| 20 May 2026
How abruptly did the Holocene Green Sahara end?
Abstract. Discussion about abrupt changes in the Sahara at the end of the African Holocene Humid Period has generally remained qualitative. Here we use a quantitative approach to characterize abruptness of vegetation changes in a transient simulation of the MPI (Max Planck Institute) Earth System model and proxy records (dust flux and pollen data) for the last 8000 years. In the simulations, we find the strongest long-term increase in simulated bare surface in a north-west to south-east oriented strip of some 400 km to 800 km in north-south direction between approximately 20°N in the west and 15°N in the east. Grid boxes in this region reveal abrupt, step-like transitions with changes in simulated bare surface, or conversely, in the total vegetation cover, which occur on average some 4 times faster than the approximately linear trend in the orbital forcing. The transitions in simulated grass and tropical tree plant functional types, i.e., in the type of the vegetation cover, appear to be up to twice as abrupt and up to twice as fast as the transition in the total vegetation cover. The reconstructed dust flux into the Atlantic shelf area at around 20°N, which we interpret as spatially aggregated changes in the openness of the landscape in the western Sahara, reveal a step-like transition with a much stronger abruptness than the simulated transition in bare surface in this region. The pollen records, however, indicate a rather gradual increase in desert-like conditions (with only one exception), but in many records, abrupt and fast transitions in Guinean, Sudanian and Sahelian phytogeographical groups occur. Our quantitative analysis thus confirms earlier propositions that there was no “collapse” of the green Sahara and that gradual changes in some proxy records and abrupt shifts in others are not necessarily contradictory. Our analysis also suggests that a gradual expansion of the Sahara during the mid-Holocene may have been accompanied by abrupt changes within the ecosystems.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Climate of the Past.
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