During his active research career, Cole studied the sources of carbon and its movements in rivers, lakes, and estuaries, with a focus on the connections between these ecosystems and their surrounding watersheds. He and colleagues showed that at a global scale, up to 50 percent of the carbon that is stored in soils is eventually exported to streams, rivers, and lakes, in the form of particles and dissolved organic matter or turned to carbon dioxide in these inland aquatic environments. The realization that inland waters play a role in the regional and global carbon cycle changed the way modelers have approached carbon sequestration on land.