Wildfire Smoke and High Heat Have Something in Common. Guess What.

Between the dangerous heat baking Texas and the Southeast, and the wildfire smoke filling the skies throughout the Upper Midwest and into the Mid-Atlantic, people across a huge part of the United States have been seeking relief from the outside world in recent days.

The two threats this week aren’t connected directly. But a common factor is adding to their capacity to cause misery.

Human-caused climate change is turning high temperatures that would once have been considered improbable into more commonplace occurrences. And it is intensifying the heat and dryness that fuel catastrophic wildfires, allowing them to burn longer and more ferociously, and to churn out more smoke.

Climate change is “the elephant in the room” that is worsening wildfires and their effects on air quality, said John C. Lin, a professor of atmospheric science at the University of Utah. As this year’s Canadian blazes have shown, climate-related disasters are becoming international affairs, not just local or regional ones, Dr. Lin said.

The weather has always been a combination of mild norms and occasional extremes, but the burning of fossil fuels is loading the dice in favor of weather on the warmer end. On Wednesday afternoon, more than 50 million Americans were living under heat advisories from the National Weather Service.

In Texas, apart from the daily temperature records that parts of the state have set this month, John Nielsen-Gammon, the Texas state climatologist, said he had also been looking at the places that have broken records for their hottest weeks.

Almost all of Southern and Western Texas’ hottest seven-day periods have occurred in the past few decades, he said, a sign of how global warming is making heat waves like those that are familiar to Texans in summertime hotter than they would be otherwise.

“While the skeptics like to point to the all-time individual maximum temperature records not having been set recently, any other temperature metric you look at is showing prominent increases, which includes persistent heat in the case of Texas,” said Dr. Nielsen-Gammon, who is also a professor of atmospheric science at Texas A&M University.

On Wednesday, President Biden delivered a speech in Chicago, which was shrouded in a soupy haze from the Canadian wildfires.

“This is part of a growing pattern of extreme weather events that we’re seeing as a result of climate change,” said Olivia Dalton, the deputy White House press secretary, “and why the president has taken such ambitious, aggressive action to tackle that threat.”

Canada’s prime minister, Justin Trudeau, has also blamed human-driven warming for increases in wildfire spread and intensity. “Year after year, with climate change, we’re seeing more and more intense wildfires — and they’re starting to happen in places where they don’t normally,” he wrote on Twitter this month, shortly before cough-inducing smoke from Canada began smothering a large section of the northeastern United States.

That encounter with smoke and haze is what first drew many Americans’ attention to the fires across their northern border. But parts of Canada have continued to grapple with burning forests even if, for a time, less of their smoke was blowing in Americans’ direction. Nearly half of the 480 fires that were raging across Canada on Wednesday afternoon were classified as uncontrolled, according to the Canadian Interagency Forest Fire Center.

Higher air temperatures add to the drying out of dead leaves, branches and other flammable matter that feeds forest fires, said Jeff Wen, a doctoral candidate in earth-system science at Stanford University who studies the societal effects of wildfire smoke. “Those drier surface fuels, once ignited, burn more intensely and more severely, really damaging ecosystems,” he said.

Already this year, carbon emissions from fires in Canada have surpassed those that fires in the country have produced in any of the past 20 years, according to the European Union’s Copernicus Atmosphere Monitoring Service. The smoke is not just drifting into the United States. It has also been making its way across the Atlantic, reaching southern Europe and the British Isles before curling toward the north and east, the monitoring service said.

Even before this year, Canada had been seeing an increase, over decades, in the area burned by wildfires, said Chelene C. Hanes, a fire scientist with the Canadian Forest Service. “The fire season is getting longer, starting earlier in the spring, going later into the fall,” she said. And, “we’re getting more of these larger fires.”

Dr. Hanes was one of two lead authors on a study, published in 2019, that described these changes from 1959 to 2015. She said she hoped to update the findings soon with information about how the relentless wildfire seasons of recent years had affected longer-term trends.

“Because it’s happening so fast,” she said. “It just seems the pace of things changing is so quick.”

Michael D. Shear contributed reporting.