Elevational dependence of global forest fires and associated aerosol optical depth: drivers and decoupling

Abstract. Forest fires have become an escalating environmental and ecological issue worldwide over the past decades. However, a knowledge gap persists in globally assessing how topography modulates wildfire behavior. Here we quantify global spatiotemporal patterns of forest fire activity and associated aerosol optical depth (AOD), together with elevation-dependent controls, using satellite observations from 2012 to 2024. The analysis reveals a slight yet significant increase in fire occurrence, accompanied by a strong positive association with fine-mode AOD (FAOD). In contrast, coarse-mode AOD (CAOD) shows little response, implying that wildfire emissions mainly contribute to the fine aerosol fraction. Forest fire occurrence declines systematically with elevation, with most fires concentrated below 600 m. In contrast, FAOD exhibits elevated mean values and increasing trends at mid-elevations (600–1400 m), revealing a decoupling between fire frequency and aerosol loading. This divergence is consistent with shifts in forest-type composition and topographically modulated smoke transport, including aerosol self-lifting driven by radiative absorption and atmospheric convection. Elevation-stratified multiple linear regression analyses incorporating the Fire Weather Index, leaf area index, temperature, wind speed, and precipitation indicate that fire activity is primarily governed by fuel availability and aridity. Precipitation exerts a consistent suppressive effect across elevations, while wind speed enhances fuel drying and fire spread at mid-elevations. Overall, these results identify elevation as a key organizing factor linking forest fires, aerosol emissions, and their underlying drivers, providing new constraints for wildfire risk assessment and fire–aerosol interactions under a changing climate.