The Great Lakes are big.

But they’re not bottomless.

A just-released report from the U.S. Geological Survey underscores both those points. The study affirms what everybody who lives in and around Michigan assumes: The Great Lakes hold a lot of water. The measure is roughly 6 quadrillion gallons, enough to cover North America, South America and Africa with a foot of water.

Yet, the report cautions that water levels in some places have been drawn down by human use and are threatened by global climate change. Advocates and policymakers should view these facts in a context far beyond the sandy shores of Lake Michigan. Water shortages have grown across the country and around the world. Climate change has altered rain and snow patterns leaving some reservoirs empty.

That makes Great Lakes water a substantial enticement — the shimmering blue gold of the young century. An abundance of fresh water could keep this region afloat economically, even as other quarters go dry. The plenitude surely makes the lakes — 90 percent of the fresh water in the United States and 20 percent of the fresh water in the world — a continuing target for people in drought, and in doubt about future reserves.

The five-year study by the U.S. Geological Survey is part of a larger Water Census project that looks at current water resources nationally, identifies trends and tries to forecast the availability of freshwater for human, economic and environmental use. The Great Lakes were the first part of that overview. The methods used here will be adapted for studies in other parts of the country.

The study found that urban and suburban water use hasn’t significantly affected water supplies on the whole, although the 2.1 billion gallons diverted by Chicago every day lowers levels in Lake Michigan and Lake Huron by 2.5 inches.

However, water has been depleted in some locations. In the Chicago-Milwaukee area groundwater levels dropped 1,000 feet because of municipal pumping and could go down another 100 feet in the next decade, the report states. Such local depletion is strong proof that even the Great Lakes aren’t inexhaustible.

Warming temperatures and shifting weather patterns pose an additional challenge and have a significant effect on water levels. Warming temperatures over the past decade have limited ice and boosted evaporation rates.

That makes it important that smart international conservation efforts be planned and executed. A number of bodies exist to accomplish those goals, including the International Joint Commission and the Great Lakes Commission. Cooperative efforts such as the Great Lakes Compact, a multi-state, U.S.-Canadian pact that limits large-scale withdrawals from the lakes, provide a good framework for addressing threats from outside and from within.

The external threats involve players far and wide who want to divert the Great Lakes for their own uses, whether for commercial gain or to quench populations that have depleted their own aquifers.

Internally, Michigan and other Great Lakes states and Canadian provinces have to develop an ethic of conservation and laws and best practices to attain those goals. Conservation measures were part of the Great Lakes Compact. More, however, can be done.

As this latest report confirms, the Great Lakes are enormous. We should work to keep them that way.

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