Addressing inconsistencies in black carbon literature

The literature describing black carbon (BC) emissions, and their effect on Earth’s climate, is growing rapidly. Unfortunately, inconsistencies in definitions; data collection and characterization; system boundaries; and time horizons have led to confusion about the relative importance of BC compared to other climate-active pollutant (CAPs). We discuss three sources of confusion: 1) Currently available BC inventories are not directly comparable to those used by the IPCC to track the greenhouse gases (GHGs) considered in the Kyoto Protocol (CO2, CH4, N2O). In particular, BC inventories often include all emissions: natural and anthropogenic in origin, controllable and non-controllable. IPCC inventories include only anthropogenic emissions. This BC accounting is appropriate for atmospheric science deliberations, but risks being interpreted as an overstatement against official Kyoto GHG inventories in a policy or control context. The IPCC convention of using 1750 as the starting year for emission inventories further complicates matters: significant BC emissions were emitted previous to that date by both human and natural sources. Though none of the pre-1750 BC emissions remain in the atmosphere today, their legacy presents challenges in assigning historical responsibility for associated global warming among sectors and regional populations. 2) Inconsistencies exist in the specific emissions sources considered in atmospheric models used to predict net BC forcing often lead to widely varying climate forcing estimates. For example, while some analyses consider only fossil fuel 1, others include both open biomass burning and fossil fuel combustion 2, and yet others include sources beyond biomass and fossil fuel burning 3. 3) Inconsistencies exist in how analyses incorporate the relationship between BC emissions and the associated cooling aerosols and processes, such as organic carbon (OC), and aerosol indirect effects (AIE). Unlike Kyoto GHGs, BC is rarely emitted in pure form and always with significant emissions of OC aerosols. The OC/BC ratio, however, is quite variable by emission source and often poorly characterized both in its current state and under intervention scenarios. In contrast, sulfur emissions, which become cooling sulfate (SO4) aerosols, are less intrinsically linked to other emissions, i.e., they can be controlled separately. Comparisons often ignore the substantial differences in uncertainties across the CAPs. These sources of confusion operate in a landscape of shifting scientific understanding of the RF from BC, including the work by Ramanathan and Carmichael (2008) indicating a BC RF that is roughly double the IPCC AR4 1 value for BC without organic carbon (OC). Doubling the impact of BC has a major impact on the relative importance of sectors for interventions. An approach is to consider post-AR4 estimates for BC, methane, etc. as part of sensitivity analyses, until a full new assessment becomes available. 1. Solomon S, Qin D, Manning M, et al. Working Group I Report: "The Physical Science Basis". Cambridge, UK and New York, NY, US: IPCC; 2007. 2. Jacobson MZ. Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols. Nature 2001;409:695-7. 3. Ramanathan V, Carmichael G. Global and regional climate changes due to black carbon. Nature Geoscience 2008;1:221-7.