Most of the water in the Great Lakes hasn't frozen over this year — largely because of a warmer-than-usual winter — and a new study shows the lakes have been losing ice cover for 40 years.

Lake Superior is the coldest of the Great Lakes. Yet only a thin layer of ice surrounds a cargo ship in the Thunder Bay, Ont., harbour. Past the breakwall, there's no ice at all.

Thunder Bay Port Authority CEO Tim Heney, a 20 year-veteran in his industry, said the lack of ice is remarkable.

Tim Heney, CEO, Thunder Bay Port Authority (Nicole Ireland/CBC)

“It's the first time I ever remember the water being open right into Thunder Bay,” Heney said. “I've never seen this before."

According to a new study published in the Journal of Climate, the Great Lakes have lost more than two-thirds of their ice cover over the last four decades. Lake Erie and Lake St. Clair lost the least (50 per cent), while Lake Ontario has lost the most ice (88 per cent). That's more than even the study's lead author, ice climatologist Jia Wang, expected.

"It's really, really huge,” he said.

Adam Cornwell, an assistant professor of geography at Lakehead University, said Wang's findings are consistent with other climate research — all of which sends a powerful message.

"What's happening on the Great Lakes is an immediate reminder that our climate is changing and the expectations that we've had in the past aren't going to hold true for the future,” Cornwell said.

Adam Cornwell, assistant professor of geography at Lakehead University. (Nicole Ireland/CBC)

“And one of those expectations is a season of ice cover on the Great Lakes."

The study noted that ice cover varies from year to year, depending on whether cold or warm systems are passing through. But it attributes the overall ice decrease to global warming — a factor that can affect fish and marine plants.

The impact isn't just environmental, according to the study. Ships could be forced to navigate lower water levels as less ice results in more evaporation.

“We could be changing to a regime even just over the next 30 years where ice-free seasons happen more often than not,” Cornwell said.