Summer might seem like an odd time to talk about hockey, but we have two good reasons: First, the National Hockey League is scheduled to start playoffs on Saturday in Edmonton and Toronto. Second, a team of researchers in Canada published a report this month looking at climate change and outdoor rinks.

After a four-month hiatus because of the coronavirus pandemic, the outcome of the N.H.L. season is anyone’s guess. The takeaway from the study is more clear: Rising global temperatures are puncturing the viability of homemade rinks because there are fewer days each winter when it’s cold enough to maintain them.

The researchers looked at backyard rinks in the Original Six N.H.L. cities — Boston, Chicago, Detroit, Montreal, New York and Toronto — and found a steady decline in the number of days when the weather was cold enough to make ice and skate on the rinks.

Toronto saw the greatest reduction in ice time. In the winter of 1942-43, the first year of the Original Six era, there were close to 60 days when Torontonians could expect high-quality skating conditions in backyard rinks. Last year, there were about 20.

"Starting in the 1980s you see this downward slope on virtually every indicator for all the cities," said Robert McLeman, an environmental scientist at Wilfrid Laurier University in Waterloo, Ontario, and one of the authors of the study. "Skating seasons are shifting toward shorter seasons and more of a yo-yo effect with a mild winter followed by a cold one. You did not see that much prior to 1980."

For the past seven years the scientists have been collecting data from outdoor rink owners in a project known as RinkWatch to determine the number of high-quality skating days available to them. To discover the conditions in the previous seven decades, they gathered data from weather stations in those towns to determine how many skateable days were available.

They used outdoor rinks as the vehicle to highlight the data because they capture the imagination, especially in Canada. Dr. McLeman said that connecting climate data to outdoor rinks in the six original N.H.L. cities made the study, published in The Canadian Geographer, an exercise in science communication as much as a research paper.

"It’s hard for individuals to sense change in average temperature," Dr. McLeman said. "It’s easier for us as humans to sense changes in our own behaviors or activities."

The scientists also measured the time period when good skating was possible. For example, in Montreal, where the N.H.L.’s Canadiens are an institution, the outdoor skating season used to begin in late November. It has mostly shifted to early December and it did not start until January a few times in recent years.

Since 1995, there have been six late starts to the outdoor rink season in Montreal, whereas in the previous 45 years there had only been only one.

Unless governments take action to sharply reduce greenhouse gas emissions, scientists say, that trend will continue. And, with fewer good skating days each winter, according to Dr. McLeman, people will be less likely to make the considerable effort to build backyard rinks.

That means hockey could become a sport for the privileged few.

"You take away the outdoor rink," Dr. McLeman said, "and only the rich kids get to play."