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.Although these boreal forests have evolved to burn and are adapted to fire, the size and intensity of the wildfires have increased as the atmosphere warms. Two of the worst wildfire seasons on record in Canada, in terms of acreage burned, occurred in the past three years, the worst of which was in 2023, when smoke crossed the border to cover East Coast cities and turned New York’s sky a deep, eerie orange. Smoky summers have become a familiar reality in Western Canada and the western United States, but eastern cities are now dealing with the same problem more often, Anabela Bonada, the managing director for climate science at the Intact Centre on Climate Adaptation, at the University of Waterloo, told me: “It’s slowly becoming a fixture.”The smoke rolled into her area, outside Toronto, yesterday and quickly reached the worst level on Canada’s air-quality health index. The fires also prompted the closure of the Boundary Waters Canoe Area Wilderness, in Minnesota, this week. First Nations communities in Ontario have been forced to evacuate. And in one particularly harrowing incident, caught on video, the crew members of a freight train in rural Ontario were surrounded by flames. They managed to escape.Canada’s wildfire season had been off to a slow start, but now more than 6 million acres have burned so far this year, on pace to become the third-highest total on record. “What is unfolding is what climate and forest scientists have been predicting for 30 years,” Werner Kurz, a retired senior research scientist at Natural Resources Canada, told me. “That as the world gets hotter and drier, we are exposing forests to more and more risk, and the old strategies of fire suppression are simply being overwhelmed.”In the particularly bad stretch from 2023 to 2025, some 8 percent of Canada’s forests burned, Bonada said. Repeated, large fires have caused a decline in the predominant species of conifers, the black spruce, shifting what vegetation regrows. These enormous swaths of boreal forest from the Yukon to Newfoundland and Labrador—more than 1 million square miles—act as massive vaults for carbon, which is locked away in billions of trees and the forest floor. But so much of those forests is going up in smoke that they have become their own notable source of greenhouse gases in recent years. A 2024 study published in Science found that carbon emissions from wildfires had increased by 60 percent in the prior two decades. Fires in boreal forests such as Canada’s were the main reason. Another study, published in Nature, found that Canadian wildfires in 2023, when more than 43 million acres burned, generated more emissions than the burning of fossil fuels in all but the three most-polluting countries.When these forest fires really get going, they can generate a destructive power that is hard to fathom. A notorious 2016 fire, known as “the Beast,” destroyed much of Fort McMurray, in an oil-producing region of Alberta, and burned across nearly 1.5 million acres. Such huge fires can generate hurricane-force winds and towering pyrocumulus clouds; when the smoke gets high in the atmosphere, it can drift across oceans and continents. Many of these fires coincide, as this week’s blazes in Ontario have, with heat waves. The town of Lytton, in British Columbia, was nearly wiped off the map in 2021 the day after it endured Canada’s hottest temperature on record, 121 degrees.“Having these temperatures in forested areas, it’s just impossible. The forests don’t remain,” John Pomeroy, a hydrologist at the University of Saskatchewan who has studied the effects of climate change, told me. He is based at a research lab in the Canadian Rockies, in Alberta. Last week, the whole area was blanketed in smoke, he said, from wildfires in British Columbia. In addition to causing harmful health impacts, wildfire smoke also darkens glaciers in the mountains. Those darker glaciers absorb more sunlight, which causes them to melt faster. The world is caught in these loops, and they’re only going to keep repeating.