Radiative Forcing Uncertainty of Black Carbon
Black carbon (BC), from incomplete combustion of fossil fuels and biofuels, is a strongly absorbing component of atmospheric aerosols that warms the atmosphere due to its absorbing properties. However, uncertainties in BC sources and properties have made it challenging to simulate, and uncertainties about its effects on climate persist. Researchers, including U.S. Department of Energy scientists at Pacific Northwest National Laboratory, conducted BC simulations using 12 different global models. The team found that at least 20% of the diversity in BC’s direct radiative forcing estimated by global aerosol models is due to differences in the simulated vertical profile of BC mass. A significant fraction of the variability comes from high altitudes, as more than 40% of the total BC radiative forcing is exerted above 5 km. The efficiency with which BC can induce radiative forcing depends on external factors, such as surface albedo, water vapor, background aerosol distributions, and, most notably, the vertical distribution of clouds. Therefore, BC above bright areas such as clouds has an enhanced positive radiative forcing (absorption effect). By combining the models’ own concentration profiles with a common 4D (spatial and temporal) efficiency profile of radiative forcing per gram of BC, the team recalculated and compared the exerted radiative forcing of BC’s direct effect at various altitudes and spatial regions. This study on the importance of BC’s vertical profile suggests that observational studies are needed to better characterize its global distribution, including in the upper troposphere.
Samset, B. H., G. Myhre, M. Schulz, Y. Balkanski, S. Bauer, T. K. Berntsen, H. Bian, N. Bellouin, T. Diehl, R. C. Easter, S. J. Ghan, T. Iversen, S. Kinne, A. Kirkevåg, J. F. Lamarque, G. Lin, X. Liu, J. Penner, Ø. Seland, R. B. Skeie, P. Stier, T. Takemura, K. Tsigaridis, and K. Zhang. 2013. “Black Carbon Vertical Profiles Strongly Affect Its Radiative Forcing Uncertainty,” Atmospheric Chemistry and Physics 13, 2423–34. DOI: 10.5194/acp-13-2423-2013.