Essay: More on this Visualization
How are the components of the human footprint measured? Earth's surface reflects some of the Sun's radiation back into space. Satellites detect this reflected radiation. Different types of land cover, such as forests, bare ground, roads, and water, reflect wavelengths of the electromagnetic spectrum differently; each has a unique "spectral signature." Much like you'd tune a radio dial to pick up a specific station, satellite analysts "tune in" to these spectral signatures. By combining signatures from distinct regions of the electromagnetic spectrum (called "bands"), scientists can precisely map Earth's altered areas. For example, water reflects very little radiation in the infrared bands while vegetated areas reflect a great deal. Urban areas are warmer than surrounding land, thus radiating more energy into space. The thermal band captures this emitted radiation to track the growth of cities. By analyzing how indicators of human activity change over time, scientists can get a dynamic, global picture of the human footprint.