The Great Lakes account for about 20 percent of the freshwater on Earth's surface. The lakes' geographical footprint is also hard to fathom. Their combined surfaces span more than 94,000 square miles, about the size of the United Kingdom. The combined measurement of the coasts of the five lakes is thousands of miles longer than either the Pacific or Atlantic coastline of the contiguous U.S.

The planet has warmed by an average of nearly two degrees Fahrenheit (one degree Celsius) since the 1880s. The Great Lakes region is on a par with this global trend: Within the basin, air temperatures have risen by an average of 1.6 degrees Fahrenheit compared with the first 60 years of the 1900s. And much of that warming has been concentrated in the winter months, nudging the ice ever closer to its tipping point.