Yet another warning
http://www.theguardian.com/environment/2013/jun/30/stephen-emmott-ten-billion

"If we discovered tomorrow that there was an asteroid on a collision course with Earth and – because physics is a fairly simple science – we were able to calculate that it was going to hit Earth on 3 June 2072, and we knew that its impact was going to wipe out 70% of all life on Earth, governments worldwide would marshal the entire planet into unprecedented action. Every scientist, engineer, university and business would be enlisted: half to find a way of stopping it, the other half to find a way for our species to survive and rebuild if the first option proved unsuccessful. We are in almost precisely that situation now, except that there isn't a specific date and there isn't an asteroid."

Then there would be a large number of people who didn't expect to be around in 2072 and didn't want to give up what they currently have in the mean time. There'd be the people who denied the asteroid existed. And then there would be the 5 Bn people who didn't even know about the asteroid and were mostly focussed on getting enough to eat and drink to survive another day.

Of course a world of 4B or 2B or 1B people in 100 years might well be a more pleasant place. But nobody will talk about the process of getting from the current 7B to the peak of 10B to a sustainable 1B. Because it ain't pretty.