世界人口将超过70亿

October 31st 2011, that's the date the U.N. says the 7 billionth person will be born on Earth,7 billion.That's a crowded planet, says Dr. Jeffrey Sachs, the director of the Earth Institute at Columbia University and author of the Price of Civilization.

It took only 12 years to go from 6 billion to 7 billion and it's expected to take maybe another 14 years to go to 8 billion. So the trajectory is still rising quickly.

Sachs says all those people mean more demands for food, more stresses on the land, and more loss of water, but that's not the biggest problem.

The big problem is that in the poorest countries, families are still having six,seven,or eight children, that what's putting this tremendous growth of population continuing because in the high-income countries, fertility rates have come down to two children on average or even less.

This map shows you birth rates across the world. Multiple births above five are centered in Africa.Most lower birth rates are in developed countries. Dr. Sachs says rapid population growth in poor countries often creates conflict, and political stress at borders.

And on top of that, cultural values and available health care in developing countries creates a barrier to proper birth control. Dr. Sachs also wrote an in-depth article on the 7 billion mark, and what it could mean for the planet itself. You can find it at cnn.com/opinion.

Jim Clancy,cnn Atlanta.