Seasonal skew of tropical savanna fires

Tropical savannas and grasslands are the most frequently burned biome in the world, and fire constitutes an important part of the ecosystem. In this ecosystem it can have both rejuvenating and destructive effects, depending on several factors including fuel conditions, weather conditions, and time of year. For centuries humanity has used fire in these landscapes for hunting, land clearance, agriculture, and most recently carbon offsetting. Land managers in locations with a monsoonal climate and frequent fire regimes such as tropical savannas use prescribed burning as a management tool in the ‘early dry season’ (EDS) shortly after the last rains of the year. Fires at this time tend to be cooler, restricted to surface level and less severe, meaning they can be controlled more easily and tend to go out at night without external input. Commonly a specific, fixed date is used to indicate when this window of safe burning has expired, set based on experience of the local or regional authority. In this work, we have defined a method of determining when this window expires on the basis of active fire hotspot data from the twin MODIS instruments from 2001 through to 2021. By using the relationship between day and night-time active fire detections, we set a flexible date for the transition between the early and late dry seasons in fire-prone savannas globally in the five major tropical savanna regions - Northern & Southern hemisphere South America (NHSA & SHSA), Northern & Southern hemisphere Africa (NHAF & SHAF), and Australia (AUST). The variability across each region was high (lowest mean standard deviation annually was 24 days in NHAF and highest was 56 in AUST). The fraction of area burned in the late dry season ranged from 15% (SHSA) to as high as 85% (AUST) on average, with many parts of Africa and Australia especially showing a significant skew towards the late dry season. This suggests potential for implementation of prescribed burning programmes to increase the amount of desirable fire in the global savanna ecosystems.