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More frequent hot weather and droughts have dealt a significant blow to crop yields, especially for key grains like wheat, barley, and maize, according to a Stanford published this week in Proceedings of the National Academy of Sciences.

The analysis finds that warming and air dryness—a key factor in crop stress—have surged in nearly every major agricultural region, with some areas experiencing growing seasons hotter than nearly any season 50 years ago. The study also pointed to two important ways that models have missed the mark in predicting impacts so far.

"There have been a lot of news stories about crop failures around the world, and often I get asked whether the impacts are happening faster than we expected," said study lead author David Lobell, the Gloria and Richard Kushel Director of Stanford's Center on Food Security and the Environment (FSE). "That motivated a closer look at what's been happening in farms around the world."

The study estimates that global yields of barley, maize, and wheat are 4% to 13% lower than they would have been without climate trends. In most cases, the losses have outweighed the benefits of increased carbon dioxide, which can improve plant growth and yield by boosting photosynthesis, among other mechanisms.

"In many ways, the changes farmers are experiencing are completely in line with what climate models predicted, so the overall impact should not be a surprise," said study co-author Stefania Di Tommaso, a research data analyst at FSE.

An unexpected twist: Climate models largely failed to predict the scale of drying in temperate zones like Europe and China. Observed increases in air dryness were far greater than projections had indicated for these regions. By contrast, U.S. farms, especially in the Midwest, experienced far less warming and drying than expected.

"These two big surprises are important to resolve," said Di Tommaso. "Of all the uncertainties in climate models, these are the two big ones that matter for global food production."

The authors note that model errors do not only matter for predicting impacts, but also for designing adaptations. Past efforts to extend growing seasons with longer-maturing crop varieties, for example, may have missed the mark because models didn't fully capture the drying trends that now threaten those very strategies.

The findings echo concerns raised in a study published in March that found U.S. agricultural productivity could slow dramatically in coming decades without major investment in climate adaptation. Taken together, the studies highlight a growing need for more accurate modeling and smarter adaptation strategies.

"Overall, I think climate science has done a remarkable job of anticipating global impacts on the main grains, and we should continue to rely on this science to guide policy decisions," said Lobell. "If anything, I think the blind spots have been on specialized crops where we don't have as much modeling, but which are very salient to consumers.

"That includes things like coffee, cocoa, oranges, and olives. All these have been seeing supply challenges and price increases. These matter less for food security but may be more eye-catching for consumers who might not otherwise care about climate change."

Lobell adds that the surprise many people express may simply be because they had been hoping the climate science was wrong, or because they underestimated the impact a 5% or 10% yield loss would have.

"I think when people hear 5%, they tend to think it's a small number. But then you live through it and see it's enough to shift markets. We're talking about enough food for hundreds of millions of people."