Assessing the climatic benefits of black carbon mitigation

Abstract

To limit mean global warming to 2 degrees C, a goal supported by more than 100 countries, it will likely be necessary to reduce emissions not only of greenhouse gases but also of air pollutants with high radiative forcing (RF), particularly black carbon (BC). Although several recent research papers have attempted to quantify the effects of BC on climate, not all these analyses have incorporated all the mechanisms that contribute to its RF (including the effects of BC on cloud albedo, cloud coverage, and snow and ice albedo, and the optical consequences of aerosol mixing) and have reported their results in different units and with different ranges of uncertainty. Here we attempt to reconcile their results and present them in uniform units that include the same forcing factors. We use the best estimate of effective RF obtained from these results to analyze the benefits of mitigating BC emissions for achieving a specific equilibrium temperature target. For a 500 ppm CO(2)e (3.1 W m(-2)) effective RF target in 2100, which would offer about a 50% chance of limiting equilibrium warming to 2.5 degrees C above preindustrial temperatures, we estimate that failing to reduce carbonaceous aerosol emissions from contained combustion would require CO(2) emission cuts about 8 years (range of 1-15 years) earlier than would be necessary with full mitigation of these emissions.

Fig. 1.

Probability distributions for the total effective RF of carbonaceous aerosols from (A) contained combustion (CC) and (B) biomass burning (BB) from the four sets of analyses considered here. The four estimates are: (i) A*, AeroCom intercomparison study with internal mixing adjustment; (ii) H, Hansen et al. (6, 12); (iii) J, Jacobson (7, 9, 13, 14); and (iv) RC, Ramanathan and Carmichael (1). The nine models in A* are shown individually and collectively. The efficacies from H are applied to A and RC. Solid lines incorporate the snow albedo effect, assuming that it is caused by CC and BB in proportion to their BC emissions, whereas dashed lines exclude the snow albedo effect. The upper x-axis on (A) indicates for each RF value the number of years earlier CO₂ emissions must be cut to 50% of 2005 levels in a 500 ppm CO₂e stabilization scenario where CC carbonaceous aerosols emissions are kept constant rather than cut to zero. For example, the CC RFe from H (with 90% confidence range) is 0.23 ± 0.18 W m^-2; if CC emissions are not cut, CO₂ will have to reach 50% of 2005 levels 8.9 years (1.5–16.7 years) earlier than otherwise. See Fig. 2 for illustration.

Fig. 2.

Target CO₂ emissions and concentrations under four CC carbonaceous aerosol emission scenarios using our “best estimate” of RF from carbonaceous aerosols of 0.22 W m^-2, as calculated using our simple atmospheric and economic model. (A) shows CO₂ emissions relative to 2005 levels (on the left axis) and as an annual emission rate (on the right axis). The solid lines in (B) show the associated CO₂ concentrations. The dashed lines in (B) indicate (on the left axis) total RF from all GHGs and carbonaceous aerosols in terms of ppm CO₂e and (on the right axis) the associated equilibrium warming assuming a 3 °C/CO₂ doubling climate sensitivity (24). The arrow (in A) indicates the 8 years difference in timing of the 50% reduction in CO₂ emissions between the full mitigation and constant emissions scenarios. The scenarios are: Full mitigation—complete elimination of emissions of carbonaceous aerosols from CC by 2100; and constant emissions—1996 carbonaceous aerosol emissions continue through 2100. “A2” and “B1” are high and low projections of carbonaceous aerosol emissions from ref.  based on IPCC SRES storylines.

Fig. 3.

Probability distribution of the OC/BC threshold above which emissions are no longer net warming, assuming an average geographic and altitudinal distribution for the emitted particles. Typical OC/BC values of different combustion sources from (3) are marked. Solid lines incorporate the snow albedo effect, assuming that it is caused by CC and BB in proportion to their BC emissions, whereas dashed lines exclude the snow albedo effect. For example, residential biofuels have a typical OC/BC ratio of 3.9; based on H, assuming an average geographic and altitudinal distribution, and including the snow albedo effect, there is a 78% chance they produce carbonaceous aerosols with a net warming effect. Although some sources (e.g., diesel engines using high sulfur fuel) coemit SO₂, the RF of the sulfate aerosol thereby formed will depend on the relative quantities of compounds emitted and whether the resulting aerosols are internally mixed (increasing RF) or externally mixed (leading the positive BC RF to be partially cancelled by the negative sulfate RF).