This fall, the water level of Lake Superior, the world's largest freshwater lake, dipped below the record it set in the Dust Bowl days of 1926. In September, the 1540-square-kilometer lake on the Canadian border was at its lowest since record-keeping began in 1860--with an average depth of 183 meters. October's level went up after several weeks of rain but was still 30 cm below the long-term average. Boats are resting on mud, docks are poking nothing but air, and fishermen and lakeside rice growers are watching their livelihoods dry up.
Some factors that explain the mess: The surface temperature of the lake has mysteriously risen 4.5°C since 1978--twice as fast as the temperature of the surrounding air. Ice, which blocks evaporation, has become rare on Superior, and precipitation has dropped 15 cm a year from the annual average of 77 cm.
Thursday, November 29, 2007