We investigate the climate response to increased concentrations of black carbon (BC), as part of the Precipitation Driver Response Model Intercomparison Project (PDRMIP). A tenfold increase in BC is simulated by nine global coupled-climate models, producing a model median effective radiative forcing of 0.82 (ranging from 0.41 to 2.91) Wm(-2), and a warming of 0.67 (0.16 to 1.66) K globally and 1.24 (0.26 to 4.31) K in the Arctic. A strong positive instantaneous radiative forcing (median of 2.10 Wm(-2) based on five of the models) is countered by negative rapid adjustments (-0.64 Wm(-2) for the same five models), which dampen the total surface temperature signal. Unlike other drivers of climate change, the response of temperature and cloud profiles to the BC forcing is dominated by rapid adjustments. Low-level cloud amounts increase for all models, while higher-level clouds are diminished. The rapid temperature response is particularly strong above 400 h Pa, where increased atmospheric stabilization and reduced cloud cover contrast the response pattern of the other drivers. In conclusion, we find that this substantial increase in BC concentrations does have considerable impacts on important aspects of the climate system. However, some of these effects tend to offset one another, leaving a relatively small median global warming of 0.47 K per Wm(-2)-about 20% lower than the response to a doubling of CO2. Translating the tenfold increase in BC to the present-day impact of anthropogenic BC (given the emissions used in this work) would leave a warming of merely 0.07 K.

The climate response to the presence of black carbon (BC) aerosol at a given altitude in the atmosphere is investigated using a global circulation model. The vertical dependence of the efficiency with which BC exerts radiative forcing (RF) through the direct aerosol effect has previously been extensively studied. Here we use the Community Atmosphere Model version 4 atmospheric component of the National Center for Atmospheric Research Community Earth System model version 1.03 to calculate the three-dimensional response to a BC layer inserted at various altitudes. Simulations have been performed both for fixed sea surface temperatures and using a slab ocean setup to include the surface temperature response. We investigate the vertical profiles of RF exerted per gram of externally mixed BC due to both the direct and semidirect aerosol effects. Associated changes in cloud cover, relative humidity, and precipitation are discussed. The precipitation response to BC is decomposed into a fast, stability-related change and a slow, temperature-driven component. 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. The global, annual mean precipitation response to BC, after equilibration of a slab ocean, is found to be positive between the surface and 900hPa but negative at all other altitudes.

The semidirect effect of black carbon (BC) is studied by using a newly proposed optical property parameterization for cloud droplets with BC inclusions. Based on Atmospheric Model Intercomparison Projecttype climate model simulations, it is found that the cloud amount can be either enhanced or reduced when BC is included in clouds. The decrease of the global annual mean total cloud amount is only about 0.023%. The 3D cloud fraction distribution, however, shows larger changes which vary with latitude. A correlation between the changes of the cloud fraction and the vertical velocity is found. The cloud water path is mainly affected by low clouds and so the impact of BC on the cloud water path is particularly strong. It is shown that the BC above clouds tends to stabilize the atmosphere and enhance the cloud amount in the boundary layer. This can be used to explain the relationship between aerosol optical depth and cloud amount according to satellite data. For BC in clouds and above, the global annual mean enhancement of solar absorption is about 0.049 W m(-2) and 0.57 W m(-2), respectively. The BC semidirect radiative forcing is estimated by subtracting the BC direct forcing from the BC total radiative forcing. The global annual mean of BC direct forcing and semidirect forcing at the top of the atmosphere are 0.264 W m(-2) and 0.213 W m(-2), respectively.