columbia University in New York linked county-level crop yields for maize, cotton and soybeans to fine-scale weather data between 1950 and 2005 and found that when temperatures rose above 29°C, yields began to drop steeply. ‘While crop yields depend on a number of factors, extreme heat is the best predictor of yields,’ Roberts said.

Using a measure of the time that crops spent above 29°C – the degree-day – they calculated that agricultural regions across the USA currently experience an average of 57 degree-days above 29°C. However, under a ‘rapid-warming’ scenario for future climate change, that figure would rise to 413 by the end of the century, leading to an 82 per cent drop in maize yields.

Even under a ‘best case’ scenario, yields are likely to drop by 30–46 per cent, according to the study.

The authors point out that the USA produces 41 per cent of the world’s maize and 38 per cent of the world’s soybeans – two crops that are among the world’s four most important food sources – so their study has widespread implications. ‘The effects of climate change on US crop production will surely be felt around the globe, especially in developing countries,’ Roberts said.

October 2009