Quantifying immediate radiative forcing by black carbon and organic matter with the Specific Forcing Pulse

摘要

Climatic effects of short-lived climate forcers (SLCFs) differ from those oflong-lived greenhouse gases, because they occur rapidly after emission andbecause they depend upon the region of emission. The distinctive temporaland spatial nature of these impacts is not captured by measures that rely onglobal averages or long time integrations. Here, we propose a simplemeasure, the Specific Forcing Pulse (SFP), to quantify climate warming orcooling by these pollutants, where we define "immediate" as occurringprimarily within the first year after emission. SFP is the amount of energyadded to or removed from a receptor region in the Earth-atmosphere system bya chemical species, per mass of emission in a source region. We limit theapplication of SFP to species that remain in the atmosphere for less thanone year. Metrics used in policy discussions, such as total forcing orglobal warming potential, are easily derived from SFP. However, SFP conveyspurely physical information without incurring the policy implications ofchoosing a time horizon for the global warming potential.Using one model (Community Atmosphere Model, or CAM), we calculate values ofSFP for black carbon (BC) and organic matter (OM) emitted from 23source-region combinations. Global SFP for both atmosphere and cryosphereimpacts is divided among receptor latitudes. SFP is usually greater foropen-burning emissions than for energy-related (fossil-fuel and biofuel)emissions because of the timing of emission. Global SFP for BC varies byabout 45% for energy-related emissions from different regions. Thisvariation would be larger except for compensating effects. When emittedaerosol has larger cryosphere forcing, it often has lower atmosphere forcingbecause of less deep convection and a shorter atmospheric lifetime.A single model result is insufficient to capture uncertainty. We develop abest estimate and uncertainties for SFP by combining forcing results from 12additional models. We outline a framework for combining a large number ofsimple models with a smaller number of enhanced models that have greatercomplexity. Adjustments for black carbon internal mixing and for regionalvariability are discussed. Emitting regions with more deep convection havegreater model diversity. Our best estimate of global-mean SFP is +1.03 ± 0.52 GJ g−1for direct atmosphere forcing of black carbon, +1.15 ± 0.53 GJ g−1 for black carbon including direct and cryosphere forcing,and −0.064 (−0.02, −0.13) GJ g−1 for organic matter. These values dependon the region and timing of emission. The lowest OM:BC mass ratio requiredto produce a neutral effect on top-of-atmosphere direct forcing is 15:1 forany region. Any lower ratio results in positive direct forcing. However,important processes, particularly cloud changes that tend toward cooling,have not been included here.Global-average SFP for energy-related emissions can be converted to a100-year GWP of about 740 ± 370 for BC without snow forcing, and 830 ± 440 with snow forcing. 100-year GWP for OM is −46 (−18, −92). Best estimatesof atmospheric radiative impact (without snow forcing) by black and organicmatter are +0.47 ± 0.26 W m−2 and −0.17 (−0.07, −0.35) W m−2for BC and OM, respectively, assuming total emission rates of 7.4 and45 Tg yr?1. Anthropogenic forcing is +0.40 ± 0.18 W m−2 and −0.13(−0.05, −0.25) W m−2 for BC and OM, respectively, assuming anthropogenicemission rates of 6.3 and 32.6 Tg yr−1. Black carbon forcing is only18% higher than that given by the Intergovernmental Panel on ClimateChange (IPCC), although the value presented here includes enhancedabsorption due to internal mixing.