Drought triggers and sustains overnight fires in North America

Abstract Overnight fires are emerging in North America with yet unknown drivers and implications. This phenomenon is significant as it challenges the traditional understanding of the “active day, quiet night” paradigm of the diurnal fire cycle and current fire management practices. Here we show that ~20% of all large fires (≥1000ha) in North America observed during 2017-2020 using geostationary satellite images and terrestrial fire records burned through a total of 1,084 nights. Overnight burning was characterized by early onset after ignition, high persistence, and a tendency to mutually reinforce extreme fires. While warming weakens the climatological barrier to general nighttime fires1, we found the major driver of overnight burning was the accumulated fuel dryness and fuel availability (i.e., drought conditions), rather than fast-reacting day-night weather fluctuations. Drought conditions tended to sustain overnight burning for periods of multiple days, and even weeks. Moreover, we show that daytime drought indicators can be used to predict overnight burning events, which could facilitate early detection and management of nighttime fires. Recently observed and predicted future increasing trends in conditions conducive to overnight burning indicate that disruption of the diurnal fire cycle may accelerate, leading to larger, more intense, and extreme fires in future.