There are two mechanisms by which aerosols may exert radiative forcing: the direct effect, whereby aerosol particles scatter and absorb solar and longwave radiation; and the indirect effect, whereby aerosol particles act as cloud condensation nuclei and modify the physical and radiative properties of clouds. Additionally for aircraft, merely flying through certain meteorological environments can result in formation of contrails (Section 3.4), which affect both solar and longwave radiation budgets. The present-day direct radiative forcing from aircraft emissions of sulfur compounds and black carbon aerosols is investigated in Sections 6.4.1 and 6.4.2; radiative forcing from the formation of contrails and the indirect effect of aerosol emissions is investigated in Section 6.4.3. Section 6.4.4 derives future RF considering our range of scenarios for fuel use. The RF models have been described previously (Section 6.3.1). A summary of radiative forcing calculations and related uncertainties is given in Section 6.4.5.
Sulfate aerosol scatters a fraction of incident solar radiation back to space, thereby leading to negative direct radiative forcing. The direct radiative forcing of pure sulfate in the longwave spectrum is likely to be negligible as a result of the size of aerosol particles and the corresponding wavelength dependence of the specific extinction coefficient (e.g., Haywood and Shine, 1997; Haywood et al., 1997a). Myhre et al. (1998) summarize 10 detailed studies of the sensitivity of direct radiative forcing from all anthropogenic sources of sulfate. With the exception of one study, sensitivities per unit column mass of anthropogenic sulfate range from -125 to -214 W g-1 SO4.
We reexamined these results by inserting a pure ammonium sulfate in a layer between 8 and 13 km in the GFDL R30 GCM and assuming an ambient relative humidity of 45% using the method of Haywood and Ramaswamy (1998). A log-normal distribution with a dry geometric mean radius of 0.05 µm and a standard deviation of 2.0 was adopted. The resulting global mean sensitivity was found to be approximately -215 W g-1 SO4, which is adopted throughout this report. Because the modeled pure sulfate particles scatter incident radiation with no absorption, the RF is not sensitive to their location relative to the tropopause. Thus, the RF is well approximated by the instantaneous radiative forcing at the top of the atmosphere even for aircraft sulfate in the lower stratosphere.
A study of the distribution of aircraft fuel burned and transported as a passive tracer from scenario NASA-1992 involved a range of global models and is presented in Chapter 3 (see also Danilin et al., 1998). The median global mean column burden of sulfate aerosol is derived in this study by adopting an emission index for sulfur EI(S) of 0.4 g kg-1 and a 50% effective conversion factor from fuel-sulfur to optically active sulfate aerosols; it is approximately 13.5 µg SO4 m-2 (Table 3-4). Thus, global mean radiative forcing from aircraft emissions of sulfate aerosol in 1992 is estimated to be -0.003 W m-2, with a likely range of -0.001 to -0.009 W m-2 (see also Table 6-1). This value is much smaller in absolute magnitude than the RF from CO2, O3, CH4, or contrails. We assume that EI(S) remains constant through 2050 and scale the sulfate RF with fuel use.
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