UNITED NATIONS — In a world filled with excess food, 20 million people are on the brink of famine, including 1.4 million children at imminent risk of death. In the face of such grim numbers, a stark question confronts the world’s most powerful: Why in 2017 can’t they avert such a seemingly archaic and preventable catastrophe?

Secretary General António Guterres of the United Nations raised the alarm Wednesday afternoon about the risk of famine in northern Nigeria, Somalia and Yemen. And this week, the United Nations declared famine in a patch of South Sudan.

“In our world of plenty there is no excuse for inaction or indifference,” Mr. Guterres said at a news conference, flanked by the heads of his aid agencies.

Each country facing famine is in war, or in the case of Somalia, recovering from decades of conflict.

What is famine?

Famine is a rare and specific state. It is declared after three specific criteria are met: when one in five households in a certain area face extreme food shortages; more than 30 percent of the population is acutely malnourished; and at least two people for every 10,000 die each day.

The chief economist for the World Food Program in Rome, Arif Husain, described it earlier this week this way: “When you declare a famine, bad things have already happened. People have already died.”

Famine was last declared in Somalia in July 2011, after an estimated 260,000 people had died, mostly in a two-month period.