One thing that happened on the road to doom was that the world figured out how to feed itself despite its rising numbers. No small measure of thanks belonged to Norman E. Borlaug, an American plant scientist whose breeding of high-yielding, disease-resistant crops led to the agricultural savior known as the Green Revolution. While shortages persisted in some regions, they were often more a function of government incompetence, corruption or civil strife than of an absolute lack of food.

Some preternaturally optimistic analysts concluded that humans would always find their way out of tough spots. Among them was Julian L. Simon, an economist who established himself as the anti-Ehrlich, arguing that “humanity’s condition will improve in just about every material way.” In 1997, a year before he died, Mr. Simon told Wired magazine that “whatever the rate of population growth is, historically it has been that the food supply increases at least as fast, if not faster.”

Somewhere on the spectrum between Dr. Ehrlich the doomsayer and Mr. Simon the doomslayer (as Wired called him) lies Fred Pearce, a British writer who specializes in global population. His concern is not that the world has too many people. In fact, birthrates are now below long-term replacement levels, or nearly so, across much of Earth, not just in the industrialized West and Japan but also in India, China, much of Southeast Asia, Latin America — just about everywhere except Africa, although even there the continentwide rates are declining. “Girls that are never born cannot have babies,” Mr. Pearce wrote in a 2010 book, “The Coming Population Crash and Our Planet’s Surprising Future” (Beacon Press).

Because of improved health standards, birthing many children is not the survival imperative for families that it once was. In cramped cities, large families are not the blessing they were in the agricultural past. And women in many societies are ever more independent, socially and economically; they no longer accept that their fate is to be endlessly pregnant. If anything, the worry in many countries is that their populations are aging and that national vitality is ebbing.

Still, enough people are already around to ensure that the world’s population will keep rising. But for how long? That is a devilishly difficult question. One frequently cited demographic model by the United Nations envisions a peak of about nine billion around 2050. Other forecasts are for continued growth into the next century. Still others say the population will begin to drop before the middle of this century. The trickiness of numbers is underscored by a look at population density. It is generally assumed that having too many people crammed into a small territory is a recipe for poverty and other social ills. Yet according to the United Nations, the three places with the highest density are Monaco, Macao and Singapore. Not one of them remotely qualifies as a desperate case.

In Mr. Pearce’s view, the villain is not overpopulation but, rather, overconsumption. “We can survive massive demographic change,” he said in 2011. But he is less sanguine about the overuse of available resources and its effects on climate change (although worries about the planet’s well-being could be a motivator for finding solutions, much as demographic fears may have helped defuse the population bomb).

“Rising consumption today far outstrips the rising head count as a threat to the planet,” Mr. Pearce wrote in Prospect, a British magazine, in 2010. “And most of the extra consumption has been in rich countries that have long since given up adding substantial numbers to their population, while most of the remaining population growth is in countries with a very small impact on the planet.”

“Let’s look at carbon dioxide emissions, the biggest current concern because of climate change,” he continued. “The world’s richest half billion people — that’s about 7 percent of the global population — are responsible for half of the world’s carbon dioxide emissions. Meanwhile, the poorest 50 percent of the population are responsible for just 7 percent of emissions.”

To some extent, worrying about an overcrowded planet has fallen off the international agenda. It is overshadowed, as Mr. Pearce suggests, by climate change and related concerns. The phrase “zero population growth,” once a movement battle cry, is not frequently heard these days; it has, for instance, appeared in only three articles in this newspaper over the last seven years.

But Dr. Ehrlich, now 83, is not retreating from his bleak prophesies. He would not echo everything that he once wrote, he says. But his intention back then was to raise awareness of a menacing situation, he says, and he accomplished that. He remains convinced that doom lurks around the corner, not some distant prospect for the year 2525 and beyond. What he wrote in the 1960s was comparatively mild, he suggested, telling Retro Report: “My language would be even more apocalyptic today.”

CLYDE HABERMAN, a regular contributor to Retro Report, has been a reporter, columnist and editorial writer for The New York Times, where he spent nearly 13 years based in Tokyo, Rome and Jerusalem. Subscribe to our newsletter here and follow us on Twitter @RetroReport.

This article first appeared in The New York Times.