Human activity augments lightning ignitions to reshape fire seasonality across all biomes on Earth

Abstract

The conjunction of lightning ignitions and dry vegetation has shaped fire regimes throughout geological time. These fire regimes have in turn influenced the traits of biotas globally. Anthropogenic ignitions, however, have radically transformed worldwide fire regimes by extending fire seasons to the limits bounded by periods of high fuel moisture while limiting the occurrence and extent of fire during periods of peak fire potential. Disaggregating contemporary human- and lightning-driven seasonal fire patterns can shed light on the magnitude of anthropogenic change to worldwide fire regimes. Here, using daily fuel moisture records and associated flammability thresholds, we identify and describe the global distribution of bioclimatic fire seasons as limited by the availability of fuels to burn. We then use a record of lightning strike density to disaggregate these bioclimatic fire seasons into ignition periods driven by anthropogenic or mixed (that is, lightning present) ignitions. Finally, we assess our ignition periods against a daily satellite burned area record and contextualize the timing of burning against vegetation productivity and the human dimension of fire. Collectively, we show that anthropogenic influences have shaped worldwide fire regimes by lengthening potential fire seasons, irrespective of local land use, fire suppression practices, lightning occurrence and biome type.