The assessment, led by Greg Asner of the Carnegie Institution for Science, is based on a combination of satellite imagery, on-the-ground field surveys, and an advanced airplane-based sensor that can accurately measure the rainforest canopy and sub-canopy vegetation at a resolution of 1.1 meters (42 inches). The approach enabled the researchers to detect changes in forest cover in areas as small as 10 square meters, roughly 100 square feet, allowing them to map thousands of small, clandestine mines that had never before been detected at scale.
Gold mining in the Peruvian Amazon. Photo by Rhett A. Butler
The results are sobering: the extent of gold mines in the Madre de Dios region of Peru increased from less than 10,000 hectares in 1999 to more than 50,000 ha as of September 2012. The rate of expansion jumped from 5,350 acres (2,166 ha) per year before 2008 to 15,180 acres (6,145 ha) per year thereafter. Surging gold prices, combined with increased access to the region granted by new roads, are thought to be driving the spike in mining activity.