The Role of Humans in Fire Dynamics: A Natural Experiment from the COVID-19 Lockdowns

Fire is an Earth-system disturbance that occurs in most terrestrial ecosystems and has widespread impacts on biogeochemical processes and human life. The geographic and temporal patterns of fire activity reflect a strong interplay of climatic, human, and vegetation factors. However, due to the complex interaction between human activities and climatic factors, the direction and magnitude of human direct impacts on fire remain poorly understood.

Taking advantage of the unique setting created by shelter-in-place orders during the coronavirus disease 2019 (COVID-19) pandemic, this study causally estimates global changes in fire occurrences due to reduced human activities in the first half of 2020 compared to average levels from 2016 to 2019. Utilizing global satellite observations of active fire detections, we constructed an aggregated global dataset at 0.5° resolution by week scales, complemented by corresponding meteorological measures, lockdown policies, and mobility indexes.

First, we assessed the average change in global fire incidence and further examined its variability across spatial, temporal, and intensity dimensions. Lockdown measures led to an average reduction of 11.8% in fire incidence worldwide. Notably, there was significant spatial heterogeneity in the direction and magnitude of human impacts on fire incidence, with changes in individual countries ranging from a 6.3-fold increase to a 5.6-fold decrease. Within groups of countries where fire incidence decreased or increased, lockdown measures exhibited contrasting temporal effects. Additionally, a greater reduction in human mobility intensified these effects in the respective directions.

Second, leveraging the attribute information of fire detection locations, we conducted separate group regression to understand the diverse pathways of human influence across landcover types, protected areas, human footprint levels, and proximity to wildland urban interfaces (WUI). Among the four landcover types, fire detections exhibited a more pronounced decline in forests and grasslands compared to shrublands/savannas and croplands. Fire within protected areas showed a larger decline on average but also experienced greater variability. A striking relationship is that areas with lower human footprint levels demonstrated a more substantial reduction in fire incidence, highlighting the critical role humans play in fire occurrences in undeveloped or low-developed lands. This relationship was further corroborated by the observed trends in relation to the distance from the nearest WUI.

Third, we examined the time-lagged effects of human activities on fire occurrences. Areas that experienced a significant reduction in fire incidents during the early lockdown stage tended to see a subsequent rebound in the later stage, thereby delaying the local fire season. Conversely, areas with increased fire detections in the early stage may experience later declines, leading to an earlier fire season. Our results indicate that reduced human activity can influence the accumulation or consumption of local fuels, consequently delaying or advancing the fire season relative to normal conditions.

Our study provides an empirical quantification of the direct effects of human activity on fire occurrences and highlights human contributions to the complex interactions between human, climate, and fire.