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For now, the only answer may be to tough things out for a while, waiting for the billions of people born during the great population booms to live out their long life, while at the same time continuing to reduce birthrates further so that things don't get thrown so far out of kilter again. But there's no telling if the earth--already worked to exhaustion feeding the 6 billion people currently here--can take much more. People in the richest countries consume a disproportionate share of the world's resources, and as poorer nations push to catch up, pressure on the planet will keep growing. "An ecologist looks at the population size relative to the carrying capacity of Earth," says Lester Brown, president of the Worldwatch Institute. "Looking at it that way, things are much worse than we expected them to be 20 years ago."
How much better they'll get will be decided in the next half-century (see chart). According to three scenarios published by the U.N., the global population in the year 2050 will be somewhere between 7.3 billion and 10.7 billion, depending on how fast the fertility rate falls. The difference between the high scenario and the low scenario? Just one child per couple. With the species poised on that kind of demographic knife edge, it pays for those couples to make their choices carefully. --Reported by William Dowell/New York, Meenakshi Ganguly/New Delhi and Dick Thompson/Washington