The water levels of Lake Huron and Lake Michigan have been falling steadily compared with those on Lake Erie, and no one knew why.
But a major report financed by the U.S. and Canadian governments suggests an answer: The fingerprints of climate change are starting to be found in the Great Lakes, the world's largest body of fresh water, causing a discernible drop in their levels.
The report, released Tuesday, estimated that Lake Huron and Lake Michigan have fallen about a quarter metre relative to Lake Erie since the early 1960s, with 40 to 74 per cent of the reduction due to recent changes in precipitation patterns and temperatures.
The alteration in climate is "the most significant factor" in the water level drop and "could be a more substantive issue for the future on the Great Lakes," said Ted Yuzyk, Canadian co-chair of the International Upper Great Lakes Study Board, which compiled the report.
Previous studies have projected a decline in the amount of water in the Great Lakes due to climate change, but the board is the first to suggest the trend is already happening.
The fall in water levels is attributed to such factors as less precipitation and the persistent, long-term decline in the lakes' ice cover each winter.
The report said generally drier weather and drought-like conditions from 1998 to 2008 in the central part of North America led to a drop of about 20 per cent in the quantity of water flowing into Lake Huron and Lake Michigan, compared with the average since 1948.
The two lakes depend on precipitation and run-off for about three-quarters of their inflow. The other quarter comes from Lake Superior, whose outflow can be partly regulated. Lake Erie, by contrast, receives nearly 80 per cent of its supplies from Lake Huron, so it hasn't been influenced as dramatically by the reduction in precipitation.
The finding that climate change is already undermining the Great Lakes is politically sensitive. The board has written to the Canadian and U.S. governments to see whether it is within its mandate to study ways to hold back some of the water in Lakes Huron and Michigan to maintain their size in the face of global warming. Mr. Yuzyk said the clarification is still being assessed.
The board was assembled by the International Joint Commission, a bi-national U.S. and Canadian organization that monitors boundary waters the two countries share.
The concerns about Lake Huron and Lake Michigan levels arose in 2005, when a Canadian environmental organization, Georgian Bay Forever, said levels were diminishing because dredging of the St. Clair River in the 1960s allowed more water to drain from the lakes. The river, which runs by the Ontario community of Sarnia, drains the two lakes and ultimately flows into Lake Erie, leading to worries that the Great Lakes had sprung a leak.
But the report said that while the riverbed experienced some erosion in the 1980s, it now appears to be stable. In addition, it said a small part of the observed water level changes were due to the way land around the Great Lakes is rebounding from the melting of glaciers that covered the area during the last ice age.
While most of the world's attention on disappearing ice has focused on the Arctic, the trend is also happening on the Great Lakes. The report said that in the past 36 winters, three of the four smallest ice covers on Lakes Huron, Michigan, Erie and Superior occurred from 1998 to 2008.
Less ice leads to increased heat input from sunlight, higher winds around the water and more evaporation, contributing to lower water levels.
The report involved more than 100 scientists and engineers and a budget estimated at $4-million.
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