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6.4.2. Direct Radiative Forcing from Black Carbon Aerosols
Tropospheric black carbon (BC) aerosol, also described as soot, primarily absorbs
incident solar radiation, which leads to positive radiative forcing. As in the
case of sulfate aerosol, the small size of the particles means that radiative
forcing in the longwave region of the spectrum is likely to be negligible. The
sensitivity of global mean radiative forcing to column loading of total anthropogenic
BC is estimated by Haywood et al. (1997a), Haywood and Ramaswamy (1998), and
Myhre et al. (1998) to range from approximately +1100 to +1850 W g-1
BC.
We reexamined results by inserting BC aerosol in a layer between 8 and 13 km
in the GFDL R30 GCM using the method of Haywood and Ramaswamy (1998). A log-normal
distribution with a geometric mean radius of 0.0118 µm and a standard deviation
of 2.0 was assumed. The resulting global mean sensitivity was found to be approximately
+3000 W g-1 BC as a result of the higher sensitivity
of the radiative forcing when the BC exists at higher altitudes above a greater
proportion of cloudy layers (Haywood and Ramaswamy, 1998). This value is adopted
throughout this report because it explicitly takes into account the effect of
the elevated altitude of the aerosol.
BC particles primarily absorb sunlight and heat the local air. Thus, unlike
BC that resides in the troposphere, BC in the stratosphere contributes negative
solar radiative forcing that is countered by induced positive longwave radiative
forcing. Thus, radiative forcing from BC aerosols is sensitive to their location
relative to the tropopause. We do not have enough information on the location
of BC relative to the tropopause, and thus our use of the instantaneous top-of-atmosphere
value overestimates the RF depending on the fraction of aircraft BC in the lower
stratosphere.
Using the aircraft fuel-burn scenarios for NASA-1992 noted above and described
in Chapter 3, we derive a global mean column burden of
BC aerosol from aircraft of 1.0 µg BC m-2 (assuming an
EI(BC) of 0.04 g kg-1; see also Table
3-4). Thus, we estimate global mean BC aerosol forcing in 1992 to be +0.003
(+0.001 to +0.006) W m-2 and assume that it linearly
scales with fuel use (see also Table 6-1). This
value is much smaller in absolute magnitude than the RF from CO2,
O3, CH4, or contrails.
6.4.3. Radiative Forcing from Persistent Contrails and Indirect Effects on Clouds
Aircraft emission of water vapor and particles, as well as the creation of
contrails, could lead to a change in global cloudiness. Some atmospheric GCM
studies that have looked at the impacts of injecting water vapor or creating
contrails (e.g., Ponater et al., 1996; Rind et al. 1996) point to the potential
importance of these effects on climate, but these pilot studies cannot be used
directly in this assessment. Persistent contrails clearly related to aircraft
are detectable, however, and their impact on radiative forcing can be evaluated.
Section 3.6 (see Table 3-9)
estimates direct radiative forcing from persistent contrails to be +0.02 (+0.005
to +0.06) W m-2 in 1992 (see also Table
6-1). This estimate is limited to immediately visible, quasi-linear persistent
contrails.
Whereas contrail formation and associated radiative forcing is an obvious and
visible consequence of aircraft activity, the secondary, indirect effect of
aerosols from aircraft on the microphysical and radiative properties of clouds
is a very complex issue that has received little attention and is very difficult
to quantify (Seinfeld, 1998). Some significant steps in quantifying the indirect
effect from anthropogenic aerosols have been made (e.g., Jones et al., 1994;
Boucher and Lohmann, 1995). The effects of aerosol particles from aircraft emissions
on clouds are more complicated because nucleation and subsequent growth of ice
crystals that make up cirrus clouds are more complex and less studied than for
water clouds. Cirrus cloud generally exert positive forcing because longwave
positive radiative forcing is of a larger magnitude than solar negative radiative
forcing. Section 3.6.5 (see Table
3-9) estimates that radiative forcing from aircraft-induced cirrus is positive
and may be comparable to contrail RF. The magnitude of this RF remains very
uncertain. No best estimate is given in Tables 6-1
and 6-2, but a range for the best estimate could
fall between 0 and 0.04 W m-2.
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