The world population is expected to grow to over 9 billion by 2050, an exponential trend that has continued for several hundred years and see no end it site.  Megaregions as people flock to cities and industry will be commonplace.  The question is how will water supplies be impacted, or impact this trend.  Interestingly it varies everywhere.  For example, China and India are not expected to reap major benefits from climate changes, so their economies will grow as will populations.  They continue to construct coal fired power plants, and impact carbon dioxide and pollution levels, which does not help the climate issues.   Recall that Beijing was basically shut down for several days recent due to smog – seems like I recall the first air pollution regulations stemming from Henry the VIII decision to move the coal plants out of London during his reign 500 years ago because of pollution, but perhaps we need to relearn history J.  Of course China and India are expected to be less affected than the more historically developed countries in the northern latitudes that have been moving to renewable and less impactful power solutions with good reason.  Aside from these two economies, the rest of the northern latitudes are likely to see changes in temperature, variation in precipitation patterns and drought frequency changes.  That has major impacts for a billion people who will see water supply shortages occur much more often, and create a whole host of “winners” and “losers” in the water supply category.  Conflicts may result from the need to change increase water supplies as desperation kicks in.  Lawrence Smith, in his book 2050, suggests that while the far northern countries, the US, Russia, the Scandanavian countries, and Canada may see more land for agriculture and more water (at least in some areas), those warmer countries in the sub-Sahara, will become more desperate and dangerous to the world order.  Water will be the new oil, and the tipping point for sustainability, akin to peak oil, needs to be developed.  The cost will be significant, but the failure will be catastrophic to global economies.  This is part of why the global pursuit of renewable power, local solutions and green jobs.  It is why the definition of sustainable water supplies continues to evolve as we understand that the impacts, or the constraints of water supplies is far more reaching than most engineers and planners have traditionally dealt with.  AWWA published a Sustainable Water CD several years ago.  It was a series of papers of different aspects of sustainability as applied to water resources.  The last paper summarized the findings and compared it to the initial paper discussion.  The conclusion was the concept is evolving.  Climate, power, agriculture, natural systems, local economies, local economic contributions to regional and national economies and politics all impact pure science recommendations for water supply allocation.  The question is can we overcome the politics to create a optimized science solution to sustain water supplies and economies.  An old Native American proverb comes to mind:  We do not inherit the Earth from our grandparents, we borrow it from our grandchildren.