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This is Scientific American's 60-Second Science. I'm Steve Mirsky. Got a minute?

"It took until about 1800 or 1825 to put the first billion people on the planet. We added the most recent billion in 12 or 13 years. We anticipate two billion more by 2050."

That's Joel Cohen, head of the Laboratory of Populations at Rockefeller University in New York. He spoke¹ February 20th at the annual meeting of the American Association for the Advancement² of Science in Washington, DC. So how many people can the earth hold?

"In the last half century, people have estimated human carrying capacities for the earth that have ranged from less than one billion to more than a trillion. They can't all be right. In fact, those numbers are political numbers, not scientific numbers. Because the question how many people can the earth support is an incomplete question, and doesn't take account of with what technologies, at what average level of well-being³, with what distribution of income, with what political and economic institutions."