LAWTON, N.D.  Whatever Dennis Miller decides to plant this year on his 2,760-acre farm, the world needs. Wheat prices have doubled in the last six months. Corn is on a tear. Barley, sunflower seeds, canola and soybeans are all up sharply.

“For once, there’s great reason to be optimistic,” Mr. Miller said.

But the prices that have renewed Mr. Miller’s faith in farming are causing pain far and wide. A tailor in Lagos, Nigeria, named Abel Ojuku said recently that he had been forced to cut back on the bread he and his family love.

“If you wanted to buy three loaves, now you buy one,” Mr. Ojuku said.

Everywhere, the cost of food is rising sharply. Whether the world is in for a long period of continued increases has become one of the most urgent issues in economics.

Many factors are contributing to the rise, but the biggest is runaway demand. In recent years, the world’s developing countries have been growing about 7 percent a year, an unusually rapid rate by historical standards.