Mid-Holocene climate at mid-latitudes: modelling the impact of the Green Sahara

During the mid-Holocene (6,000 years ago) the Northern Hemisphere experienced a reinforcement of the monsoonal regime, which led to the so-called “African Humid Period” (AHP) and to the greening of the Sahara region. Paleoclimate archives also show a gradual cooling of north-eastern Atlantic and the warming of the western subtropical Atlantic, eastern Mediterranean and northern Red Sea during the Holocene. These changes were likely accompanied by a positive-to-negative transition of the AO/NAO phase from mid-late Holocene into the pre-industrial period, leading to climate impacts in North America, Europe, the Mediterranean and Siberia.

However, inconsistencies still exist between proxies and model simulations of the Holocene climate. To explain the limitations of climate models, several studies pointed out the role of the vegetation feedback at tropical and higher latitudes. The objective of this study is to investigate the impact of the Green Sahara on the Northern Hemispheric mid-latitude atmospheric circulation and associated climate variability during the African Humid Period. To this aim, vegetated Sahara with reduced dust emission is prescribed into a coupled ocean-atmosphere model (the Green Sahara experiment).

Model simulations show a sizable impact on the main circulation features in the Northern Hemisphere when the Green Sahara is prescribed, especially during boreal summer, when the African monsoon develops. This study provides a first constraint on the Green Sahara influence on northern mid-latitudes, indicating new opportunities for understanding mid-Holocene climate anomalies in North America and Eurasia. However, inconsistencies between proxies and model simulations still persist in the Green Sahara experiment, indicating that more accurate simulations of the MH climate modifications are needed (e.g. prescribing realistic vegetation at mid and high latitudes, considering seasonal cycle in vegetation cover).