When the boreal forests of Canada catch on fire, no one can do anything about it in many cases. The forests are part of Earth’s largest land biome, a greenbelt of wilderness that encircles the globe, and they’ve been suffering from the planet’s thermostat being jacked up. Wood-boring pests that flourish in milder climates have swept north and east, through tens of millions of acres. Droughts and dwindling snowpack have stressed the trees. They are ready to burn.

Many people simply don’t grasp the sheer magnitude of the boreal forest or what it would take to manage fires across its enormous area, Jed Kaplan, a professor in the Department of Earth, Energy, and Environment at the University of Calgary, told me: “You can’t control these fires. You cannot put personnel, fire engines, over an area that is the size of the entire American South, or something like that. It’s just way too big of an area.”

And so the fires spread, pouring out smoke that washes over the residents of faraway cities. The “Ontario Armageddon” (as one wildfire newsletter called it), along with several large fires burning in northern-Minnesota forests, has left Toronto with some of the worst air quality in the world this week and has shrouded New York City in a sickly gray haze. Canada has 869 active fires at the moment, and most are burning in wilderness areas where authorities monitor them but don’t try to put them out.