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Nearly 100 million people live in and depend on the Sahel for agriculture and natural resources. The region is sensitive to natural climate and environment variations caused by the seasonal movement of the tropical rainbelt. In the paleoclimate record, insolation plays a clear role on West African Monsoon strength, but responses to other forcings like temperature, greenhouse gases, ice volume, and land surface cover are unclear due to the lack of highly resolved, terrestrial records that span major global and regional shifts through time. Here we present leaf wax precipitation and vegetation records from several targeted study windows throughout the last 25 million years, derived from long-chain n-alkane hydrogen (δDwax) and carbon (δ13Cwax) isotopes, respectively, in a sediment core from ODP Site 959 in the Gulf of Guinea, where westerly winds and major river systems transport Western Sahel-sourced material. Analyses of trend and variability document a range of rainfall and vegetation responses to orbital forcings in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene. We find that both the climate and environment was more variable in times of higher CO2 and global temperatures, suggesting an increase in ecosystem instability moving forward into the future. Because of the high resolution and temporal coverage of these new biomarker isotope records, we can examine relationships between precipitation and vegetation fluctuations, even prior to C4-expansion when there was a strong correlation despite minimal variation in δ13Cwax in a C3 world. Further, we find a wetting trend throughout the record, demonstrating that vegetation on long timescales was decoupled from hydroclimate and that the terrestrial ecosystem may face aridification, contradicting some model projections. 

Wildfires are essential to terrestrial ecosystems, playing a crucial role in nutrient and carbon cycles, particularly in highly seasonal environments like the Western Sahel. Their occurrence is linked to complex feedback mechanisms between climate, landscape structure, vegetation and the carbon cycle. It is therefore central to understand wildfire dynamics in the context of paleoclimatic and environmental change. Here we present a record of 3 to 7 ringed polyaromatic hydrocarbons (PAHs), from five targeted study windows throughout the last 25 million years from ODP Site 959 in the Gulf of Guinea. The time windows target the effects of orbital forcings of the West African Monsoon on wildfire and vegetation responses in different boundary conditions in the Oligocene, Miocene, Pliocene, and Pleistocene, including shifts in global temperatures, greenhouse gas concentrations, and regional land surface. Orbitally resolved PAH biomarkers can provide insight into fire activity and be coupled with changing precipitation patterns and biomes. We discuss PAH sources and how wildfire frequency is linked to the observed drying trend, climate variability, and vegetation expansion throughout the Cenozoic in the Western Sahel. These findings are central for understanding future wildfire dynamics in the vulnerable Western Sahel region in the light of global warming. 

The recent decline in Horn of Africa rainfall during the March–May “long rains” season has fomented drought and famine, threatening food security in an already vulnerable region. Some attribute this decline to anthropogenic forcing, whereas others maintain that it is a feature of internal climate variability. We show that the rate of drying in the Horn of Africa during the 20th century is unusual in the context of the last 2000 years, is synchronous with recent global and regional warming, and therefore may have an anthropogenic component. In contrast to 20th century drying, climate models predict that the Horn of Africa will become wetter as global temperatures rise. The projected increase in rainfall mainly occurs during the September–November “short rains” season, in response to large-scale weakening of the Walker circulation. Most of the models overestimate short rains precipitation while underestimating long rains precipitation, causing the Walker circulation response to unrealistically dominate the annual mean. Our results highlight the need for accurate simulation of the seasonal cycle and an improved understanding of the dynamics of the long rains season to predict future rainfall in the Horn of Africa.