Looking back over the twenty-first century, future historians may well see it as even more tumultuous than the century that preceded it. In the twentieth century the human population more than tripled, average life expectancy at birth more than doubled, and real gross domestic product increased nearly forty times. Huge numbers of people experienced very real improvements in living standards as economic growth spread around the world, yet huge numbers were living in extreme poverty when the century ended. Political systems from Nazism to Communism rose and fell, while democracy in its various forms survived. Well over 200 million people died in wars and conflicts, and nuclear energy was unleashed with the specific purpose of annihilating many tens of thousands in a flash. Seemingly magical technologies in transportation, communication, entertainment, and computation proliferated, as did enormous cities and corporations with global reach. Societies were transformed for better and worse, as was the planet itself.