In a paper published Thursday in Science, demographers from several universities and the United Nations Population Division conclude that instead of leveling off in the second half of the 21st century, as the UN predicted less than a decade ago, the world's population will continue to grow beyond 2100. (Read "Population Seven Billion" in National Geographic magazine.)

And for the first time, through the use of a "probabilistic" statistical method, the Science paper establishes a range of uncertainty around its central estimate-9.6 billion Earthlings in 2050, 10.9 billion by 2100. There's an 80 percent chance, the authors conclude, that the actual number of people in 2100 will be somewhere between 9.6 and 12.3 billion.

That range "is the truly