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Improved Attribution of Climate Forcing to Emissions

Science  30 Oct 2009:
Vol. 326, Issue 5953, pp. 716-718
DOI: 10.1126/science.1174760

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  1. Fig. 1

    Radiative forcing from 1750 to 2000. Numerical values within the figure give the net forcing (instantaneous at the tropopause). Uncertainties in the abundance-based values are 0.16 for CO2, 0.05 for methane, +0.15 to –0.10 for ozone, 0.20 for sulfate, 0.10 for nitrate, and 0.05 for stratospheric water (5). For emissions-based values, we estimate uncertainties by adding the forcing uncertainties for each component in quadrature, yielding 0.14 for methane, 0.04 for CO+VOCs, 0.09 for NOx, 0.23 for sulfate, and 0.10 for ammonia. AIE are not included. All forcing values are from this work except those from CO2 and stratospheric water, which are based on the IPCC AR4 (5).

  2. Fig. 2

    The 100-year GWPs for methane, CO, and NOx (per Tg N) as given in the AR4 and in this study when including no aerosol response, the direct radiative effect of aerosol responses, and the direct+indirect radiative effects of aerosol responses. The AR4 did not report uncertainties for methane or CO and gave no mean estimate for NOx. The range for the GWP of CO is from the third IPCC assessment and encompasses values reported up through the AR4. Our calculations for the shorter 20-year GWP, including aerosol responses, yield values of 79 and 105 for methane, 11 and 19 for CO, and –335 and –560 for NOx, including direct and direct+indirect radiative effects of aerosols in each case. The 100-yr GWPs for SO2 (per Tg SO2) and ammonia would be –22 and –19, respectively, including direct aerosol radiative effects only, and –76 and –15 adding indirect aerosol radiative effects. GWPs for very short-lived NOx, SO2, and ammonia will vary widely by emission location and timing, and hence global values are of limited use.