LONDON (MCT) — There are plenty of ways to cut your carbon footprint, whether it's driving less or buying an energy-efficient refrigerator. But the British Medical Journal, in an editorial last month, urged a more controversial one: having fewer children.

With 60 million people already living in one of the most densely populated countries in the world, the journal said, British couples should aim to have no more than two children as part of their contribution to worldwide efforts to reduce carbon emissions, stem climate change and ease demands on the world's resources.

Limiting family size is "the simplest and biggest contribution anyone can make to leaving a habitable planet for our grandchildren," the editorial's authors said.

Family planning as a means to reduce climate change has been little talked about in international climate forums, largely because it is so politically sensitive. China's leaders, however, regularly argue that their country should get emission reduction credits because of their one-child policy, and many environmentalists — and even a growing number of religious and ethics scholars — say the biblical command to "be fruitful and multiply" needs to be balanced against Scripture calling for stewardship of the Earth.

Increasingly, "a casual attitude toward global warming ought to be viewed as a sin," argues James Nash, director of the Churches' Center for Theology and Public Policy, a Washington-based research group that studies the relationship between Christian faith and public policy.

The appeal to have fewer children sounds a bit odd in Europe, where one of the biggest worries these days is plunging birthrates. German women today bear an average of just 1.3 children, fewer than women in China, where the one-child policy is fast weakening. Even British women are giving birth to just 1.9 children on average, a level below that needed to produce a stable population.

In the U.S., a Census Bureau study released last week said that women are not only waiting longer to have children, those ages 40 to 44 who do have an average of 1.9 children each, more than one child less than women in that group did 30 years ago.

But each child born in a rich country like Britain or the United States is likely to be responsible for 160 times as much carbon emitted as a child born in Ethiopia, said John Guillebaud, a British family-planning doctor, university professor and one of the authors of the British Medical Journal editorial. With efforts to cut emissions likely to go only so far, cutting births may be the best option, he said.

"We're not Big Brother. We're not for pushing people," he insisted in an interview. "We just think deciding how big a family to have should take into consideration our descendants."

At the current projected rates of growth, the world's population, now at 6.7 billion, is expected to reach about 9 billion by 2050. Environmentalists argue that a population that large will dramatically overtax the world's resources and likely lead to growing conflict as well as potentially crippling climate change, particularly as poorer parts of the world develop and begin using more resources.

Most of the expected growth in population is projected to come in less-developed parts of the world, particularly Asia, where 60 percent of the world's people live today, and Africa, where birthrates are the highest in the world.

Worldwide, population growth has been declining in recent years and even in much of Asia and Africa "the drop in fertility rate has been quite amazing," said Werner Haug, director of the United Nations Population Fund's technical division. Despite falling international investment in family planning, Thailand today has a European-like birthrate; Kenyan women, who once averaged eight children, are now having five.

Overall, Asia's birthrate, excluding China, averages 2.8 children per woman, and Africa's 5.4, well down from the past, said Carl Haub of the Washington-based Population Reference Bureau, an independent organization that analyzes demographic data.

But because any birthrate above 2.1 children per couple — the approximate replacement level, allowing for some untimely deaths — will produce ever-expanding growth, even Asia is still set to "grow like wildfire," Haub said.

The problem is worst in places such as northern India, where literacy, education and access to birth control are poor and poverty levels and population numbers are already high. If those conditions continue, runaway growth could push India toward a population of 2 billion people, Haub said. Sub-Saharan Africa, at expected growth rates, is likely to nearly triple its population by 2050, also to about 2 billion people, he said.

Even in the United States, birthrates, which had fallen to around 1.85 children per non-Hispanic white woman, are now about 2.1 children per U.S. couple thanks to large-scale Hispanic migration. Hispanic women give birth to three children on average, Haub said, and the U.S. Census Bureau projects that by 2050, Hispanics will increase to 30 percent of the U.S. population, while non-Hispanic whites will drop to 46 percent.

In a nation where Texas' 23 million people account for more greenhouse gas emissions than all 720 million Sub-Saharan Africans, even small rates of U.S. population growth may have a disproportionate impact on global warming, said the U.N.'s Haug.

Freedom to choose family size is a cherished right in most places in the world, and trying to promote smaller families for the sake of the environment is fraught with peril. A variety of cultures and religions — from Hinduism to Catholicism, Mormonism to orthodox Judaism — either discourage birth control or promote large families or the birth of sons.

Governments in countries from Australia to Italy pay parents to have more babies in an effort to keep their economies on track and their retirement programs funded, though studies show such efforts have limited effectiveness.

Reducing family size can be most difficult in the places with the biggest need for it, experts say. In India, a botched government sterilization campaign in the 1970s has left family planning a largely taboo political subject. And in Africa, where a large family is still a point of pride for most men, women struggle to persuade their husbands to reduce births.

Just as important, in places where climate change hardly registers as a major concern and big families are the norm, "will women really see two children as an ideal? Or will aspects of the culture keep the birthrate above that?" Haub asked.

He and other experts say the best way to cut the world's birthrate is simply to push ahead with what has worked best in the past: education of both men and women, access to information about birth control options and to a variety of the options themselves, and better health care to give parents confidence that children born will survive to adulthood.

That simple recipe is hardly an easy one, Haub admits.

"In the long run it's the answer, but education takes an extremely long time," he said.

The good news is that in surveys by his group, many women in the fastest-growing areas of the world say their last pregnancy was not one they wanted. That suggests that if barriers to providing contraception can be overcome, women will use it.

"We have to make it possible for every couple who goes to bed tonight in the world to not have a baby if they don't want one," said Guillebaud, an environmentalist and doctor whose clinic has performed 40,000 vasectomies in Britain.