By William Crooks
Local Journalism Initiative
Environment and Climate Change Canada (ECCC) presented new findings Wednesday showing that climate change has significantly increased the likelihood and severity of heat waves across the country during summer 2025 — in some cases making them at least 10 times more likely to occur.
Speaking at a bilingual media briefing held via Zoom, Jennifer Smith, National Warning Preparedness Meteorologist, and Dr. Nathan Gillett, Research Scientist, presented data from Canada’s rapid extreme weather attribution system. The findings suggest a clear human fingerprint in many of the most intense heat events experienced nationwide over the past season.
Diverse and dangerous heat across regions
“Heat touched every part of Canada this summer,” said Smith, “but it didn’t happen all at once.” She explained that the timing and characteristics of heat events varied regionally. Western Canada experienced major heat waves in May and again in late August and September, with daytime highs reaching 40°C in B.C.’s Fraser Canyon. In contrast, Eastern Canada saw a condensed stretch of heat from late June to mid-August, with several regions setting records.
Maple Plains, P.E.I., set an all-time provincial high of 38.1°C on Aug. 12. Newfoundland’s Avalon Peninsula experienced its longest-ever heat warning: seven consecutive days. “That’s what sticks in people’s memories,” Smith added, emphasizing that Canadians don’t live the monthly averages — they live the extremes.
She also noted that in the Northwest Territories and Nunavut, peak heat arrived later than usual, with fewer overall warnings issued despite multiple five-day events along the Mackenzie River Valley.
Quantifying climate change’s role
Dr. Gillett described how the attribution system uses global climate models to compare simulations of pre-industrial climate conditions (without human influence) to present-day climates influenced by greenhouse gas emissions.
Of the 12 heat waves analyzed this summer, 11 were found to be “much more likely” — at least twice as likely — due to human-induced climate change. One event, the Aug. 13 heat wave in Atlantic Canada, was classified as “far more likely,” meaning at least 10 times more likely due to climate change.
“That heat wave was the rarest and most extreme of the season,” said Gillett. “It peaked at around four degrees hotter than the typical hottest day of the year.” He explained that it occurred against a backdrop of already dry conditions, exacerbating wildfire risks.
Gillett displayed a graph showing the Atlantic event as a one-in-100-year occurrence in today’s climate — and far rarer under pre-industrial conditions. “That event was made at least 10 times more likely by human-induced climate change,” he said.
Media Q&A reveals future direction
During the media question period, journalists probed further. In response to a question, Gillett explained how the program has evolved since its 2024 pilot. “We refined our system this year,” he said. “We now focus on how observed heat wave temperatures compare to the hottest day of the year for that region, which is a shift from how we approached the analysis last year.”
He also confirmed that ECCC has begun analyzing precipitation extremes using smaller geographic regions (89 across Canada, versus 17 for heat waves). Early results indicate that precipitation events are also becoming more frequent and intense due to climate change, though they require higher-resolution models to analyze effectively.
Gillett addressed timing concerns as well, saying, “We are running the system daily, but it does take time to translate findings into public-facing results. We’re working on releasing these more quickly in the future.”
Asked how researchers distinguish human influence from natural variability, Gillett explained that while solar cycles and volcanoes do affect the climate, their contribution is dwarfed by the warming effect of human-released greenhouse gases. “By far, the dominant factor driving the warming we’ve observed is increases in greenhouse gases from human activity,” he said.
Fewer warnings but longer events
In response to questions about alert patterns, Smith noted that the overall number of heat warnings this year may have been lower than previous years in some regions, but some warnings lasted longer. “We saw very long-duration events, like in Newfoundland and Labrador,” she said. “Previously they may have had warnings for one or two days; this year they had a seven-day event.”
In Western Canada, despite hot daytime highs, fewer heat warnings were issued because overnight lows remained cooler, preventing the events from meeting full warning criteria.
Science for safety and adaptation
“Heat is one of Canada’s deadliest weather hazards,” Smith reminded participants. Gillett echoed this concern, referencing the 2021 Western Canada heat dome, which caused over 600 deaths. He stressed the importance of attribution science for public safety and policy planning.
“These results can help us better plan for, respond to, and rebuild after weather emergencies,” Gillett said. “Understanding how risks are changing can inform decisions to protect Canadians and help plan climate change adaptation strategies.”