That can't go on forever, but worryingly, our current models of economic growth don't allow for any other way. Thanks, Thought Bubble. And then there is our astonishingly robust health. Although much of the world has been ravaged by HIV-AIDS for the past three decades, there's been a relative lack of global pandemics since the 1918 flu. And that's particularly surprising given increased population density and more travel between population centers. China has seen 150 million people leave the countryside for cities in the last 20 years. This was Shanghai in 1990, and this is Shanghai in 2010.