Extreme heatwave sweeps US and Canada, bringing record temperatures and air quality warnings

A record-breaking heat wave spread from the US Mountain West into the eastern United States and parts of Canada on Tuesday, placing more than 100 million people under heat alerts as soaring temperatures, dangerous humidity and worsening wildfire smoke raised health concerns across the continent.

Hot, dry conditions have also fueled intense wildfires in southwestern Ontario and northern Minnesota, with smoke expected to worsen air quality across parts of the US Midwest and Northeast in the coming days.

The heat dome gripping North America has already broken all-time temperature records in several western US states. Billings, Montana, reached 111F (44C), surpassing its previous record of 108F, while Salt Lake City climbed to 109F, exceeding its former record of 107F.

Extreme heat and humidity continue to grip the Mountain West and are now spreading toward the densely populated East Coast, which experienced another severe heatwave earlier this month. The hot conditions have also extended into parts of Canada, including Ontario and, to a lesser extent, Montreal.

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"Above average temperatures and dangerous levels of heat are forecast to enter the Northeast on Tuesday before the most intense heat occurs on Wednesday and expands into the Mid-Atlantic," the US National Weather Service said.

Cities from Richmond, Virginia, to Boston, Massachusetts, are expected to see temperatures rise into the upper 90s and approach 100F, placing daily high-temperature records at risk.

In Canada, Ottawa and Toronto are forecast to reach highs of 100.4F on Tuesday, with humidity making conditions feel even hotter.

"Hot and humid air can also bring deteriorating air quality and can result in the Air Quality Health Index approaching the high-risk category," Environment and Climate Change Canada said.

Smoke from wildfires burning hundreds of miles away in northern Quebec and northwestern Ontario turned Montreal's skies yellow on Tuesday morning.

A heat dome is a large area of high pressure in which sinking air suppresses cloud formation and rainfall, allowing heat to accumulate over days or even weeks.

New climate reality

"We know that heat domes are, of course, a natural part of the climate system," Marc Alessi, a climate scientist at the Union of Concerned Scientists, said during a briefing.

"But the climate system now is fundamentally different due to fossil fuel-driven climate change," he said. "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 you know what fossil fuel-driven climate change looks like."

An analysis by the World Weather Attribution group found that the hot and humid conditions during the East Coast heatwave earlier this month, coinciding with US Independence Day celebrations on July 4, would have been "virtually impossible" without human-caused climate change.

Scientists also say a developing "super" El Nino in the equatorial Pacific may be contributing to the extreme conditions.

US forecasters expect El Nino to reach its peak between October and December at potentially record-breaking strength, with its greatest warming impacts likely to be felt in 2027.

Combined with human-driven climate change, the previous El Nino helped make 2023 the second-hottest year on record, followed by 2024, which became the hottest year ever recorded.