Remote impacts of the mid-Holocene Green Sahara

The mid-Holocene (MH: 6,000 years before present) is a key time slice for paleoclimate studies, and is one of the two entry cards for participation in the current Paleoclimate Modelling Intercomparison Project (PMIP4). The MH was characterized by high boreal summer insolation, leading to an intensification of the Northern Hemisphere monsoons. In northern Africa, the strengthening of the West African Monsoon was further amplified by nonlinear feedbacks, resulting in the development of vegetation referred to as the “Green Sahara”. The vegetation and land surface changes over northern Africa had various remote effects impacting the global climate through teleconnections.

In this study, we analyse outputs from five fully coupled global climate models to identify the  remote impacts of the Green Sahara on global climate. Through the difference of two sets of mid-Holocene simulations – with and without the Green Sahara – we isolate the effect of the northern African vegetation and land cover changes on South American hydroclimate and tropical modes of climate variability such as the El Niño Southern Oscillation and the Atlantic Niño. Using an atmosphere-only climate model, we further investigate the Saharan-Arctic teleconnection invoked to explain the Arctic cooling concurrent with Saharan desertification. We quantify proxy-model agreement through metrics such as the Cohen’s Kappa index and the Root Mean Square Error to assess if the inclusion of the Green Saharan changes leads to greater coherence of model simulations with proxy reconstructions. Our results demonstrate the critical role of the Green Sahara in modulating the MH climate.