IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

2.9.3 Global Mean Radiative Forcing by Emission Precursor

The RF due to changes in the concentration of a single forcing agent can have contributions from emissions of several compounds (Shindell et al., 2005). The RF of CH4, for example, is affected by CH4 emissions, as well as NOx emissions. The CH4 RF quoted in Table 2.12 and shown in Figure 2.20 is a value that combines the effects of both emissions. As an anthropogenic or natural emission can affect several forcing agents, it is useful to assess the current RF caused by each primary emission. For example, emission of NOx affects CH4, tropospheric ozone and tropospheric aerosols. Based on a development carried forward from the TAR, this section assesses the RF terms associated with each principal emission including indirect RFs related to perturbations of other forcing agents, with the results shown in Figure 2.21. The following indirect forcing mechanisms are considered:

  • fossil carbon from non-CO2 gaseous compounds, which eventually increase CO2 in the atmosphere (from CO, CH4, and NMVOC emissions);
  • changes in stratospheric ozone (from N2O and halocarbon (CFCs, HCFC, halons, etc.) emissions);
  • changes in tropospheric ozone (from CH4, NOx, CO, and NMVOC emissions);
  • changes in OH affecting the lifetime of CH4 (from CH4, CO, NOx, and NMVOC emissions); and
  • changing nitrate and sulphate aerosols through changes in NOx and SO2 emissions, respectively.

For some of the principal RFs (e.g., BC, land use and mineral dust) there is not enough quantitative information available to assess their indirect effects, thus their RFs are the same as those presented in Table 2.12. Table 2.5 gives the total (fossil and biomass burning) direct RFs for BC and organic carbon aerosols that are used to obtain the average shown in Figure 2.21. Table 2.13 summarises the direct and indirect RFs presented in Figure 2.21, including the methods used for estimating the RFs and the associated uncertainty. Note that for indirect effects through changes in chemically active gases (e.g., OH or ozone), the emission-based RF is not uniquely defined since the effect of one precursor will be affected by the levels of the other precursors. The RFs of indirect effects on CH4 and ozone by NOx, CO and VOC emissions are estimated by removing the anthropogenic emissions of one precursor at a time. A sensitivity analysis by Shindell et al. (2005) indicates that the nonlinear effect induced by treating the precursors separately is of the order of 10% or less. Very uncertain indirect effects are not included in Table 2.13 and Figure 2.21. These include ozone changes due to solar effects, changes in secondary organic aerosols through changes in the ozone/OH ratio and apportioning of the cloud albedo changes to each aerosol type (Hansen et al., 2005).

Table 2.12. Global mean radiative forcings since 1750 and comparison with earlier assessments. Bold rows appear on Figure 2.20. The first row shows the combined anthropogenic RF from the probability density function in panel B of Figure 2.20. The sum of the individual RFs and their estimated errors are not quite the same as the numbers presented in this row due to the statistical construction of the probability density function.

 Global mean radiative forcing (W m–2)a   
 SAR (1750–1993)  TAR (1750–2005) AR4 (1750-2005) Summary comments on changes since the TAR  
Combined Anthropogenic RF  Not evaluated  Not evaluated  1.6 [–1.0, +0.8]  Newly evaluated. Probabilitydensity function estimate 
Long-lived Greenhouse gases (Comprising CO2, CH4, N2O, and halocarbons)  +2.45 [15%] (CO2 1.56; CH4 0.47; N2O 0.14; Halocarbons 0.28)  +2.43 [10%] (CO2 1.46; CH4 0.48; N2O 0.15; Halocarbons 0.34b)  +2.63 [±0.26] (CO2 1.66 [±0.17]; CH4 0.48 [±0.05]; N2O 0.16 [±0.02]; Halocarbons 0.34[±0.03])  Total increase in RF, due toupward trends, particularly inCO2. Halocarbon RF trendis positiveb  
Stratospheric ozone  –0.1 [2x]  –0.15 [67%]  –0.05 [±0.10]  Re-evaluated to be weaker 
Tropospheric ozone  +0.40 [50%]  +0.35 [43%]  +0.35 [–0.1, +0.3]  Best estimate unchanged.However, a larger RF couldbe possible 
Stratospheric water vapourfrom CH4 Not evaluated  +0.01 to +0.03  +0.07 [±0.05]  Re-evaluated to be higher 
Total direct aerosol  Not evaluated  Not evaluated  –0.50 [±0.40]  Newly evaluated 
Direct sulphate aerosol  –0.40 [2x]  –0.40 [2x]  –0.40 [±0.20]  Better constrained 
Direct fossil fuel aerosol (organic carbon)  Not evaluated  –0.10 [3x]  –0.05 [±0.05]  Re-evaluated to be weaker 
Direct fossil fuel aerosol (BC)  +0.10 [3x]  +0.20 [2x]  +0.20 [±0.15]  Similar best estimate to the TAR.Response affected by semi-direct effects 
Direct biomass burning aerosol  –0.20 [3x]  –0.20 [3x]  +0.03 [±0.12]  Re-evaluated and sign changed.Response affected by semi-direct effects 
Direct nitrate aerosol  Not evaluated  Not evaluated  –0.10 [±0.10]  Newly evaluated 
Direct mineral dust aerosol  Not evaluated  –0.60 to +0.40  –0.10 [±0.20]  Re-evaluated to have a smalleranthropogenic fraction 
Cloud albedo effect  0 to –1.5 (sulphate only)  0.0 to –2.0 (all aerosols)  –0.70 [–1.1, +0.4] (all aerosols)  Best estimate now given 
Surface albedo (land use)  Not evaluated  –0.20 [100%]  –0.20 [±0.20]  Additional studies 
Surface albedo (BC aerosol on snow)  Not evaluated  Not evaluated  +0.10 [±0.10]  Newly evaluated 
Persistent linear contrails  Not evaluated  0.02 [3.5x]  0.01 [–0.007, +0.02]  Re-evaluated to be smaller 
Solar irradiance  +0.30 [67%]  +0.30 [67%]  +0.12 [–0.06, +0.18]  Re-evaluated to be less than half 

