Climate Number: Five Trillion Gallons
While commonly considered two separate lakes, Lakes Michigan and Huron are actually hydrologically one body of water – they are connected at the Straits of Mackinaw and rise and fall in unison. Since 1980, Lake Michigan-Huron has been warming and annually averaged surface temperatures are now 3.6 degrees Fahrenheit warmer than they were in the late 1970′s. This warming has been accompanied by less wintertime ice cover and more evaporation from the Lake’s surface. While about half of this 25 percent increase in evaporation is accounted for by increases in summertime evaporation, more water is evaporating from the lakes during the spring, fall and winter seasons as well. The decline in ice cover means that there is now more liquid water exposed to late fall and winter winds, which pick up the moisture from the Lake and deposit it down-wind in the form of lake effect snow. Increases in Great Lake lake effect snow have been observed during this same period when ice cover has declined. This 25 percent annual increase in evaporation from the Lake means that over five trillion more gallons of water are going into the atmosphere from the lakes each year than during a typical year in the 1970′s. This increase helps to explain why lake levels have been declining over the last few decades despite above average regional precipitation totals. Water levels in Lake Michigan-Huron have been hovering around long-term record lows since 2000.
For Comparison: Five trillion gallons is about the same amount of water stored in the reservoir created by China’s Three Gorges Dam on the Yangtze River. Five trillion gallons is also about how much water is allocated annually to seven western states and Mexico under the 1922 Colorado Water Compact, which is based on estimates of the annual flow of the Colorado River at that time.
Seasons: Winter, Spring, Summer, Fall
Sources: Hanrahan, JL et al. “Connecting past and present climate variability to the water levels of Lakes Michigan and Huron.” Geophysical Research Letters 37 (2010): L01701 and National Geographic: Global Action Atlas. Colorado River Project Summary (2009). Accessed Online 1 February 2010
