Like rivers, lakes may experience dwindling water supplies as the world warms. In the United States, for example, scientists have been concerned about the shrinking of the Great Lakes. In 2007, Lake Superior's water level was more than 1.5 feet (45cm) below its long-term mean. In part this drop was because of a drought, but the loss of water also seems to be due to long-term trends. Lake surface temperatures have risen by 4.5°F (2.5°C) since 1979, causing an increase in the rate of evaporation. Specifically, evaporation has increased by 1.8 inches (4.6mm) per year since 1978, while precipitation has fallen by .16 inches (4.1mm).5
The temperature rise on the lake is greater than air temperature rise over the same period. University of Minnesota researcher Jay Austin has connected the change in lake temperature to "a reduction in winter ice cover on the lake."6 Ice reflects sunlight; without the ice cover, the lake absorbs more energy from the sun, and water temperatures rise. Ice also puts a cap on the lake that prevents evaporation. With less ice, evaporation increases. Thus, there is a feedback effect: As global warming causes air temperature to rise, there is less ice on the lake, which causes water temperature to rise further.
Since 2007, the Midwest has experienced increased rainfall and colder weather. As a result, according to reporter Sheri McWhirter, "Water levels in the Great Lakes are rising after receding for a decade."7 Lake Huron and Lake Michigan were 10 inches higher in June than the previous year. Despite such short term gains, however, McWhirter also noted that average lake levels are still expected to decline by about 3 feet (1m) over the course of the century. University of Michigan researcher Don Scavia explains that "Climate projections say the lakes will go up and down around a decreasing average. . . . The lows will be lower than in the past, and the highs will be lower than in the past."8
Less water in the Great Lakes can be problematic. When lake levels were at historic lows in 2007, merchant ships had to carry less cargo or risk being beached. In places like Africa, however, the effect of global warming on lakes has raised the specter of environmental devastation. In Lake Tanganyika in eastern Africa, for example, air temperatures above the lake rose by 1.1°F
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