| 23 Feb 2026
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.
Wildfires have become an escalating environmental and ecological concern worldwide over recent decades. This study uses satellite observations to investigate the influence of elevation on global forest fire activity and the associated aerosol optical depth (AOD). The results reveal a decoupling between fire frequency and aerosol loading, characterized by a decline in forest fire occurrence with elevation, while FAOD exhibits elevated mean values and increasing trends at mid-elevations. This divergence is attributed to variations in forest-type composition and topographically modulated smoke transport. In addition, elevation-stratified multiple linear regression analyses suggest that fuel availability and aridity are the primary factors controlling forest fire activity at mid-elevations. Overall, the manuscript is well organized and clearly presented, and the observational analyses and interpretations are generally convincing. The topic is relevant to understanding fire–aerosol interactions and their dependence on topographic factors. The paper could be considered for publication after the following issues are addressed.
1. In this study, AOD is decomposed into FAOD) and CAOD to analyze their relationships with forest fire activity. However, the manuscript does not sufficiently explain the scientific rationale and significance of this separation. The authors should clearly clarify the motivation and importance of distinguishing FAOD and CAOD in the context of wildfire–aerosol interactions. A brief explanation in the methodology or introduction or discussion would help readers better understand why this decomposition is necessary.
2. The manuscript reports no significant correlation between CAOD and forest fire pixel counts. However, several previous studies have suggested that wildfire activity can be associated with increases in dust aerosols, which contribute to coarse-mode optical depth. Therefore, the authors should discuss this issue in more detail. In particular, it would be helpful to explain why such relationships are not detected in the present analysis.
3. FWI is widely used as a representative indicator of meteorological conditions favorable for wildfire occurrences. However, the elevation-dependent regression analysis in this study indicates a relatively weak independent relationship between FWI and forest fire activity. This result is somewhat unexpected and deserves further discussion. The authors should provide additional explanation for this finding.
4. In Line 357, the manuscript states that “fewer than 50 grid cells above 1500 m exhibiting statistically significant trends may compromise the representativeness and accuracy of the computed global FAOD trend.” Later in the manuscript, it is mentioned that “the limited number of grid cells (<10) between 1300 and 2100 m reduces statistical robustness.” The criteria used here appear inconsistent. The authors should clarify why different thresholds (50 vs. 10 grid cells) are used in these two contexts and explain the reasoning behind these choices.
5. In Section 2.5 (Correlation and multivariate regression analysis), the manuscript indicates that linear regressions are applied to individual 2° × 2° grid cells, while the elevation-based analysis uses variables resampled to 1° × 1° grids. The use of different spatial resolutions in related analyses may introduce inconsistencies. The authors should clarify the rationale for using two resolutions and discuss whether the results are sensitive to this choice. If possible, using a consistent spatial resolution across analyses would improve methodological clarity.
6. Several minor formatting issues should be corrected. For example: Line 217: “two-tailed t test”should be formatted as “two-tailed *t* test” (with the “t” in italics). Line 453: “+2.89 % yr⁻¹”should be formatted as “+2.89% yr⁻¹”.