Extratropical lightning fires burn increasingly more severe than human-ignited fires

Fires ignited by human and lightning occur at distinct environments and thus diverge during their developing processes. A global characterization of fires by their ignition cause will inform fire forecast and prediction but is currently prohibited by a lack of ignition cause in global fire inventories. Here we develop a machine-learning classification system and ascribe the ignition cause of 65.17 million global, satellite-detected fire events during 2012-2024. According to this fire inventory, extratropical lightning fires exhibit longer duration, larger burned area and hotter flame, compared with human fires. Despite their contribution to only 2.4% of fire occurrence, lightning fires are responsible for 10.9% of extratropical burned area and 47.6% of that consumed by large fires over 100 km². This disproportionate abundance of lightning fires in the regime of most severe burning is attributable to synchronized seasonality of lightning ignition and burning conditions, as well as their scarcer accessibility to firefighting practices. Due to their closer linkage to the elongating fire-favorable weather, extratropical lightning fires has elongated by about 0.24 days decade^-1, outpacing human fires. With projected hotter, dryer, and stormier extratropical summers, our results provide a direct support for a future of severer lightning fires.