Bimodal fire regimes unveil a global-scale anthropogenic fingerprint

Aim: While fire is recognized as an integral part of the Earth system, the ability of humans to shape fire regimes both spatially and temporally remains poorly understood. Our goals were to identify the extent of fire regimes exhibiting two annual fire seasons and to investigate the environmental correlates of such regimes at the global scale. Location: All areas of the globe exhibiting relevant fire activity, at 0.5 degrees spatial resolution. Time period: 2002-2012. Major taxa studied: (not applicable). Methods: The modality of fire seasonality at the global scale was classified using a 10-year record of satellite-derived fire activity and model fitting of circular statistical distributions. The main environmental correlates controlling global fire regimes were then analysed over bimodal and unimodal areas using the Kolmogorov-Smirnov test. Results: About 25% of the global land surface with relevant fire activity has two significantly distinct fire seasons per year, with at least one of these seasons occurring under sub-optimal fire weather conditions. In these bimodal areas, population density and the fraction of fires occurring in actively managed land, especially in croplands and pastures, are significantly higher than in neighbouring unimodal areas. Results reveal that through these land-use and management practices humans have a strong influence on global patterns of fire seasonality. Main conclusions: We identified a bimodal seasonality pattern, previously unreported at the global scale, and show that it reveals an anthropogenic fingerprint on fire regimes. Insights into where and when fire is actively employed as a land management tool enhance our understanding of the role of fire in the Earth system, and highlight the need to better understand how fire practices may change in the future.