Today there are 7.3 billion people on planet Earth, according to the United Nations.

If you think that's a lot ... just wait.

A new U.N. report forecasts the biggest growth spurt in history. By the year 2030, the report predicts, Earth's population is expected to jump to 8.5 billion. And by the end of the century, the projection is 11.2 billion. That's about 6 percent higher than earlier forecasts.

Billions of people will be added in what are already the poorest parts of the world — 3 billion in Africa alone.

The other standout in this global population report is India — on track to surpass China as the most populous nation on earth in just seven years.

Why the population superboom?

"The reason is that fertility rates are not falling as fast as previously anticipated," says Robert Walker, president of the Population Institute.

Indeed, Africa has the highest birth rate in the world. The average woman bears 4.7 children compared to 2.5 globally. (In the U.S., by contrast, the rate is 1.9. In Europe, where population is expected to decline by 2100, the rate is 1.6. Japan is even lower: 1.4.)

The population growth in Africa is also being driven by improvements in health. Fewer kids are dying, AIDS is no longer a death sentence and Africans are living longer than ever before.

The burgeoning numbers pose huge challenges for already poor countries: more competition for jobs, water and land; heightened political friction; more food shortages.

The West African nation of Niger, for example, which has the world's highest fertility rate — 7.6 births per woman — is already a regular recipient of international food aid.

"If Niger is already having great difficulty feeding its population when it's 18 million, what's going to happen in 35 years when its pop is 68 million," Walker asks. The answer, he says, is that Niger "probably will not be able to feed itself."

In many of the countries with the highest birth rates, says John Bongaarts of the Population Council, governments could do a lot more to improve access to contraception.

"Government leaders much prefer to open new clinics or hospitals and airports rather than talk about sex and contraception," he says.

But Bongaarts points to two African nations that have greatly expanded access to family planning over the past decade: Ethiopia and Rwanda. In the 1980s Rwanda had the second highest birthrate in the world: 8 or 9 children per woman. That rate has now been reduced by half. Ethiopia has made birth control available through rural clinics, and reported rates of contraception use have risen from just 7 percent just 10 years ago to 30 percent today.

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