Abstract
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 9 global coupled-climate models, producing a model-median effective radiative forcing (ERF) 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 hPa, 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 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.
Highlights
As a strong absorber of shortwave radiation, black carbon (BC) emitted to the atmosphere has an influence on global and regional climate [Bond et al, 2013]
The globally averaged change in BC burden varies among the models, with CanESM2 [E] and HadGEM2-ES [E] showing the largest changes and CESM-CAM5 [E] showing the smallest change (Fig. 2 and Table 1)
While the very strong climate responses HadGEM2-ES [E] and CanESM2 [E] can at least partly be a result of climate feedbacks on BC concentrations, CESM-CAM5 [E] and MIROC-SPRINTARS [E] have climate responses that are more similar to the concentration-driven models
Summary
As a strong absorber of shortwave radiation, black carbon (BC) emitted to the atmosphere has an influence on global and regional climate [Bond et al, 2013]. The semi-direct effect can have opposite signs depending on where the BC is located in relation to the altitude of the cloud layer [Koch and Del Genio, 2010]. This means that inter-model variation in both BC and cloud fields will greatly influence estimates of this effect, as found for instance in Hodnebrog et al [2014]. Samset and Myhre [2015] found substantial inter-annual differences in semi-direct RF in simulations using the CESM-CAM4 model, primarily due to differences in cloud fraction, and pointed to the importance of cloud field representation and location relative to BC for estimates of the semi-direct effect. Chen and Penner [2005] identify aerosol burden and cloud fraction to be the most important sources of model disagreement in estimates of indirect aerosol effects
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