For decades, the world was often swimming in surplus food because farmers were so productive. But rising demand has caught up, and reserves have become so tight that global food markets are vulnerable to even minor shocks. Many analysts say that higher, more volatile prices may be here to stay.

The new dynamic reflects in part the rising demand for meat in developing countries such as China, which has almost single-handedly driven up prices for the soybeans it imports for animal meal, as well as the increasing use of corn for ethanol. Today, at least a third of the U.S. crop goes for making fuel. In addition, there is spreading concern that climate change may make weather less settled and more disruptive to growers.