Changes in biomass burning in Africa since the last glacial maximum: a new continental-scale paleo-synthesis and interrogation of the climatic and human drivers of shifting fire regimes

Fires on Earth are changing in response to human activities, both through direct ecosystem management and indirect climate change-induced warming and associated shifts in regional rainfall patterns. While increased burning in forested systems often captures international media attention, the decline in burning in grassy systems––especially African savannas––receives less focus, despite their dominant contribution to total global burned area and fire emissions. Forecasting future fire activity and its impacts on local ecology and livelihoods, as well as global climate feedbacks, requires a robust mechanistic understanding of the complex interactions between climatic conditions, ecosystem functioning, human activities, and fire across a range of climate states not captured by modern satellite-based observations.

This study focuses on Africa, whose environments span a diversity of climates and ecologies, from some of the driest and most sparsely vegetated regions on Earth (such as the Sahara) to some of the wettest and most biologically productive (such as the Congo Rainforest). These two ends of the rainfall gradient experience non-existent to infrequent burning. However, the most expansive biomes in Africa are tropical savannas and grasslands where precipitation is intermediate and highly seasonal, supporting rapid vegetation growth during wet seasons and drying and abundant fires in the dry season. As a result, burning in Africa constitutes more than half of all global burned area each year. Robust histories of how fires have changed in Africa through time are therefore essential to understanding changes in biomass burning at a global scale. In addition to its broad scope of environments and outmatched contributions to total global burning, Africa also has the longest history of human fire use and land-use change, making it an ideal testing ground for interrogating the combined roles climate shifts and human behaviors play in shaping fire regimes through time.

Here, we present a new synthesis of African paleofire activity inferred from the accumulation of both physical and molecular proxies (e.g., charcoal and polycyclic aromatic hydrocarbons) within climate archives spanning multiple depositional contexts (e.g., lacustrine, marine, and peat sediments) which record biomass burning across a host of ecosystems. Our new reconstruction spans the last 24 thousand years, within which we focus on four key time periods: the Last Glacial Maximum and deglaciation, the mid-Holocene African Humid Period, the late-Holocene rise of metallurgy and agriculture, and the post-industrial era. We evaluate trends in biomass burning during these intervals, and, by comparison to paleoclimate and archeological datasets, we assess the extent to which these patterns are driven by climatic and/or human influences at continental, regional, and biome scales.