Aug 06 2007
Shrinking Lake Superior Also Heating Up
Here’s an interesting article from National Geographic News, Shrinking Lake Superior Also Heating Up.
Excerpt: When Jay Austin arrived in Minnesota two years ago, he was surprised to find “remarkably few” studies on Lake Superior’s temperature. So Austin, an assistant professor at the Large Lakes Observatory and the University of Minnesota, set out to do one.
His result was striking: As the climate over the Great Lakes region warms, Lake Superior is heating up at nearly twice the climatic pace.
One buoy in the lake recorded a surface temperature up to 75 degrees Fahrenheit (24 degrees Celsius) in summer, unprecedented for a lake that’s notoriously cold year-round.
The fact that the water temperature is rising should mean water in the lake should rise, all things being equal. Of coarse, all things aren’t equal. The water height in the lake depends upon many factors; offhand I can think of the following:
thermal expansion, precipitation, evaporation, runoff, usage. The the height of the lake can be thought of as
H = T + P + E + R + U (where T, P, E, R, and U are the changes in height due to thermal expansion, precipitation, evaporation, runoff, and usage respectfully)
As I said before, as temperature increases, the T-term above will increase causing H to increase. But an increase in temperature will also increase evaporation, leading to a decrease in H. I’m not sure how temperature changes would change runoff, R, but their effects should be minor. Similarly, how changes in temperature will affect precipitation rates is not clear, but judging from the last couple decades, there and elsewhere, an increased temperature will lead to less precipitation, causing a drop in lake levels. The usage, U, refers to changes in human consumption of the water. This term is probably very small.
So if the lake levels are decreasing and the temperature of the lake is increasing we can compare the rates of some of these terms. Assuming that both runoff and usage will be small compared to the other terms, the precipitation and evaporation terms must be decreasing the lake levels at a rate faster than the thermal expansion is increasing the lake levels.
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