USA Today: In the 1890s, New York City was swamped -- not by a storm but something smellier, horse manure. Horses, the primary mode of transportation, dropped more than a million pounds each day, causing a sanitation crisis. No one found a fix, and some estimated the streets would eventually be buried several feet deep. Then, "shift happened," says Harvard chemist Daniel Nocera. The automobile arrived, and almost overnight, it replaced horses and cleaned up the streets. Hailed as an environmental savior,...