I just added Empty Planet: The Shock of Global Population Decline by Darrell Brickner and John Ibbitson to the Recommended Reading Section.  Their well supported theory is that the global population will continue to rise for a short period of time, then level off and eventually decline.

If they are correct it will have a major impact on the world in terms of wars, the severity and therefore impact of climate change, famine, geo-politics, migration and the global economy.  The winners in this will be the countries that have the optimal immigration policies for their situation.