Just over 200 years ago, there were one billion humans on Earth; now there are seven billion, and, sometime this century, the world population is expected to reach ten billion. Stephen Emmott, head of Computational Science at Microsoft Research, explains what this steadily growing human population means for the earth: deforestation, desertification, species extinction, growing threats to food and water. His book Ten Billion analyzes the issues our growing population will bring about.