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

Abstract:

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.