Fourteen of the world's twenty biggest cities are currently experiencing water scarcity or drought. Four billion people, it is estimated, already live in regions facing water shortages at least one month each year--that's about two-thirds of the planet's population. Half a billion are in places where the shortages never end. Today, at just one degree of warming, those regions with at least a month of water shortages each year include just about all of the United States west of Texas, where lakes and aquifers are being drained to meet demand, and stretching up into western Canada and down to Mexico City; almost all of North Africa and the Middle East; a large chunk of India; almost all of Australia; significant parts of Argentina and Chile; and everything in Africa south of Zambia.