When you think of climate change, you might think of wildfires, mudslide, hurricanes. But all across Canada and the northern U.S., shorter, warmer winters are quietly threatening a sporting way of life.

This story appears in the April 22, 2019, issue of Sports Illustrated. For more great storytelling and in-depth analysis, subscribe to the magazine—and get up to 94% off the cover price. Click here for more.

The most famous backyard in Canada is on a quiet street in the north end of Brantford, Ont. It was here, behind the modest gable-roofed home at 42 Varadi Avenue, that the boy who would become the Great One spent winters skating outside on an ice rink built by his father, developing the inimitable creativity that would define his game.

For hours in the cold, a young Wayne Gretzky would toil on that patch of ice frozen over grass known as "Wally's Coliseum," named for Wayne's father, Walter. (Before the Gretzkys moved in, Walter inspected the backyard to make sure it was spacious and flat enough for a rink.) Often Wayne practiced alone. Pretending he was Gordie Howe, he would skate around tin cans, fake passes and fire shots, taking aim at a turned-over picnic table.

Though Walter's backyard rink has since been replaced by an in-ground pool, the home remains sacred for Gretzky devotees, hockey's own Church of the Nativity. Once, Walter watched through the window as a woman tore a chunk of grass from the front lawn, dashed back to her car and sped off. It's one reason Brantfordians venerate outdoor skating, either in their own backyards or on community park rinks.

It's not just Brantford, though, and it's not just the Gretzkys. All across Canada and the northern U.S., outdoor hockey—played on frozen ponds, backyard ice and community rinks—has long enlivened dreary winters. In future stars (and mere beer-league hobbyists) it nurtures adaptiveness and flair. A line from Roch Carrier's children's book, The Hockey Sweater, once graced the back of Canada's five-dollar bill: "The winters of my childhood were long, long seasons. We lived in three places—the school, the church and the skating rink—but our life was on the skating rink." Bobby Orr once called backyard rinks "the heart and soul of hockey."

In recent years, the NHL has turned the sport's outdoor roots into a nostalgia-soaked—and wildly successful—marketing tool by way of the Winter Classic and Stadium Series. Many of the players starring in this spring's playoffs grew up playing outdoors, where the game is at its most accessible. The origins of ice hockey are murky—several Canadian towns claim to be the modern game's birthplace—but the "great northern sport," as SI once called it, was played outdoors for years, possibly decades, before the first indoor game was played on March 3, 1875, in Montreal.

That heritage is now threatened by rising global temperatures. Since 1950, Ontario has warmed by 1.4° Celsius, or 2.5° Fahrenheit, according to the Ontario Centre for Climate Impacts and Adaptation Resources. That number could increase to between 2.5° and 3.7° Celsius by 2050. While the globe as a whole has warmed an average of 1° Celsius since the start of the Industrial Revolution in the 19th century, the Northern Hemisphere's winters, in particular, are warming rapidly: In the United States, average winter temperatures in every state have warmed at least 1° Fahrenheit since 1970, and in four hockey-mad states—Alaska, Minnesota, Vermont and Wisconsin—winters have warmed by more than 5°.

As average temperatures continue to rise, the number of days suitable for skating will fall. Lori-Dawn Cavin, who manages Brantford's 27 community rinks, considers 40 skating days a good winter. Since 2005, when the parks and recreation department started keeping records, the number of skating days has fluctuated wildly each year. In the winter of '13–14, for instance, the city's rinks combined for 1,017 skating days, or an average of 51 per rink. But in '11–12, the parks managed only 17 combined skating days. In the season that just concluded, the Brantford rinks combined for 351 skating days; only two had 40.

But the evidence isn't just anecdotal. Data collected by RinkWatch, a citizen science project cofounded by Robert McLeman, an environmental scientist at Wilfrid Laurier University in Ontario, indicates that the number of high-quality skateable days in Toronto has dropped precipitously in the last few decades. From the late 1950s to the early '80s, Toronto averaged between 35 and 42 of those days; recently, that number has fallen by 25% and projects to fall another 33% by the year 2090.

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