“When the well is dry, we learn the worth of water,” observed Benjamin Franklin, one of America’s founding fathers, more than two centuries ago, long before anyone imagined the need to sustain water ecosystems or that fresh water could become scarce across the planet. Today, for the first time in human history, the “global well” is starting to go dry—and we are all about to learn the painful lessons of what happens when societies run short of history’s most indispensable resource.