Overpopulation

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A lot of Australia is either desert or at most semi-desert, if you were to over-impose a map of Australia over the Sahara desert, that would show you how much habitable land there is in Australia.

'Habitable' ≠ 'inhabitable'. You can build on desert as easily as fertile plains.

http://en.wikipedia.org/wiki/Dubai#Geography nice little jewel of the desert.

Anyway, that's not the point: we get more than half our food from overseas... and water probably not dissimilar. There is no reason you can't build a city anywhere (as long as you've figured out how to build there), since there's always a way or six to make it work

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If you listen to David Attenborough you'll get it. Sure technologies etc will help to put a sticking plaster on the problem. Fact is though there's a finite amount of people, and other animals that a finite amount of resources = the fresh water supply, can support. Currently that's 9bn. We have 40 years left to make that stretch. We should buy ourselves more time, but we will reach the limit some time.

40 years to find a way to drink seawater. Someone'll get it done eventually.

So nature has a limit where population cannot exceed it. <insert science fiction fantasy here>

Only a fantasy till it's done (ie: submarines). Jules verne comes to mind as science fantasies becoming a reality. I see no reason it can't be done (converting seawater into drinking water) given enough money, time, and effort by people qualified to try.