2011 Literature Review Archives - Radiative Forcing

Bond, T.C., C. Zarzycki, M.G. Flanner and D. M. Koch. 2011. Quantifying immediate radiative forcing by black carbon and organic matter with the Specific Forcing Pulse. Atmos. Chem. Phys., 11:1505-1525.

A new metric – the Specific Forcing Pulse (SFP) - is proposed for assessing the climate warming or cooling effects of short-lived climate forcers generally.  When used to assess the impact of black carbon (BC) and organic matter (OM) emissions for major source-region combinations, all major BC sources (with co-emitted OM) are indicated to have net warming effects. However, important aerosol related cloud effects, which tend towards cooling, were not included in this study.

Given the slow progress towards reducing global emissions of long-lived greenhouse gases (GHGs), particularly CO2, attention is increasingly turning to the potential to mitigate near term warming by reducing emissions of short-lived climate forcers (SLCFs) that have a climate warming effect.  Since SLCFs have short atmospheric lifetimes, atmospheric concentrations of these species are not homogeneous and neither are the resultant climate effects, with effects depending on both the timing (e.g. season) and region of emission. This makes a global average metric such as the Global Warming Potential (GWP), developed for long-lived GHGs with uniform distribution, a poor means of evaluating the effects of SLCFs. Bond and colleagues propose a new metric, the Specific Forcing Pulse (SFP), that can be applied to species that remain in the atmosphere for less than one year, and that better captures the distinctive temporal and spatial nature of the effects of such species. It quantifies the impact per emission from a particular region either for a global total or within a specific region or latitudinal band. In this paper, they present values of SFP for atmospheric and snow forcing of black and organic carbon for 23 source-region combinations obtained from a single model, and then recommend a method for using multiple models and specific model enhancements (e.g. for aerosol aging) to provide best estimates of the SFP and associated uncertainties.  Their best estimate of global mean SFP for BC including direct and cryospheric forcing is +1.15 ±0.53 gigajoules per gram (GJ/g), a warming effect, while that for OM was estimated at -0.064 (-0.02, -0.13) GJ/g, a cooling effect, although values varied depending on the region and timing of emission. Of note is the order of magnitude difference between the cooling per mass of OM and the warming per mass of BC; BC is far more effective in interacting with visible radiation than OM. The lowest OM:BC mass ratio required to produce a neutral direct top-of-the-atmosphere forcing effect is ~15:1 for any region; higher ratios can be expected to induce cooling and lower ratios, warming. On this basis, all major BC sources (with co-emitted OM) are indicated to have net warming effects. However, as the authors note, important processes, particularly aerosol-induced changes to clouds that tend toward cooling, have not been included in these estimates. Also, SFP values depend critically on emission rates and therefore should be examined in light of uncertainties about emission inventories.