Human-caused fires limit rainfall in tropical Africa

Abstract:

It is well established that smoke particles modify clouds, which in turn affects climate. However, no studies have quantified the temporal dynamics of aerosol-cloud interactions with direct observations. Here, for the first time, we use temporally-offset satellite observations from northern Africa between 2006 and 2010 to quantitatively measure the effect of fire aerosols on convective cloud dynamics. We attribute a reduction in cloud fraction during periods of high aerosol optical depths to a smoke-driven inhibition of convection. We find that higher smoke burdens limit upward vertical motion, increase surface pressure, and increase low-level divergence -- meteorological indicators of convective suppression. The resulting decrease in rainfall is substantial and may increase human ignitions during the dry season. These results are corroborated by climate simulations that show a smoke-driven increase in regionally averaged shortwave tropospheric heating and decrease in convective precipitation during the fire season. Our results suggest that, in tropical regions, anthropogenic fire initiates a positive feedback loop where increased aerosol emissions limit convection, dry the surface and enable increased fire activity via human ignition.