A historic, record-shattering heat wave driven by a persistent heat dome is pushing eastward across North America this week, placing more than 100 million people across the United States and Canada under official heat warnings and triggering dangerous wildfire and air quality risks across the continent. What began as an unprecedented heat event in the western U.S. has spilled over into densely populated regions of the East Coast, Mid-Atlantic and southern Canada, following a pattern scientists link directly to fossil fuel-driven climate change.
By Tuesday, the heat dome had already toppled long-standing all-time temperature records across western states. Billings, Montana hit 111 degrees Fahrenheit (44 degrees Celsius), breaking its previous record by three full degrees, while Salt Lake City, Utah reached 109 degrees Fahrenheit, topping its old benchmark by two degrees. While extreme heat and dry conditions remain locked in the Mountain West, the mass of high pressure is now moving toward populated eastern corridors that already saw deadly brutal heat earlier this month, stretching from Virginia up through New England and into Canada’s most populous regions including Ontario and Montreal.
The U.S. National Weather Service projects that above-average, dangerous heat will reach the Northeast by Tuesday, with peak high temperatures hitting the region on Wednesday before spreading into the Mid-Atlantic. Major cities from Richmond, Virginia to Boston, Massachusetts will see highs climb into the upper 90s to near 100 degrees Fahrenheit, putting daily and all-time temperature records at risk of being broken. In Canada, the country’s capital Ottawa and largest city Toronto are both forecast to hit 100.4 degrees Fahrenheit on Tuesday, with the real feel temperature climbing even higher due to oppressive humidity.
The combination of unseasonable heat, dry conditions and trapped hot air has already exacerbated wildfire risk across North America. Fierce wildfires are burning out of control in southwestern Ontario and northern Minnesota, while smoke from hundreds of miles away blazed in northern Quebec and northwestern Ontario turned the sky over Montreal a hazy yellow by Tuesday morning. Environment and Climate Change Canada warned that the hot, stagnant air will degrade air quality across much of the region, pushing the national Air Quality Health Index into high-risk categories that threaten vulnerable populations. While cooler temperatures are expected to reach eastern regions by the end of the week, the heat dome will remain anchored over the U.S. Mountain West for the entire remainder of July.
Meteorologists explain that heat domes form when large areas of high pressure trap sinking air near the surface, preventing cloud formation and precipitation and allowing heat to build steadily over days or even weeks. While heat domes are a naturally occurring climate phenomenon, climate scientists emphasize that modern extreme heat events like this are directly amplified by decades of fossil fuel emissions that have warmed the planet.
“We know that heat domes are, of course, a natural part of the climate system,” said Marc Alessi, a climate scientist at the Union of Concerned Scientists, during a recent briefing. “But the climate system now is fundamentally different due to fossil fuel-driven climate change. Our atmosphere is much warmer. Our oceans are much warmer. They’re releasing a lot more heat into the atmosphere, and this heat dome is an example of what fossil fuel-driven climate change looks like.”
A recent analysis from the international research group World Weather Attribution found that the extreme heat and humidity that hit the East Coast earlier this month, timed with the U.S. 250th Independence Day celebrations, would have been “virtually impossible” to occur without human-caused climate change.
Scientists also note that a developing “super” El Niño in the equatorial Pacific Ocean is adding additional pressure that is amplifying the current heat dome. Record-warm sea surface temperatures in the central Pacific are shifting the paths where tropical storms form and release energy, which in turn distorts the jet stream over the western U.S. and allows hot high-pressure air to become trapped near the surface. U.S. climate forecasters project that this El Niño will peak between October and December of this year at potentially record-breaking intensity, with the largest temperature impacts following in 2027. When combined with long-term human-caused climate change, the previous El Niño event helped make 2023 the second-hottest year on record globally, and 2024 the hottest year ever recorded.