Summary/Abstract
Heatwaves are the deadliest type of extreme weather, with hundreds of thousands of people dying from heat-related causes each year. Extreme heat is most deadly earlier in the year, when people have not acclimated to the heat, and vulnerable people are exposed to high temperatures for the first time. Heatwaves as observed in March 2026 in Western North America are still rare events, even in today’s climate which has warmed by 1.3°C due to the burning of fossil fuels, with a return period of about 500 years. As this assessment partly includes forecast data, to prevent an overestimation of the extremeness of the event the authors use a return period of 100 years throughout the analysis.
Observation-based data products show a strong increase in the likelihood and intensity of heat waves in the region, suggesting that such events have become about 4°C warmer as a best estimate, and that events as warm as in March 2026 would have been virtually impossible without human-induced climate change. Climate models strongly underestimate this observed trend but still show a significant increase in extreme heat. The authors combine models and observations, giving equal weight to both lines of evidence, and found an estimated increase in intensity of 2.6°C for such events, with an increase in likelihood of a factor of about 800. This means that without climate change it would have been virtually impossible for the event to occur.
The authors also combine climate models and observation-based products in the same way to study changes in only the past 10 years, during which time the world has warmed by approximately 0.4°C. In this short time, the authors found an increase in intensity of such heat events by 0.8°C and an increase in likelihood of a factor of about 4. They also investigated changes in heat extremes at other times of year and across the region. They found that while increases are evident across all months, the most substantial warming signal for heat extremes in this region occurs in March. In this month, temperatures in the current climate are as much as 6°C higher in parts of the region than those observed under a baseline climate approximately 1.3°C cooler. An abundance of weather stations with long records across the region also show a majority of strongly increasing trends with warming, which provides an additional line of evidence to the findings from this analysis.