One of the most pressing problems is sustainably arable land. The most productive farmland in the world has been progressively losing topsoil because our modern mechanized agriculture methods are wasteful. This topsoil was deposited over millions of years by wild grasslands, then eroded away over 150 years. And as the world's population grows, much of the remaining potentially productive farmland each year gets paved over by exurbs. http://world.time.com/2012/12/14/what-if-the-worlds-soil-run...