Canadian Weather Analysis Shows a Hot Summer, but Not a Record Breaker

Environment and Climate Change Canada shared the recent results from the Rapid Extreme Weather Attribution System on September 17, 2025. Canada experienced extreme heat from coast to coast, though the duration and dryness varied considerably by region.

The Rapid Extreme Weather Attribution System models today's climate conditions with those of the 19th century prior to the Industrial Revolution by measuring levels of atmospheric gases to determine the extent to which human activity is influencing extreme weather events.

Overall, the summer of 2025 was the 11th warmest on record, a welcome relief from some of the record-breaking summer heat of the past few years. Heat waves in many regions featured hot days followed by cooling periods, which lowered the overall average temperature.

“When you average it out over a whole month, those extremes tend to disappear,” said Jennifer Smith, national warning preparedness meteorologist with the Meteorological Service of Canada and Environment and Climate Change Canada on a webinar discussing the results of the analysis. “But Canadians didn't live the averages: they lived the extremes. They felt the sleepless nights and the long, hot afternoons.”

In some regions, such as western Canada, there were high heat events at the beginning and the end of the summer, including a long-duration heat event in Alberta in late August that lasted into early September. In eastern Canada, there were high heat events from late June to mid-August, giving residents the impression of a short summer.

Heat arrived later than usual in northern Canada, including the Northwest Territories and Nunavut, and there were fewer heat warnings than usual in the region. In Atlantic Canada, extended drought conditions exacerbated both regular and extreme heat events.

Climate Change is Impacting Summer Heat in Canada

Climate researchers analyzed 10 of the hottest heat waves across Canada in July and August 2025, including the longest heat event of the summer in British Columbia. These events lasted in some regions from August 23 to September 9 and featured temperatures as high as 10°C above normal daily high temperatures. Of the 10 events, nine were determined to have been two-to-10 times more likely to have occurred because of anthropogenic climate change.

“The most extreme events have the biggest change in frequency because of climate change,” said Dr. Nathan Gillett, a research scientist with Environment and Climate Change Canada, on the webinar. “We saw that this year, for example, with the Atlantic Canada heat wave increasing in frequency by at least ten times, and that was the rarest event of the season.”

Attribution System Adapts to Changing Climate

After its introduction in the summer of 2024, the Rapid Extreme Weather Attribution System is evolving to meet the challenge of ongoing extreme weather events exacerbated by climate change.

“We have refined our system this year,” said Gillett. “We're focusing on how the temperatures in a given heat wave compared to the hottest day of the year in previous years, which is slightly different from how we were doing the analysis last year. We've refined exactly how we use the climate model simulations. We focus on simulations with the same level of global warming that is observed in the real climate.” According to Gillett, this reduces the effects of differences between models, which should help to continue to improve the estimates.

In January 2025, Environment and Climate Change Canada began extreme cold attribution analysis for the coldest extreme temperatures in Canada each winter. As global warming increases, extreme cold events will decrease. The analysis of two extreme cold events in January and February 2025 found they were both much less likely to be the result of human influence on the climate.

Later in 2025, the system will begin tracking extreme precipitation events, which are considered much more likely to occur as a result of human-induced climate change.

The Rapid Extreme Weather Attribution System is part of Environment and Climate Change Canada's ongoing efforts to provide information to support climate change adaptation, encourage environmental studies, and help Canadians plan for and respond to extreme weather emergencies.