North American Wildfire Potential Burning Hours Increased 36% Since 1975

Annual potential burning hours for wildfires in North America have increased by 36 per cent between 1975 and 2024, according to a study published in Science Advances that analysed nearly 9,000 fires larger than 200 hectares using hourly satellite observations from 2017 to 2023. California now experiences approximately 550 more potential burning hours annually than in the mid-1970s, with parts of south-western New Mexico and central Arizona recording increases of up to 2,000 additional hours.

Fires in boreal, temperate and sub-tropical forests are lasting later into the night and starting earlier in the day, driven by hotter and drier conditions. Western mountain regions and boreal forests experienced the longest burning periods, with approximately one-third of active days exceeding 12 hours. Around 60 per cent of fires reached peak intensity within 24 hours of detection. The number of extreme fire days with 12 to 24 hours of potential burning has expanded from 81 to 233 in some boreal regions. Spring and autumn months are seeing the largest relative increases in burning hours, between 48 and 57 per cent, compared to summer's 24 per cent relative growth.

Night-time cooling that historically suppressed fire activity is weakening. About 14 per cent of active fire days now see peak intensity occurring at night, significantly narrowing the window for effective firefighting. Temperature and relative humidity are identified as the first-tier drivers of burning probability, with hotter and drier conditions increasing both ignition risk and fire spread. Researchers identify asymmetric diurnal warming, where night-time temperatures rise at a different rate to daytime temperatures, as limiting the nocturnal cooling that once allowed forests and firefighting crews to recover.

Recent fires illustrate the shift toward around-the-clock fire activity: the 2023 Maui fire in Hawaii, the 2024 Jasper fire in Alberta and the 2025 Los Angeles fires in California all demonstrated extended nighttime burning patterns. Co-author Xianli Wang notes that wildfires during drought conditions create a vicious circle where warmer atmospheric conditions draw moisture from soil and vegetation, increasing fuel flammability while reducing the rate at which dead vegetation can regain moisture. Shrinking quiet periods are straining firefighting resources by reducing safe intervention windows and increasing the proportion of time when conditions favour rapid fire spread.

Key figure — 36% increase in annual potential burning hours from 1975 to 2024