Calculating the risk of slowing crop yields


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According to a new study co-authored by Stanford professor David Lobell, the chance of a worldwide slowdown in agricultural yield growth in the next two decades is significantly higher due to global warming.

Lobell and co-author Claudia Tebaldi, a senior researcher at the National Center for Atmospheric Research, set out to estimate the odds of a steep drop in global wheat and corn yield progress under several climate scenarios. The study, “Getting caught with our plants down: the risks of a global crop yield slowdown from climate trends in the next two decades” appeared in Environmental Research Letters.

Lobell said he was motivated to pursue the study based on questions posed by stakeholders and decision makers in governments and the private sector.

“I’m often asked whether climate change will threaten food supply, as if it’s a simple yes or no answer,” Lobell said. “The truth is that over a 10 or 20 year period, it depends largely on how fast the Earth warms, and we can’t predict that very precisely. So the best we can do is try to determine the odds.”

Lobell and Tebaldi calculated the chance of a 10 percent global yield loss from climate change over the next 20 years, which would represent a severe impact on food supply, enough to roughly halve the rate of yield growth.

The short time frame of the study was deliberate, Lobell said. “Many studies have looked at climate and agriculture trends over the coming 50 or 100 years. But the next two decades are when most of the global population growth, and dietary shifts driven by a growing middle class, will occur. The growth rate of food demand will be higher during this time than at any other time in the next century.”

Without human-induced global warming – in other words, in a world with only natural climate variability – the likelihood of a yield drop that large is only 1 in 200. But when the team accounted for global warming, they saw the odds jump to 1 in 10 for corn and 1 in 20 for wheat. “In this study, we did not try to estimate the most likely impacts of climate change on crops,” Lobell said. “Rather, we estimated the likelihood of a really major impact, not because we want to scare people, but because there are many people who want to be prepared for all contingencies.”

“The point of the paper is to move from hand-waving about scenarios of what could go wrong, to specific and transparent estimates of the actual odds,” Lobell said. “The odds are not very high, but they are significant and a lot bigger than they used to be. The people asking these questions are accustomed to planning for scenarios with much less than a 10 percent chance of happening, so it will be interesting to see whether this study has any effect on how they operate.”

Lobell adds that organizations working toward global food security, and related issues such as conflict prevention, are most interested in the next 20 years because their decisions rarely consider the more distant future.  “As scientists, we might prefer to work on time scales in which the answers are clearer, but we also want to be responsive to the actual concerns and questions that decision makers have.”

Lobell is associate professor of Environmental Earth System Science at Stanford and associate director of the Center on Food Security and the Environment. He is also a senior fellow at the Stanford Woods Institute for the Environment and the Freeman Spogli Institute for International Studies.


David Lobell:

Laura Seaman, Communications and External Relations Manager, Center on Food Security and the Environment: