Drought and overwintering fires drive consecutive large fire seasons in boreal North America

Boreal North America experienced unprecedented consecutive extreme fire seasons during 2023–2025, marked by record-breaking burned area and emissions. We identify a series of interconnected mechanisms that enabled these back-to-back extremes. A multi-year drought—the most severe in at least 45 years—lowered the water table and desiccated organic soils, allowing more fires to persist over winter under the snowpack. In 2024 and 2025, 14–22% of the burned area in boreal North America originated from overwintering fires. Spring terrestrial water storage anomalies predicted subsequent fire activity, indicating that hydrological monitoring can provide early warning months in advance. These multi-year carryover effects challenge current fire danger systems that treat seasons independently and demonstrate that multi-year climate variability can override long-term wetting trends predicted under climate change.