10 October 2016

Soot from Biomass Burning Affects Clouds, Increasing Drought over Africa

Scientists find that carbonaceous aerosols could suppress regional rainfall over tropical Africa through the aerosol-cloud interaction mechanism.

Science

Subtropical North Africa has been subject to extensive droughts in the late 20th century, linked to changes in the sea surface temperature. Aerosol indirect effects also appear to play an important role in suppressing regional rainfall over tropical Africa.

Impact

Between 1950 and 1980, the Sahel region, a large swath of the African continent located between the tropical rainforests and the Saharan desert, has experienced a large reduction in rainfall.  Various factors affecting climate have been suggested as the driver of this drastic change in Sahel rainfall.  This research points to carbonaceous aerosols, carbon-containing particles emitted from biomass burning, as one mechanism that suppresses regional rainfall through their interactions with clouds.

Summary

Subtropical North Africa has been subject to extensive droughts in the late 20th century, a phenomena often linked to changes in the sea surface temperature. However, climate model simulations that are forced by observed SSTs cannot reproduce the magnitude of the observed rainfall reduction. Scientists at the Department of Energy’s Pacific Northwest National Laboratory and collaborators show that aerosol indirect effects on clouds provide an important positive feedback mechanism that increases agreement with rainfall changes. Model results from two sets of sensitivity experiments were designed to distinguish the role of aerosol direct/semi-direct and indirect effects on regional precipitation. The results indicate that changes in cloud properties because of the presence of carbonaceous aerosols are a key mechanism to explain the reduced rainfall over subtropical North Africa.

Contact
Philip J Rasch
Pacific Northwest National Laboratory (PNNL)
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