Conversation: Humanity is on course for a population greater than 11 billion by the end of this century, according to the latest analysis from the UNs population division. In a simple sense, population is the root cause of all sustainability issues. Clearly if there were no humans there would be no human impacts. Assuming you dont wish to see the complete end of the human race a desire that is shared by some deep green thinkers and Bond super-villians then the issue is whether there is an optimal number...