2005-05-20 / News

Climate Change Affects Lake Superior Seasons

The water level of Lake Superior rises and falls every year with the changing of the seasons. But a recent study from Lake Superior State University now shows that this rising and falling has become progressively weaker over the past five decades, and that climate change is the primary culprit.

“The amount of water flowing into and out of Lake Superior has changed significantly over the past 50 years,” said John D. Lenters, an assistant professor and hydroclimatologist at Lake Superior State University (LSSU). “The result has been a noticeable impact on the lake’s water levels, including a 20 percent decrease in the annual range between high and low levels.”

Prof. Lenters first discovered these peculiar seasonal changes in Great Lakes water levels in the spring of 2000, when he was a research associate at the University of Wisconsin-Madison. This more recent study, however, carries the investigation further by looking at the causes of the seasonal shifts. The focus of the study is on Lake Superior, but similar shifts have been found for the other Great Lakes as well.

In his research, Prof. Lenters found that from 1948 to 1999, the amount of precipitation and river runoff entering Lake Superior during autumn increased significantly.

For example, he said, “On average, Lake Superior receives about 70 percent more precipitation during the month of October than it used to 50 years ago. For a lake as large as Lake Superior, this amounts to more than three cubic kilometers of additional water for a one-month period.”

The research results also show that Lake Superior has been receiving less precipitation and river runoff during the spring months. Mr. Lenters attributes some of the decrease in runoff to changes in snowmelt, since winter and spring air temperatures have increased by an average of 2.5 degrees F over the same 51-year interval.

“The changes in spring and fall nearly offset each other,” he said. “Exactly why there are two counteracting trends at different times of the year is a bit of a mystery. It may be purely coincidental. But it’s a good thing they do offset each other, because otherwise we would have seen even more dramatic changes in lake level over the past 50 years as a result of these trends in precipitation and runoff.”

Mr. Lenters found other changes in the Lake Superior water balance besides precipitation and runoff. Evaporation over the lake has been increasing during the summer and early autumn, but decreasing during the winter and early spring. Even the evaporation of water over land areas of the basin has changed.

“On an annual basis, the Lake Superior watershed is losing 20 percent more water to evapotranspiration than it did five decades ago,” he said. “This is a significant change.”

The study also shows that the outflow of water through the St. Marys River has decreased since 1948, primarily during the autumn months. Although Lake Superior has been regulated since 1921, Prof. Lenters said that the changes in outflow do not appear to be owing to the influence of regulation. Rather, he said the drop in outflow is related to the fact that autumn lake levels have also dropped.

“And that,” he said “is fundamentally due to the changes in climatic factors such as precipitation and evaporation.”

Mr. Lenters said it is too early to tell whether these changes in Lake Superior’s water balance are related to global warming, but notes the results of this recent study show the potential for such a connection.

“The increases in temperature in this region are consistent with some of the 50-year changes in runoff and evaporation,” he said, “although they don’t help to explain the precipitation trends. We still need to do much more research to fully understand what’s causing these large changes in the lake’s water balance.”

Prof. Lenters teaches in the LSSU School of Environmental and Physical Sciences and is director of the LSSU Honors Program. He presented the results of his research at the fall meeting of the American Geophysical Union in San Francisco. The complete study is reported in a special Lake Superior issue of the Journal of Great Lakes Research, published by the International Association of Great Lakes Research (2004, Volume 30, Supplement 1, pp. 20-40).

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