The middle class in poor countries is the fastest-growing segment of the world’s population. While the total population of the planet will increase by about 1 billion people in the next 12 years, the ranks of the middle class will swell by as many as 1.8 billion. Of these new members of the middle class, 600 million will be in China. Homi Kharas, a researcher at the Brookings Institution, estimates that by 2020 the world’s middle class will grow to include a staggering 52 percent of the global population, up from 30 percent now. The middle class will almost double in the poor countries where sustained economic growth is lifting people above the poverty line fast. For example, by 2025, China will have the world’s largest middle class, while India’s will be 10 times larger than it is today.

While this is, of course, good news, it also means humanity will have to adjust to unprecedented pressures. The rise of a new global middle class is already having repercussions. Last January, 10,000 people took to the streets in Jakarta to protest skyrocketing soybean prices. And Indonesians were not the only people angry about the rising cost of food. In 2007, higher pasta prices sparked street protests in Milan. Mexicans marched against the price of tortillas. Senegalese protested the price of rice, and Indians took up banners against the price of onions. Many governments, including those in Argentina, China, Egypt, and Russia, have imposed controls on food prices in an attempt to contain a public backlash.

These protesters are the most vociferous manifestations of a global trend: We are all paying more for bread, milk, and chocolate, to name just a few items. The new consumers of the emerging global middle class are driving up food prices everywhere. The food-price index...