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Afforested Sahara in Northern Africa?

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Atmospheric Feedbacks from an Irrigated, Afforested Sahara in Northern Africa
Tronje Peer Kemena1 Katja Matthes1,2 Thomas Martin1 Sebastian Wahl1 Andreas Oschlies1,2

Afforestation of the Sahara has been proposed as a climate engineering method to sequester a substantial amount of carbon dioxide, potentially effective to mitigate climate change. Earlier studies predicted changes in the atmospheric circulation system. These atmospheric feedbacks raise questions at the self- sustainability of such an intervention, but have not been investigated in detail. Here, we investigate changes precipitation and circulation in response to Saharan large-scale afforestation and irrigation with NCAR’s CESM-WACCM Earth system model.
Model results show a Saharan temperature reduction by 6 K and weak precipitation enhancement by 267 mm/yr over the Sahara. Only 26% of the evapotranspirated water re-precipitates over the Saharan Desert, considerably large amounts are advected southward to the Sahel zone and enhance the West African monsoon (WAM).

Different processes cause circulation and precipitation changes over North Africa. The increase in atmospheric moisture leads to radiative cooling above the Sahara and increased high-level cloud coverage as well as atmospheric warming above the Sahel zone. Both lead to a circulation anomaly with descending air over the Sahara and ascending air over the Sahel zone. This leads together with changes in the meridional temperature gradient to a southward shift of the inner tropical convergence zone. The strengthening of the Tropical easterly jet and the northward displacement of the African easterly jet is associated with a northward displacement and strengthening of the WAM precipitation.

Our results promote complex atmospheric circulation feedbacks, which reduce the precipitation potential over an afforested Sahara and enhance WAM precipitation.