A groundbreaking United Nations study concludes that the earth's population will increase by three billion, to around nine billion, over the next 300 years. The figures are substantially lower than previous estimates, largely because of a projected decline in developing world fertility rates.

In announcing the results of the new study, U.N. Population Division chief Joseph Chamie cautioned that no one knows the future. Even small shifts in birth, death or migration patterns can have enormous effects on the size of the population.

Still, planners and many others need the best guesses about what the earth will be like hundreds of years from now. So Mr. Chamie and his staff set to work on three models - high, low, and medium scenarios.

The result was a high variant in which the population jumps to 36 billion, and a low variant of around two billion, if birth rates decline. Mr. Chamie, however, said a medium course of slow, steady population growth seems most likely.

"These projections are less than we've had in the past. So there's a bright side of the news. That the world's population that we anticipate according to our medium scenario, if you assume these assumptions of about a two child norm, which we see most people expressing, we'll end up with a world less than 10 billion by the 24th century, 2300," he says.

Mr. Chamie said among the most interesting findings is that Europe's population is shrinking fast. Fifty years ago, Europe made up 25 percent of the world's population. Today it is half that. He said European leaders are concerned at figures showing that the decline will likely continue far into the future.

"In many of the industrialized countries, reproduction, fertility levels are well below replacement. Some countries such as Japan, Italy, the Russian Federation, Germany, France, Spain have expressed concern about these low levels and what they mean in terms of a declining and aging population," he says.

Mr. Chamie predicted that China, where fertility dropped from six children per family to two in one generation, will be forced to abandon their one-child policy or face a rapidly declining population.

"I think they're moving away from a strict one child policy to a bit more flexibility. Because if China keeps its current level of 1.8 children, it would decrease from approximately today, 1.3 billion, it would go down to 400 million people, if it kept its current fertility of 1.8 children for 300 years, it would be more than halved," he says.

At the other end of the spectrum are countries such as Niger, Yemen and Pakistan, which are experiencing astronomical growth. Many of these rapidly-growing countries are Islamic, prompting some to wonder if there might be a Muslim population explosion. But Mr. Chamie expressed confidence that the natural laws of economic prosperity will act as a control.

"I'd like to dispel right now that Muslims don't have higher fertility because they're Muslims. Iran's fertility is two. Tunisia's fertility is two. And we see all over the world, Indonesia, Malaysia, it's coming down. The same forces that operate on a Christian, a Hindu, a Jew, are operating on Muslims," he says.

Mr. Chamie says the study revealed a few other interesting predictions. Today, for instance, the median age in the world is 26. Three hundred years from now, it will nearly double, to 48.

And the population of the United States, which has shot up from 76 million in 1900 to nearly 300 million today, will taper off somewhat. According to Mr. Chamie's prediction, the U.S. population in 2300 will be about 520 million.