Some 450 million people in the world are now confronting water-shortage problems. That’s grim enough — but experts meeting this week in Stockholm to discuss water scarcity say the number could grow to 2.7 billion within 25 years. North Africa, the Middle East, Pakistan, and parts of India and China, as well as areas in southern Europe, are most hurt by current shortages. Parts of the U.S., however, aren’t far behind. Warmer temperatures, the loss of wetlands to sprawl, and the growing demands of agriculture are accelerating shortages across the U.S. Major U.S. cities could go dry in 10 to 20 years.