# Large-Scale Modeling of Absorbing Aerosols and Their Semi-Direct Effects
## 3 Results
### Radiative Effects
#### 3.2.2 Top of Atmosphere Radiative Effects
The positive forcing is mainly due to solar wavelengths (+0.19 Wm\({}^{-2}\) in the global mean), while the average radiative effect at the thermal infrared is negative (\(-\)0.1 Wm\({}^{-2}\)). For the 'high BC' sensitivity case GFAS the semi-direct effect is much higher, with 0.26 Wm\({}^{-2}\) in the global mean. This high value is mostly controlled by positive forcing in the tropical regions of Africa and South America (Figure 4d) where high biomass burning emissions contribute to aerosol absorption. The overall BC SDE forcing is usually estimated to be negative in the global annual mean (e.g., [2]), in this model study this is not the case even for the total aerosol effect SDE is positive in the global annual average where SDE (all aerosol) = (+0.08 \(\pm\) 0.47) Wm\({}^{-2}\). This is within the range of earlier studies that evaluate the semi-direct effect for anthropogenic aerosols., which are summarized in [PERSON] and [PERSON] [14] to be between \(-\)0.4 and +0.1 Wm\({}^{-2}\). [PERSON] and [PERSON] [23] find \(-\)0.1 Wm\({}^{-2}\) as globally averaged semi-direct aerosol effect, and [PERSON] et al. [22] derive global averaged net semi-direct effects of \(-\)0.09 or +0.081 Wm\({}^{-2}\) for aerosol represented by seven or three modes in the model, respectively. The regional distribution of the positive effect is evident in the biomass burning regions in the Sahel and South American regions, but also over parts of the Sahara desert for the solar part of the spectrum (not shown). This overall positive SDE agrees with an overall reduction of cloud cover. It is however comparably small and highly variable, with (\(-\)0.25 \(\pm\) 0.24)% cloud cover change for the global mean in the BASE case. Again, the global average values of the effect of the absorbing aerosol are due to averaging of both positive and negative values for cloud cover differences. The SDE is the consequence of changes in cloud cover due to changes in atmospheric heating rates by aerosol absorption. This relationship is evident in the scatter plot shown in Figure 5, where the solar part of the SRE of BC is shown in dependence of the cloud cover differences between the model experiments BASE and noBC. Decreasing cloud cover is related to a positive radiative forcing due decreased backscatter of incoming solar radiation. The linear regression coefficient between the changes in SDE to cloud cover is (\(-\)1.1 Wm\({}^{-2}\))/(% cloud cover) (r\({}^{2}\) = 0.69). The instantaneous direct radiative effect is positive (Figure 4b), with high positive values evident over the Sahara and North Atlantic. While over the Sahara the positive BC forcing is possibly due to the high surface albedo, the positive forcing of the BC over the Atlantic downwind of the Sahara is partly due to the reflectivity of underlying mineral dust aerosol. Due to the BC radiative effect the dust emission itself is slightly decreased from 1590 Mt/year to 1530 Mt/year when BC forcing is included. Thus BC forcing reduces dust emissions (and in consequence dust load and AOD, decreasing the negative dust radiative effect at TOA) by about 4%. This change is too small to explain the positive TOA BC forcing over the tropical North Atlantic. Instead, the comparably highly reflecting dust aerosol caused an additional positive radiative effect compared to the case when dust aerosol is excluded. Excluding dust aerosol from the model (simulation noDUST and noABS, which are similar to BASE and noBC, but excluding mineral dust) leads to a lower radiative effect of BC, which is evident in the difference between experiments noDust and noAbs (Figure 6, Table 2). Here, the effect of BC is considered that would occur if all mineral dust was removed from the atmosphere. Positive BC forcing is reduced to from +0.51 Wm\({}^{-2}\) to +0.24 Wm\({}^{-2}\) for DRE and from +0.42 Wm\({}^{-2}\) to +0.27 Wm\({}^{-2}\) for IDR, and the SDE is actually negative in the global average with \(-0.03\) Wm\({}^{-2}\).