Thanks Bar Sinister, this from the National Post link,
"At one extreme, if the world’s fertility rate were to fall to the same level as that of Shanghai, at around 0.8 per couple, then by the early 22nd century population would be falling so fast that it would be under a billion by 2150.
If it were closer to the European Union average of 1.5 then population would fall below 5 billion around 2140 and 3 billion by 2200. In contrast, maintaining the current rate of 2.5 would see it top 15 billion by 2100."
was a real surprise. I didn't think that the example of Shanghai would be as low as it is. There is a lot of "ifs" in an estimate of future population growth. I don't think it would take people long, if conditions were good, to move back up to a sustainable population and beyond. Having a policy now to curb growth until we get to that point would be a really good idea because that 15 Billion figure is going to really hurt the chances of getting close to anything like sustainability.
If filling in the blanks works, at a 2 child per couple replacement rate, we could still end up with 10 Billions around the turn of the century. I think if that happens, life and all the crutches we use to make it easier, is going to get very expensive.