# Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects
## 4 Conclusions  
Global semi-direct effects of BC aerosol from fossil fuel and biomass burning were simulated as residuals of the total direct radiative effect and the instantaneous direct forcing using the  
Figure 8: (**a**) Simulated 8-year averaged differences in cloud cover in the European region in July resulting from the BC radiative effect; and (**b**) differences in dust optical thicknesses in eight-year average February conditions. Both results are based on the differences of results for BASE and noBC. ECHAM6-HAM2 global aerosol climate model. While the resulting semi-direct effect as consequence of changes in cloud cover was overall positive, the SDE only about 20% compared to the IDR. The results fall within the range of earlier results on the overall semi-direct effects of anthropogenic aerosols, while here we focused on absorbing aerosols from both anthropogenic and natural sources. The global average values of the effect of the absorbing aerosol are small due to averaging of both positive and negative values for radiative effects. The most robust regional feature for the semi-direct effect of absorbing aerosol is the positive effect evident in the biomass burning regions in the Sahel and central Africa where positive forcing of more than 5 Wm\({}^{-2}\) occurs. This positive SDE is related to a reduction of cloud cover. Elsewhere, the semi-direct aerosol effect may be regionally positive or negative, and varies for different model setups. The presence of dust impacts on the positive aerosol effects where BC from biomass smoke is transported above mineral dust-containing air layers. In effect, the reflective dust leads to an albedo increase of otherwise dark ocean surfaces and thus an enhancement of the positive radiative effect in regions downwind of major dust sources. This is for example the case in the tropical Atlantic region, where both biomass smoke and Saharan dust plumes are transported off the African continent in the northern hemisphere winter months. In this model study about 40% of the positive BC global annual mean IDR can be attributed to the presence of dust aerosol. Since as known from paleoclimate records dust loads may considerably change in different climates, possible future increases or decreases in dust export from desert regions due to changes in vegetation cover or dust-producing wind system may have considerable impacts on TOA forcing by absorbing BC aerosol. The results on the semi-direct aerosol effects may be dependent on the model resolution. Not only cloud cover, but also dust aerosol that is sensitive to surface winds, may change at higher model resolutions. In particular it cannot be excluded that the response to heating rate changes of cloud coverage is dependent on model resolution. The repetition of the model experiments with resolution of T106 or higher is beyond the scope of this study, but should be addressed in future work. As a substitute for such high-resolved global model investigations the results of individual earlier regional studies using a spatial resolution of 28 km show surprising agreement in specific features presented such as the positive semi-direct aerosol effect in Europe in the summer months. Overall there is cancellation of positive and negative effects resulting from different responses of cloud cover in different regimes. Additionally, the model variability is high, which makes the results uncertain. An improvement of the significance of the results would be desirable by making longer simulations to reduce those uncertainties. The global model simulations were performed with fixed sea surface temperatures as boundary condition. To understand the response of the full climate system to the heating rate changes by the absorbing aerosols, simulations with a fully coupled aerosol-climate Earth system model should be performed that includes consideration of sea surface temperature changes. Still, the results provide insights into the rapid adjustment of atmospheric conditions to absorbing aerosol forcing. Conceptualization: I.T. and B.H.