While news reports and disaster movies remind us about tipping points for Arctic melt and sea level rise, some things closer to home get less attention. Take food supply: new modelling studies show that there are climate tipping points here too, beyond which crop yields will collapse.

Wolfram Schlenker at Columbia University, New York, and Michael Roberts at North Carolina State University in Raleigh used a high-resolution dataset of weather patterns from 1950 to 2005 to discover how yields of three key US crops would respond to increasing temperatures.

“The single best predictor of a year’s yield is the amount of time temperatures exceed about 29 °C and the extent to which they do so,” they say.

“Below this, warmer temperatures are beneficial for yields, but the damaging effects above 29 °C are staggeringly large.”


Maize massacred

Overall, the results suggest that yields of maize, cotton and soybean drop by roughly 0.6 per cent for each “degree-day” spent above 29 °C.

A degree-day is a measure devised by the team to indicate by how much 29 °C is exceeded and the time spent above that threshold. At present, agricultural regions across the US spend an average of 57 degree-days above 29 °C during the growing season.

That’s likely to rise as the world warms. Using a model of future climate change the researchers found that the number of degree-days above 29 °C in a growing season could rise to 413 by the end of the century if we do not cut greenhouse gas emissions. This would cause maize yields to fall by 82 per cent.

Even if we reduce emissions by 50 per cent by 2050 relative to 1991 levels – a target that governments are struggling to agree on – yields could still fall by between 30 and 46 per cent.

“These estimates of yield loss are amongst the best we have,” says David Bohan, head of the Ecosystem Dynamics and Biodiversity group at Rothamsted Research, UK.

States of hunger

The US is the world’s largest producer and exporter of crops, accounting for around 40 per cent of global maize and soybean production.

“If US yields go down a lot, it could drive up prices of staple food commodities all around the world”, say the researchers. “Almost surely the poor would suffer far more that the US would.”

David Pimentel at Cornell University in Ithaca, New York, argues that curbing biofuel production would tackle starvation and high food prices far quicker than curbing greenhouse gas emissions. “Some 66 per cent of the world population malnourished, yet the US turns 33 per cent of its corn crops into biofuels,” he says.