For people who are looking for ways to reduce their "carbon footprint," here's one radical idea that could have a big long-term impact, some scientists say: Have fewer kids. A study by statisticians at Oregon State University concluded that in the United States, the carbon legacy and greenhouse gas impact of an extra child is almost 20 times more important than some of the other environment-friendly practices people might employ during their entire lives — things like driving a high mileage car, recycling, or using energy-efficient appliances and light bulbs. "In discussions about climate change, we tend to focus on the carbon emissions of an individual over his or her lifetime," said study team member Paul Murtaugh. "Those are important issues and it's essential that they should be considered. But an added challenge facing us is continuing population growth and increasing global consumption of resources." Reproductive choices haven't gained as much attention in the consideration of human impact to the Earth, Murtaugh said. When an individual produces a child – and that child potentially produces more descendants in the future — the effect on the environment can be many times the impact produced by a person during their lifetime. A child's impact Under current conditions in the United States, for instance, each child ultimately adds about 9,441 metric tons of carbon dioxide to the carbon legacy of an average parent – about 5.7 times the lifetime emissions for which, on average, a person is responsible. The impact doesn't only come through increased emissions of carbon dioxide and other greenhouse gases — larger populations also generate more waste and tax water supplies. Other offbeat environmental impacts have been in the news recently:

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The impact of having children differs between countries. While some developing nations have much higher populations and rates of population growth than the United States, their overall impact on the global carbon equation is often reduced by shorter life spans and less consumption. The long-term impact of a child born to a family in China is less than one-fifth the impact of a child born in the United States, the study found. However, as the developing world increases both its population and consumption levels, this equation may even out. "China and India right now are steadily increasing their carbon emissions and industrial development, and other developing nations may also continue to increase as they seek higher standards of living," Murtaugh said. Not advocating law The researchers note that they are not advocating government controls or intervention on population issues, but say they simply want to make people aware of the environmental consequences of their reproductive choices. "Many people are unaware of the power of exponential population growth," Murtaugh said. "Future growth amplifies the consequences of people's reproductive choices today, the same way that compound interest amplifies a bank balance." Murtaugh's findings are detailed in a 2009 issue of the journal Global Environmental Change.