“Some people still regret that; some applaud it,” said Joel E. Cohen, who heads the Laboratory of Populations at Rockefeller University in New York. “I’m not sure we need demographic goals but we need forward thinking.”

Well, concerns about population seem to be creeping back. As the threat of climate change has evolved from a fuzzy faraway concept to one of the central existential threats to humanity, scholars like Professor Cohen have noted that reducing the burning of fossil fuels might be easier if there were fewer of us consuming them.

“Population wouldn’t be the whole story but it could make a big difference,” Mr. Cohen said.

An article published in 2010 by researchers from the United States, Germany and Austria concluded that if the world’s population reached only 7.5 billion people by midcentury, rather than more than nine billion, in 2050 we would be spewing five billion to nine billion fewer tons of carbon dioxide into the air.

This alone would deliver 16 to 29 percent of the emission reductions needed over the next four decades to keep the global temperature from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit) above that of the late 19th century, the threshold scientists predict could lead to severe disruptions to the climate.

Slower population growth could bring other benefits. The World Resources Institute has been looking into how the world will feed itself in 2050 without busting the carbon budget.

On current demographic and economic projections, food production would have to increase 70 percent by 2050. “Population growth is responsible for about one-half of increased food consumption,” said Tim Searchinger of the World Resources Institute. “The other half comes from higher incomes and richer diets.”

Much of the expected population growth is set in stone, but sub-Saharan Africa, expected to add 1.2 billion people by 2050 on top of its current 900 million, is an exception.

If fertility in sub-Saharan Africa slowed more rapidly than projected — declining to 2.1 children per woman in 2050 from 5.4 today — feeding the most undernourished region in the world would be a lot easier. And sparing African forests and woodlands from even greater deforestation would substantially reduce the amount of carbon entering the atmosphere.

For all the benefits of slower population growth, population policies remain a highly touchy subject.

In the 1970s and 1980s, rich nations’ support for population control in poor countries smacked of just another form of colonialism. Coercive population control — like India’s forced sterilizations, which were abandoned after they led to the collapse of Indira Gandhi’s government in 1977, or China’s one-child policy, which remains in place — is now widely considered a blatant violation of human rights.

Even China’s one-child policy is undergoing re-examination in Beijing because of the skewing of the country’s sex ratio — countless pregnancies have been aborted and millions of girls have been killed or left to die by parents who had hoped for a boy — and the tearing of the traditional safety net from so many elderly Chinese being forced to rely on only one child for support. Economists at the International Monetary Fund have even welcomed Africa’s fast-rising population as an opportunity to increase its pace of economic growth.

Population growth is only one factor — and not necessarily the most important one — contributing to global climate change. Over the course of the 20th century, emissions of carbon dioxide grew 180 percent faster than the population in poor countries and 60 percent faster than the population in rich ones. Shifting the world economy into more sustainable energy sources and away from fossil fuels is still the most promising strategy.

“There is a strong case to be made that the world faces sustainability issues whether it has nine billion people, seven billion people or four billion people,” said John Wilmoth, who directs the United Nations Population Division. “Nobody can deny that population growth is a major driving factor, but in terms of the policy response, what are you going to do?”

Yet there are ways to make a difference on the population front that do not depend on coercive governments straying into people’s bedrooms.

Access to education is critical. Across human history, fertility rates have fallen when it has made economic sense for families to have fewer children. Education — especially of girls — has played a powerful role in expediting the decline.

Across much of the developing world, more educated women have fewer children, and their offspring are more likely to survive. The spread of public education was accompanied by plummeting fertility rates in such disparate places as Brazil and Iran.

The other obvious tool is access to reproductive health. In the developing world, 222 million women have an unmet need for modern contraception, according to one study. Providing them with it, at a relatively small cost of $4 billion a year, could prevent 54 million unintended pregnancies.

These are hardly new ideas. The U.N. population conference in Cairo 20 years ago suggested pretty much this approach.

But we are not there yet. Out of every 1,000 children born in the Democratic Republic of Congo, for example, 99 die before the age of 5. In Nigeria, it is 123. A third of the girls in Mali are not enrolled in elementary school. Neither are 60 percent of Liberian girls.

By contrast, 95 percent of Guatemalan girls are enrolled in elementary school, as are 97 percent of Cambodian girls. In Bangladesh and Bolivia, among the poorest countries outside Africa, only about 40 of every 1,000 children die before they reach 5 years old.

During the General Assembly next month, the United Nations plans to hold a meeting to mark the anniversary of the Cairo conference. And it is organizing another to discuss new commitments to mitigate climate change. Perhaps delegates will notice the connection.