Global warming, popularly known as the "greenhouse effect," could further decrease the clarity of California's Lake Tahoe, already compromised because of land-use changes in the area, says M.L. Shelton, a UC Davis geography professor. Shelton presented his findings last Friday at the annual meeting of the Association of American Geographers. Shelton analyzed the influence of five selected climate change scenarios in which he altered temperature and precipitation to represent conditions that likely would accompany a perturbed climate system. Should the future hold a warm, moist climate, runoff from the upper Truckee River into Lake Tahoe would increase by as much as 25 percent. That river contributes more than 24 percent of the lake's annual inflow and is already an area with extensive environmental disturbances. If, on the other hand, the future brings a warm, dry climate, runoff from the river could be reduced by as much as 33 percent.