Humans leave a mark on Earth that is detectable from space. By analyzing satellite images, scientists can measure the extent of deforestation, agriculture, urbanization, and other features. Satellite analysts estimate that humans directly affect 83 percent of our planet’s land surface, leaving a dwindling percentage of land truly “wild.” This huge influence is called the “human footprint.”

The Pearl River delta near Hong Kong is a dramatic example of the human footprint. Humans have continuously occupied the region for thousands of years, and the region’s cities are among the fastest-growing urban centers ever observed. Scientists use the same techniques illustrated for the Pearl River delta to study human-altered surface features around the world, thus mapping the global human footprint.