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," Ms. 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, Mr. 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, Mr. Pope said, there are reasons for keeping a low profile on the issue.

"Look at Planned Parenthood," he said, recalling the group's bruising 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.

By tackling such pregnancies, the fertility rate could be brought down to about 1.9 births per woman, slightly below replacement level yet high enough to ease concerns about economic stagnation and support for the elderly, said John Bongaarts, a demographer with the Population Council, a research group in New York.

Dr. 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."