Environmental Engineering Reference
In-Depth Information
both hemispheres. The enhancement of O
3
results in a positive globally averaged
radiative forcing and a reduction in radiative forcing from reduced CH
4
concentra-
tions (reduced by approximately 2-4 per cent).
That the globally averaged O
3
radiative forcing has an approximately equal and
opposite magnitude to the reduction of CH
4
radiative forcing has resulted in the
erroneous conclusion, in many quarters, that there is no climate effect, despite the
fact that the IPCC report clearly stated otherwise (IPCC, 1999, section 6.5.1). It
would be more correct to postulate a partial cancellation of the CO
2
radiative forc-
ing induced by aircraft as both CO
2
and CH
4
are, more or less, well mixed across the
hemispheres because of long lifetimes. This is a theme picked up in the concluding
section of this chapter.
Effects of CO
2
emissions
The effects of CO
2
are much easier to assess than other aircraft emissions, since they
are a simple function of fuel burn. Moreover, there is no difference between the emis-
sions of CO
2
from aircraft or any other source in terms of radiative effect. Since
CO
2
has an atmospheric residence time of many decades, it is well mixed in the atmo-
sphere and can be treated as an additional CO
2
source in climate model simulations.
The calculated radiative forcing in 1992 was 0.018Wm
-2
, with an uncertainty of
±30 per cent that arises from uncertainties in the carbon cycle and the radiative
transfer calculations themselves (IPCC, 1999).
Effects of water vapour emissions
Wa t e r v a p o u r,
per se
, is a strong greenhouse gas; without it, the temperature of the
Earth's atmosphere could not sustain life. The troposphere has relatively high con-
centrations of water vapour as a result of the natural hydrological cycle. The upper
troposphere and lower stratosphere tends to be drier, and the mid to upper strato-
sphere very dry. Thus, any introduction of water into the stratosphere tends to warm
the atmosphere. Subsonic aircraft introduce some water vapour from the combus-
tion of kerosene; but this perturbation has been found to have a small effect, as the
background concentrations of water vapour are rather high in the lower stratosphere,
relative to the amount of water introduced by current and projected subsonic aircraft
fleets. Thus, the radiative forcing is small, of the order 0.0015Wm
-2
(IPCC, 1999).
Similarly, Ponater et al (1996) and Rind et al (1996) conducted studies of the direct
effects of water vapour from aircraft using global climate models and concluded that
the direct radiative effect was negligible. Emissions of water vapour into the strato-
sphere, particularly the mid-upper stratosphere, have a different effect, which is dis-
cussed later.
Effects of particle emissions on contrails and cirrus clouds
One of the impacts of aircraft on climate that has stimulated much interest and recent
research is that of contrails and cirrus clouds. The idea that contrails can affect cli-
mate is not new: potential climate effects of contrails were discussed between the late
1960s and the early 1980s (eg Appleman, 1966; Chagnon, 1981). However, these
studies of contrails did not receive much attention. The potential effects of contrails
