Record-shattering March temperatures in Western North America virtually impossible without climate change
An unusually early spring heatwave is developing across the southwestern United States (US), with temperatures that are more typical of summer than mid-March (AccuWeather, 16 March 2026). Driven by a strong, slow-moving high pressure system, called a ‘heat dome’, the event is causing temperatures to rise 11-17℃ (20-30℉) above average across parts of California, Nevada and Arizona (BBC, 17 March 2026). In many areas, temperatures are expected to exceed 37.8℃ (100℉). In Phoenix, temperature forecasts show multiple consecutive days of around 41.1℃ (106℉), a huge increase over the previous all-time March record of 100℉.
The heatwave poses a significant public health threat, particularly given its timing early in the season. The persistent heat dome is creating stable atmospheric conditions, suppressing cloud formation and precipitation, while allowing temperatures to climb, thus expected to cause persistent heat. Extreme heat is the leading cause of weather-related fatalities in the US. Limited overnight cooling and the lack of air-conditioning is expected to add to the heat stress, particularly for vulnerable populations in these parts who are not acclimatised to the heat, including tourists from cooler countries (CNN, 17 March 2026). This heat dome is breaking records for this time of the year, and is in some ways comparable to the persistent system that caused the 2021 Pacific Northwest (PNW) Heatwave. The PNW event occurred later in the year, in June, with temperatures of up to 48.9℃ (120℉) causing hundreds of fatalities (BBC, 17 March 2026).
The impacts of this early-season heatwave are likely to extend beyond health and have environmental implications. High temperatures are expected to accelerate snowmelt in these parts, including the mountains of Colorado where the snowpack levels are already lowest since 1981 due to the preceding warm winter, and the Sierra Nevada region in California, where although snowfall was average, the high heat is likely to drive rapid snowmelt. Early snowmelt in these parts can reduce water availability during the summer months, increasing the risk of water shortages, prolonging and intensifying dry seasons and increasing wildfire danger (Gergel et al., 2017; Uzun et al., 2021)
Key Messages
- 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 we 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. We combine models and observations, giving equal weight to both lines of evidence, and find 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.
- We 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, we find an increase in intensity of such heat events by 0.8°C and an increase in likelihood of a factor of about 4.
- We also investigated changes in heat extremes at other times of year and across the region. We find 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.
- Heat action plans that reduce heat-related deaths are increasingly being implemented across the region, which is encouraging. However, there remains an urgent need for an accelerated roll-out of heat action plans in light of increasing vulnerability driven by the intersecting trends of climate change and population ageing, paying also special attention to the mental health impacts of extreme heat (Stewart-Ruano et al., 2025). Cities are hot-spots for heat risk, so urban planning needs to focus on measures to reduce the urban heat island effect, such as increasing cooling green and blue spaces.