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Editor's note: This story is part of a series of online exclusives about natural phenomena and human endeavors we'd like to see come to an end. They are connected with the September 2010 special issue of Scientific American called "The End".

Every day, about 350,000 people are born and 150,000 die. Run this loop for a few decades, and the United Nations projects that we're on track to increase global population by about one-third by 2050.

Most of that growth will happen in the poorest countries on Earth. Despite their poverty, those two billion people will add to the atmosphere at least three times the current greenhouse gas emissions of the U.S. This fact alone has given the efforts to slow population growth new urgency: the U.K.'s Optimum Population Trust has calculated that paying for contraception in the developing world is approximately four times more cost-effective per ton of greenhouse gases saved than to fund, for example, renewable energy projects.

Yet access to contraception will not solve the problem on its own. According to Werner Haug, director of the Technical Division of the United Nations Population Fund, the drivers of high fertility are economic and social inequality between the sexes, along with the lack of family planning coupled with high infant mortality, which drives people to have many children to ensure that enough will survive.

Solving these problems requires improvements in a host of circumstances, such as the availability of vaccinations, improved women's health, better access to education for women, and the pursuit of overall economic development. Prosperity, history has shown, is the key to shifting from the high birth and death rates common to the developing world to the low rates that accompany industrialization.