The Amu Darya and Syr Darya rivers of Central Asia flow across deserts to empty into the Aral Sea. Under Soviet rule, so much water was diverted from the rivers for agricultural purposes that salinity levels rapidly rose and the sea shrank. There was an upsurge in dust storms containing toxic salt residue, and a new desert began to replace the sea. At the same time, agricultural runoff rendered the drinking water unfit for human consumption. In...