The United Nations Population Fund estimates that in one week, on October 31, 2011, the world's population will reach 7 billion. Just 200 years ago, there were only 1 billion people on the planet, and over the next 150 years, that number grew to 3 billion. But in the past 50 years, the global population has more than doubled, and the UN projects that it could possibly grow to 15 billion by the year 2100. As the international organization points out, this increasing rate of change brings with it enormous challenges. Meeting the basic needs of so many will mean growing, shipping, and distributing more food while providing more clean water, health care, and shelter -- all without inflicting too much further damage on our environment.