Notes: a For the AR4 column, 90% value uncertainties appear in brackets: when adding these numbers to the best estimate the 5 to 95% confidence range is obtained. When two numbers are quoted for the value uncertainty, the distribution is non-normal. Uncertainties in the SAR and the TAR had a similar basis, but their evaluation was more subjective. [15%] indicates 15% relative uncertainty, [2x], etc. refer to a factor of two, etc. uncertainty and a lognormal

distribution of RF estimates.

b The TAR RF for halocarbons and hence the total LLGHG RF was incorrectly evaluated some 0.01 W m–2 too high. The actual trends in these RFs are therefore more positive than suggested by numbers in this table (Table 2.1 shows updated trends).

Table. 2.13. Emission-based RFs for emitted components with radiative effects other than through changes in their atmospheric abundance. Minor effects where the estimated RF is less than 0.01 W m–2 are not included. Effects on sulphate aerosols are not included since SO2 emission is the only significant factor affecting sulphate aerosols. Method of calculation and uncertainty ranges are given in the footnotes. Values represent RF in 2005 due to emissions and changes since 1750. See Figure 2.21 for graphical presentation of these values.

 CO2  CH4  CFC/ HCFC  N2O  HFC/ PFC/SF6  BC- direct  BC snow albedo  Organic carbon  O3(T)a  O3(S)b  H2O(S)c  Nitrate aerosols  Indirect cloud albedo effect  
Component emitted Atmospheric or surface change directly causing radiative forcing 
CO2 1.56d                         
CH4 0.016d 0.57e             0.2e   0.07f     
CFC/HCFC/halons     0.32g             –0.04h       
N2      0.15g           –0.01h       
HFC/PFC/SF6         0.017g                 
CO/VOC 0.06d 0.08e             0.13e         
NOx   –0.17e             0.06e     –0.10i Xj 
BC            0.34k 0.1l           Xj 
OC               –0.19k         Xj 
SO2                         Xj 


a tropospheric ozone.

b stratospheric ozone.

c stratospheric water vapour.

d Derived from the total RF of the observed CO2 change (Table 2.12), with the contributions from CH4, CO and VOC emissions from fossil sources subtracted. Historical emissions of CH4, CO and VOCs from Emission Database for Global Atmospheric Research (EDGAR)-HistorY Database of the Environment (HYDE) (Van Aardenne et al., 2001), CO2 contribution from these sources calculated with CO2 model described by Joos et al. (1996).

e Derived from the total RF of the observed CH4 change (Table 2.12). Subtracted from this were the contributions through lifetime changes caused by emissions of NOx, CO and VOC that change OH concentrations. The effects of NOx, CO and VOCs are from Shindell et al. (2005). There are significant uncertainties related to these relations. Following Shindell et al. (2005) the uncertainty estimate is taken to be ±20% for CH4 emissions, and ±50% for CO, VOC and NOx emissions.

f All the radiative forcing from changes in stratospheric water vapour is attributed to CH4 emissions (Section 2.3.7 and Table 2.12).

g RF calculated based on observed concentration change, see Table 2.12 and Section 2.3

h 80% of RF from observed ozone depletion in the stratosphere (Table 2.12) is attributed to CFCs/HFCs, remaining 20% to N2O (Based on Nevison et al., 1999 and WMO, 2003).

i RF from Table 2.12, uncertainty ±0.10 W m–2.

j Uncertainty too large to apportion the indirect cloud albedo effect to each aerosol type (Hansen et al., 2005).

k Mean of all studies in Table 2.5, includes fossil fuel, biofuel and biomass burning. Uncertainty (90% confidence ranges) ±0.25 W m–2 (BC) and ±0.20 W m–2 (organic carbon) based on range of reported values in Table 2.5.

l RF from Table 2.12, uncertainty ±0.10 W m–2.


Figure 2.21. Components of RF for emissions of principal gases, aerosols and aerosol precursors and other changes. Values represent RF in 2005 due to emissions and changes since 1750. (S) and (T) next to gas species represent stratospheric and tropospheric changes, respectively. The uncertainties are given in the footnotes to Table 2.13. Quantitative values are displayed in Table 2.13.