15b) reveals partly positive values of the solar cloud radiative forcing. This effect can be seen in situations where the direct part of solar irradiance is not attenuated, while diffuse radiation is enhanced by scattering processes in surrounding clouds.
Solar radiations are generally scattered and absorbed by aerosols causing direct radiative forcing which warms up the planet and indirect forcing through their important role in cloud physics causing planet to cool down.
The clear sky total (direct + diffuse) aerosol radiative forcing at the surface and top of the atmosphere was calculated in the shortwave (0.2-4.0 [micro]m) region using the SBDART model, developed by Ricchiazzi et al.
Since RCP 2.6 is supposed to be a stabilization scenario with a target radiative forcing of 2.6 W[m.sup.-2] or equivalently with a total GHG (plus aerosol effects) concentration of 450 ppm C[O.sub.2]-eq, we assume that the radiative forcing of other GHGs and aerosols is kept constant at 0.65 W[m.sup.-2] and the remainder is due to [CO.SUB.2].By using an approximate formula (13) for the radiative forcing of C[O.sub.2]
In this article, we used the results from 19 state-of-the-art climate models to span the largest possible range of intermodel differences and the uncertainties of GHG radiative forcing. We also used recent empirical studies to narrow the uncertainty range associated with temperature-diarrhea regression coefficients in WHO04.
Recently however, despite the relatively lower contributions of air transport to the overall anthropogenic radiative forcing, aviation is under unprecedented pressure to reduce its environmental footprint per passenger kilometers.