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Agronomist Roger Elmore suspected trouble in July, when Iowa’s cool summer nights didn’t get as cool as usual.

The evening heat, he knew, could mean a smaller corn harvest at a time when global food markets are so tight that anything less than a bumper U.S. yield can send prices higher.

The world now faces a crisis in food prices that has its roots in those warm Iowa evenings. Since last summer, several events – floods in Australia, blistering drought in Russia, the threat of a poor winter wheat crop in China – have compounded concerns about the food supply and pushed world prices to the highest levels measured since the U.N. Food and Agricultural Organization began calculating its index in 1990.

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.