Major U.S. environmental groups have dodged the subject of population control for decades, wary of getting caught up in the bruising politics of reproductive health.

Yet, virtually alone, the Center for Biological Diversity is breaking the taboo by directly tying population growth to environmental problems through efforts like giving away condoms in colorful packages depicting endangered animals. The idea is to start a debate about how overpopulation crowds out species and hastens climate change — just when the world is welcoming Baby No. 7 Billion.

"Wrap with care, save the polar bear," reads one of the packages. "Wear a condom now, save the spotted owl," says another.

Kieran Suckling, executive director of the center, a membership-based nonprofit organization in Tucson, Ariz., said he had an aha moment a few years ago. "All the species that we save from extinction will eventually be gobbled up if the human population keeps growing," he said.

In the United States, the birth rate has fallen steadily since the baby boom, from 3.6 births per woman in 1960 to 2.0 today, or just under the replacement level, at which a population replaces itself from one generation to the next. Yet even at that rate, demographers estimate that the country will grow from 311 million people now to 478 million by the end of the century, because of both births and immigration.

The highest birth rates — from five to more than six births per woman — are occurring in a handful of nations in Africa and Asia, including Nigeria and Yemen.

Yet among large economies, the United States is second only to Australia in the amount of carbon dioxide it emits per capita, according to the latest figures from the federal Energy Information Administration.

"Every person you add to the country makes all these tremendous demands on the environment," said Joel E. Cohen, chief of the Laboratory of Populations at Rockefeller University and Columbia University.

But experts are reluctant to suggest an ideal birth rate. "There isn’t any magic number," Cohen said.

As recently as the 1970s, the subject of population control was less controversial, partly because the baby boom years had given rise to concerns about scarcity of resources, some population experts and environmentalists said. Then came China’s coercive one-child policy and a rise in social conservatism in the United States, combined withB the country’s aversion to anything perceived as restricting individual freedoms, be it the right to bear arms or children.

Some groups also fear whipping up anti-immigrant sentiment and opposition to family planning. Immigration now accounts for about one-third of the growth rate in the United States.

"We see reluctance and fear to deal with this issue," said Jose Miguel Guzman of the United Nations Population Fund.

Groups contacted for this article generally declined to discuss the issue or did not return calls.

The Center for Biological Diversity’s condom campaign, begun on college campuses last year, now includes video ads in Times Square and lobbying in Washington for more family planning services. It is an aggressive strategy even for the center, which is best known for barraging federal agencies with lawsuits intended to protect species and ecosystems.

The condom campaign is intended to raise awareness and help reduce unintended pregnancies. "Reproduction is always going to be a matter of free will," said Randy Serraglio, the manager of the campaign. "This is about getting people to make the connection."

A study published last year in the Proceedings of the National Academy of Sciences showed how slowing the country’s population growth rate to 1.5 births per woman from 2.0 could result in a 10 percent drop in greenhouse gas emissions by midcentury and a 33 percent drop by the end of the century.

But the notion that curbing births is an effective way to control emissions is not an easy sell.

When Oregon State University released a study two years ago calculating the extra carbon dioxide emissions a person helps generate by choosing to have children, the researchers received hate mail labeling them "eugenicists" and "Nazis."

The study, which also calculated the impact of a birth beyond the child’s lifetime "should the offspring reproduce," said that each American child generated seven times as much carbon dioxide over time as one child in China, and 169 times as much as one in Bangladesh. Reducing car travel, increasing recycling and making homes more energy efficient would have a fraction of the impact on emissions that reducing the birth rate would, it found.

"There are important consequences to having children, and we tried to quantify them," said Paul A. Murtaugh, an associate professor of statistics and one of the study’s co-authors. "It should be on the table. It needs to be."

Some groups, like the World Wildlife Fund and Conservation International, said theyB worked on population-related issues mostly internationally. The president of the National Audubon Society declined an interview without explanation. The chairwoman of the Green Group, a loose association of several dozen environmental organizations, did not return calls or emails.

The Natural Resources Defense Council president, Frances Beinecke, said her group focused on addressing climate change through energy strategies and conservation efforts. "Particularly in this economic environment, we’re not in a position to just add, add, add," Beinecke said of her group’s agenda.

Kevin Knobloch, president of the Union of Concerned Scientists, said the research on reducing emissions by cutting birth rates was not yet "robust" enough to make a convincing case for a clear way forward.

A country’s carbon footprint does not necessarily shrink when the birth rate drops, Knobloch said. In India and China, he pointed out, smaller families have consumed more as their incomes rose — a common trend in developing countries. "It gets complex very quickly," he said.

Carl Pope, the chairman of the Sierra Club, said his organization now had one population officer on staff who was working on international reproductive health services. In this country, Pope said, there are reasons for keeping a low profile on the issue.

"Look at Planned Parenthood," he said, recalling the group’s battle with Republican lawmakers over federal financing last spring. "There’s a huge atmosphere of intimidation. The moment you say ‘family planning,’ immediately somebody pulls out abortion."

The 2.0 fertility rate in the United States is higher than the rates in other developed countries, including Germany and Japan (1.3), Canada (1.6) and Britain (1.8), according to figures from the United Nations.

John Seager, president of the group Population Connection, said organizations had been more assertive about lobbying the Obama administration for money to finance family planning services overseas.

Unintended pregnancies account for roughly half of all annual births in the United States, according to studies by the Guttmacher Institute, which is based in New York and promotes reproductive health worldwide.

Bongaarts described the inaction by environmental groups as a missed opportunity. "The global warming community is staying away from anything having to do with population," he said, "and that’s frustrating."

© 2011 The New York Times Company