Previously, the vertical dependence of the efficiency with which BC exerts radiative forcing (RF) through the direct aerosol effect has been extensively studied. All model calculations agree that it strengthens by more than an order of magnitude between the surface and the top of the troposphere. The vertical dependences of the BC semidirect and indirect effects are however much poorer known, as are its full impact on temperature and precipitation.
In this talk, we summarize recent advances in the understanding of BC climate impacts as a function of altitude. We show simulations with the National Center for Atmospheric Research Community Earth System model, running CAM4 and CAM5, where a BC layer has been inserted at various model layers. Both direct and semidirect RF per gram BC is shown as a function of altitude, as is the resulting climate response in a slab ocean setup.
We find that while the efficiency of BC to exert positive RF due to the direct effect strengthens with altitude, as in previous studies, it is strongly offset by a negative semidirect effect. The net radiative perturbation of BC at top of atmosphere is found to be positive everywhere below the tropopause and negative above.
Further, we decompose the precipitation response to BC into a fast change related to atmospheric stability, and a slow, temperature-driven component. The global, annual mean precipitation response to BC, after equilibration of a slab ocean, is found to be positive between the surface and 900 hPa but negative at all other altitudes.
Finally, we set limits on the total climate impact of present day BC emissions, based on combinations of the above model results with measured concentration profiles.