Cross-posted from Yale Environment 360. In a mere half-century, the number of people on the planet has soared from 3 billion to 7 billion, placing us squarely in the midst of the most rapid expansion of world population in our 50,000-year history -- and placing ever-growing pressure on the Earth and its resources. But that is the past. What of the future? Leading demographers, including those at the United Nations and the U.S. Census Bureau, are projecting that world population will peak at 9.5 billion to 10 billion later this century, and then gradually decline as poorer countries develop. But …