Crude world birth rate:
I copied this out of Wikipedia.

http://en.wikipedia.org/wiki/Birth_rate downloaded July 7, 2010.  The numbers represent births per thousand of the population.  Some of the numbers are historical and some are the predicted values.  So what does it mean?  If at equilibrium the average person lives 70 years, then the number of deaths per thousand per year is about 14.3.  So if everybody were going to have children, that would also be the birth rate at equilibrium.  But generally in the rich world they say it takes about 2.1 babies per woman to have a stable population, which means 1.05 per person.  So we have to multiply our crude birth rate by 1.05 which would give us a break even number of about 15.

Of course this does not allow for how many people are young and how many are old, nor does it take into account that birth rates and replacement rates vary from place to place.  But it’s a clue.

So we expect to drop below 15 births per thousand people in the years 2030 to 2035 or about 20 years from now.  Usually the experts do not take into account the fact that fertility continues to decline across the board, but assume that people of a given age will have the same fertility in the future as they do now.  They continue to attribute declining fertility to increasing economic development, which or course is indirectly true; rich people can get around more and meet more potential mates than can poor people.  I suspect their estimate is optimistic.  I suspect the world crude birth rate will fall below replacement in about 15 to 20 years. 

The crude birth rate leaves much out.  But it is a landmark of sorts.

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