As the world’s population moves to cities (at the rate of a million new denizens a week), the shift is having some interesting effects on the world’s population totals. Stewart Brand noted that cities are population “sinks.” People move into town and the birth rate goes down to the replacement level and keeps on dropping.

This has some interesting implications. Depending on who you ask, world population is going to go up to at least 9 billion people. It’s after it hits that level, though, that things get interesting. Brand mentioned the possibility that if the birth rate goes just under the replacement level of 2.1 children per woman to 1.85, by the year 2300 the world population is somewhere between two and three billion. I’m assuming that he is citing this PDF from the UN. Even though 2300 is a long way off, check out page 3 of that report; by 2200 it’s already down to 3.2 billion.

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Talk about a jump in per capita GDP …

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