Confronting the Global Climate Response to Black Carbon Aerosols with its Uncertainty

Black carbon aerosols (BC) modulate global temperatures and the hydrological cycle as well as regional climate. However, their radiative forcing is not well-constrained observationally and recent estimates of just the direct forcing ranges from 0.08 to 1.27 W/m2 - the upper limits of which puts BC second only to carbon dioxide in terms of radiative forcing. Consequently, the climate impacts of these heterogeneous short-lived forcing agents are highly uncertain. To establish the uncertainty in the climate response to BC, we conduct a suite of idealized experiments with the DOE/NCAR CESM1.0 model with the atmosphere component (CAM4) coupled to a Slab Ocean Model (SOM) forced separately with increasing BC concentrations covering a large swath of the estimated range of current BC radiative forcing. We find that the increase in BC results in global warming - with a sensitivity of 0.22 K/W/m2 including the semi-direct effects, decrease in global precipitation - despite the increase in global temperatures, a northwards shift of the ITCZ - along with an increase in cross-equatorial southwards energy transport, tropical expansion in the Northern Hemisphere - associated with BC induced mid-latitude warming, and an increase in precipitation during the Indian Monsoons - with the enhancement of the meridional tropospheric gradient, among other responses. Further, these global responses are near-linear functions of the increase in BC concentration, suggesting that the climate response to BC aerosols can be readily estimated if the uncertainty in BC can be constrained.