Water scarcity is the next big climate crisis. Water stress – not just scarcity, but also water-quality issues caused by pollution – is already driving the first waves of climate refugees. Rivers are drying out before they meet the oceans, and ancient lakes are disappearing.

Fourteen of the world’s twenty megacities are now experiencing water scarcity or drought conditions. It’s increasingly clear that human mismanagement of water is dangerously unsustainable, for both ecological and human survival. And yet in recent years some key...