Valley formation aridifies East Africa and elevates Congo Basin rainfall

East African aridification during the past 8 million years is frequently invoked as a driver of large-scale shifts in vegetation(1) and the evolution of new animal lineages, including hominins(2-4). However, evidence for increasing aridity is debated(5) and, crucially, the mechanisms leading to dry conditions are unclear(6). Here, numerical model experiments show that valleys punctuating the 6,000-km-long East African Rift System (EARS) are central to the development of dry conditions in East Africa. These valleys, including the Turkana Basin in Kenya, cause East Africa to dry by channelling water vapour towards Central Africa, a process that simultaneously enhances rainfall in the Congo Basin rainforest. Without the valleys, the uplift of the rift system leads to a wetter climate in East Africa and a drier climate in the Congo Basin. Results from climate model experiments demonstrate that the detailed tectonic development of Africa has shaped the rainfall distribution, with profound implications for the evolution of African plant and animal lineages.

Automated summary

The aridification of East Africa during the past 8 million years is often considered as a driving force behind changes in vegetation and the evolution of new animal lineages, including hominins. This study uses numerical model experiments to show that the valleys in the East African Rift System play a central role in creating dry conditions in East Africa. These valleys channel water vapor towards Central Africa, resulting in increased rainfall in the Congo Basin rainforest and drier conditions in East Africa. The findings suggest that the tectonic development of Africa has shaped rainfall distribution and influenced the evolution of African plant and animal lineages.