A Standardized Global Climate Model Study Showing Unique Properties for the Climate Response to Black Carbon Aerosols

M. Sand, T. Iversen, P. Bohlinger, A. Kirkevåg, I. Seierstad, Ø. Seland, A. Sorteberg
2015 Journal of Climate  
The climate response to an abrupt increase of black carbon (BC) aerosols is compared to the standard CMIP5 experiment of quadrupling CO 2 concentrations in air. The global climate model NorESM with interactive aerosols is used. One experiment employs prescribed BC emissions with calculated concentrations coupled to atmospheric processes (emission-driven) while a second prescribes BC concentrations in air (concentration-driven) from a precalculation with the same model and emissions, but where
more » ... e calculated BC does not force the climate dynamics. The difference quantifies effects of feedbacks between airborne BC and other climate processes. BC emissions are multiplied with 25, yielding an instantaneous top-of-atmosphere (TOA) radiative forcing (RF) comparable to the quadrupling of atmospheric CO 2 . A radiative kernel method is applied to estimate the different feedbacks. In both BC runs, BC leads to a much smaller surface warming than CO 2. Rapid atmospheric feedbacks reduce the BC-induced TOA forcing by approximately 75% over the first year (10% for CO 2 ). For BC, equilibrium is quickly re-established, whereas for CO 2 equilibration requires a much longer time than 150 years. Emissiondriven BC responses in the atmosphere are much larger than the concentration-driven. The northward displacement of the intertropical convergence zone (ITCZ) in the BC emission-driven experiment enhances both the vertical transport and deposition of BC from Southeast Asia. The study shows that prescribing BC concentrations may lead to seriously inaccurate conclusions, but other models with less efficient transport may produce results with smaller differences. Corresponding author address: M. Sand,
doi:10.1175/jcli-d-14-00050.1 fatcat:djolqsiwjrcszerryrw3qfxpui