Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Computed results for global radiative forcing by contrails depend on assumed values for
contrail cover and mean optical depth of contrails. Neither is well known. Here, the
computed global contrail cover was normalized to yield 0.5% observed cover by line-shaped
contrails over Europe, guided by satellite data. Because satellite data mainly reveal thicker
contrails, a larger cover resulting from the presence of optically thin contrail cirrus might not
be accurately detected. Therefore, the results for larger t values (0.3 to 0.5) are considered
in combination with the given contrail cover (0.1% globally) as being most representative for
real forcing conditions. Hence, in the diurnal and annual mean, a global 0.1% increase in
thin contrail cloud cover causes a net heating of the Earth-atmosphere system of
approximately 0.02 W m -2 . The difference between the largest and smallest net forcing
values in Table 3-8 suggests an error bound on the order of 0.01 W m -2 . Based on Section
3.5, the actual global cover may (with 2/3 probability) be 2 to 3 times smaller or larger than
the derived line-shaped contrail cover. The optical depth value is likely known to a factor of
2 to 3. Thus, for assumed Gaussian behavior of individual uncertainties, the radiative forcing
value may differ from the best estimate by a factor of about 3 to 4.
Based on computations using estimates of line-shaped contrail occurrence for the 1992 fuel
scenario (0.1% cover), the best estimate of global-mean radiative forcing is positive, has a
value of about 0.02 W m -2 with an uncertainty factor of 3 to 4, and may range from 0.005 to
0.06 W m -2 for present climate conditions. Certainly, the state of our understanding is only
fair. Future investigations may result in considerable changes to the best estimates.
Global radiative forcing by contrails obviously is much smaller than that attributed to other
anthropogenic changes in the past century-1.5 W m-2 , which represents about the median
of the range of values given in IPCC (1996) but comparable to the forcing by past CO2 emissions by aircraft (Brasseur et al., 1998; see Chapter 6). A mean radiative forcing of 0.02
W m-2 induces a vertically averaged heat source in the troposphere equivalent to
approximately 0.0002 K day-1. The atmosphere reacts to this heat source in a complex
manner (Ponater et al., 1996) and may take decades to reach a steady-state temperature
response. In steady state, the mean surface temperature may increase by about 0.01 to
0.02 K globally if climate sensitivity from contrail forcing is comparable to that of well-mixed
greenhouse gases (IPCC, 1996; see Section 6.2.1). In comparison to the global mean
value, annually averaged zonal mean values are larger by a factor of about 5, and regional
values are larger by a factor of up to 40 (see Figure 3-21). The pattern of the climate
response differs from the pattern of radiative forcing, in particular at small regional scales.
Zonal mean steady-state temperature changes of between 0.01 and 0.1 K appear to be
possible for present contrail cover when previous studies as scaled to the cover and
radiative forcing found here (Liou et al., 1990; Ponater et al., 1996; Strauss et al., 1997).
Regional contrail forcing may have short-term consequences for the daily temperature
range in such a region if contrail forcing persists for at least a day. These temperature
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Figure 3-22: Trend in overall efficiency of propulsion h (solid circles),
computed from aircraft specific fuel consumption (SFC) data (open
circles; data as in Figure 7-9), according to h = V(Q SFC)-1, with V as
the aircraft speed (~240 m s-1 ) and Q as the specific heat of
combustion of aviation fuels (43 MJ kg-1). Solid circles also denote
the critical altitude z (right axis) above which contrails form (for 100%
relative humidity in the mid-latitude standard atmosphere) for the
years 1960 to 2010.