I just watched a podcast on the future of the Green movement featuring Stewart Brand and it had two fascinating insights.

First, there's a very good chance that by the year 2300, the Earth's population could be back at the same level as the 60's.  We've recently witnessed a doubling of the Earth's population, but this could be the last time this ever happens.

The chart above is from the UN Population in 2300 meeting that was help in late 2003.  The "Low" scenario you see reflects what happens when people move to cities.  Turns out that in cities, birth rates rapidly drop below the 2.1 children/woman needed for replacement.  We're seeing that already in Japan, Italy (1.2) and Spain (1.15) -incidentally, those are two of the lowest levels every seen.

Since people are rapidly moving to cities (this should be the year when the percentage of the world's population living in cities crosses 50%), we're seeing population growth drop precipitously even in developing countries.  Phillip Longman has written a book on this and there's a great article in Foreign Affairs.

Keep in mind that any prediction that looks out this far and is based on compounding will be highly sensitive to small changes in growth rates - but despite that, it's mind-boggling to imagine a world with a declining population.

Incidentally, all this city growth is happening primarily in slums - which leads to the next interesting point of the podcast.  It turns out that slum dwellers are incredibly environmentally sound.  They recycle just about everything and despite lack of running water, etc. they likely have a much lower environmental footprint than your average city.  Without realizing it, they are some of the "greenest" people around.