A bird's-eye view of the health of coral reefs

Almost three-quarters of the world's coral reefs are thought to be deteriorating as a consequence of environmental stress. Until now, it has been possible to evaluate reef health only by field survey, which is labour-intensive and time-consuming. Here we map live coral cover from the air by remote imaging, a technique that will enable the state of shallow reefs to be monitored swiftly and over large areas. It is predicted that coral reefs will suffer mounting stress associated with a global increase in atmospheric carbon dioxide over the coming decades and from local disturbances such as overfishing and disease. The most obvious effect of such stress is a decline in living coral cover, so a temporal change in cover is a good indicator of the state of health of a coral reef. However, the measurement of coral cover by field survey is impractical on the scale of hundreds to thousands of square kilometres.