Humanitarian organizations that cater to the needs of the displaced find new challenges. For instance, said Sophie Delaunay, the executive director of the United States chapter of Doctors Without Borders, heightened insecurity had increasingly made it difficult for aid workers to reach the displaced in places like South Sudan and Syria. “Either they are stuck or NGOs can’t go because it’s too risky,” she said.

Other times, she added, there are remarkably few aid agencies that operate even in accessible places where huge numbers of displaced people congregate, like the airport in Bangui, the capital of Central African Republic.

The movements of refugees are a glimpse into the trouble spots of the world. In 1975, the agency counted just over 3.6 million refugees, with the largest number from Ethiopia. By 1992, there were nearly 18 million refugees worldwide, with over 4 million of them from Afghanistan alone. By 2004, the total number had dipped to about nine million, but by then refugees from Darfur had begun to flee Sudan.

The shifting flows of the displaced reflect also the changing pattern of war, which has gone from pitting countries against each other to warring factions vying for control within countries, often with guns and gunmen from abroad, as in the case of both Congo and Syria. “The nature of displacement is very different,” said Alexander Betts, a professor of refugee studies at Oxford, in a telephone interview. “The cases of displacement are very different, and the needs of the displaced population are very different.”

The total number of refugees also include the long-term displaced, like the five million Palestinians who were uprooted starting in 1948. Also among the 51 million are over a million asylum seekers, the largest numbers of them in Germany, the United States and South Africa.