VIENNA (Reuters) - China says its one-child policy has helped the fight against global warming by avoiding 300 million births, the equivalent of the population of the United States.

But delegates at U.N. climate change talks in Vienna said on Thursday birth control is unlikely to find favor as a major policy theme, partly because of opposition by the Catholic Church and some developing nations trying to increase their population.

Some scientists say that birth control measures far less draconian than China’s are wrongly overlooked in the fight against climate change, when the world population is projected to soar to about 9 billion by 2050 from 6.6 billion now.

“Population is clearly an important factor,” said Yvo de Boer, head of the U.N. Climate Change Secretariat, at U.N. talks trying to plan a new deal to combat climate change after 2012.

China, which rejects criticism that it is doing too little to confront climate change, says that its population is now 1.3 billion against 1.6 billion if it had not imposed tough birth control measures in the late 1970s.

The number of births avoided equals the entire population of the United States. Beijing says that fewer people means less demand for energy and lower emissions of heat-trapping gases from burning fossil fuels.

“This is only an illustration of the actions we have taken,” said Su Wei, a senior Foreign Ministry official heading China’s delegation to the 158-nation talks from Aug 27-31.

He told Reuters that Beijing was not arguing that its policy was a model for others to follow in a global drive to avert ever more chaotic weather patterns, droughts, floods, erosion and rising ocean levels.

But avoiding 300 million births “means we averted 1.3 billion tonnes of carbon dioxide in 2005” based on average world per capital emissions of 4.2 tonnes, he said.

GERMANY

A country emitting 1.3 billion tonnes a year would rank just ahead of Germany on a global list of emitters behind only the United States, China, Russia, India and Japan.

Beijing introduced its one-child policy in the late 1970s. The rules vary across the country but usually limit families to one or, at most two, children.

“Population has not been taken seriously enough in the climate debate,” said Chris Rapley, incoming head of the Science Museum in London.

He favors a greater drive for education about family planning to avoid unwanted births and slow population growth.

But tougher birth control runs into opposition from the Roman Catholic Church, and from some developing nations which favor rising birth rates and have per capita emissions a fraction of those in rich nations.

Harlan Watson, the chief U.S. negotiator, said that high immigration to the United States makes it harder to slow its rising emissions.

“It’s simple arithmetic,” he said. “If you look at mid-century, Europe will be at 1990 levels of population while ours will be nearing 60 percent above 1990 levels. So population does matter,” he said.