In the years since, new scientific research has checked those assumptions.

For one, a group of young scientists has pioneered more sophisticated ways of analyzing the relationship between agriculture and climate. People like David Lobell at Stanford and Wolfram Schlenker at Columbia have used elaborate statistical techniques to get a detailed picture of what heat does to crop yields. Their work suggests that rising heat stress in some major growing areas is already putting a drag on production, and raises the possibility of much more serious effects as global warming continues.

Scientists had long hoped that the effect of heat and water stress on crops might be offset by the very thing driving global warming: the sharp increase of carbon dioxide in the air. The gas is the main food supply for plants, and a large body of evidence suggested that the ongoing rise could boost crop yields.

But a lot of that evidence came from tests in artificial environments like greenhouses. Younger scientists, who insisted on testing crops in open-air conditions more closely resembling the real world, found that the bump in yield, while certainly real, was not as high as expected. And it may not be high enough to offset other stresses from global warming.

None of this work can be called definitive — experts say we need more studies, in more types of crops, under a wider variety of growing conditions. Because the body of science is so incomplete, our forecasts of future food supply are primitive, and that means the Yokohama report will certainly not be the last word.

The scientists writing the intergovernmental panel’s report appear to have taken the recent science seriously. The draft suggests they intend to serve notice on world leaders that the risks could be substantial.