Alarm bells are being raised over the lack of ice on the Great Lakes this year.

A report issued by the nonprofit group Climate Central notes all five lakes have experienced a decades-long decrease in ice cover.

The strongest trend is on Lake Superior, where frozen lake coverage has diminished by roughly 30% since 1973.

Melissa Widhalm, associate director of the Midwestern Regional Climate Center, says conditions can vary year-to-year, but the long-term trend creates too much uncertainty.

“And so it’s making people want to know, how much are we actually going to get this year? Are we going to be able to build that ice back?” says Widhalm.

The report notes shrinking ice cover and fewer frozen days have a ripple effect on outdoor recreation, educational activities and ecosystem health.

Shipping seasons might be extended, but the lack of ice potentially leads to lower water levels.

The lakes hold one-fifth of all fresh water on Earth’s surface and supply drinking water to over 30 million people across the eight U.S. states and Ontario.

The report adds the decrease in ice cover coincides with a rise in average winter temperatures across the same region.

“When you have warmer air temperatures leading to warmer water temperatures, that’s going to slow down the speed that that ice develops,” says Widhalm.

The report adds the Great Lakes are not alone.

It notes the surface waters of hundreds of lakes across the globe have warmed 0.4 to 0.8°F per decade between 1970 and 2010.

The duration of lake ice cover across the Northern Hemisphere has also declined by over two weeks annually as lakes freeze later in winter and thaw earlier in spring.

(With files from Mike Moen/Minnesota News Service)