Humans alter the climate by emitting greenhouse gases, by changing planetary albedo, and by altering atmospheric chemistry. Between 1900 and 2000, humans' emissions of carbon into the atmosphere increased fifteen-fold, while the numbers of people increased less than fourfold. One way to help mitigate global change is to reduce our ‘carbon footprint’. Unsustainable human population growth, however, may overwhelm those efforts. Understanding the relationship between human population growth and global change factors is complex and evolving. To start, understanding human contributions to global change requires an understanding of human population patterns at many scales.