Environmental Engineering Reference
In-Depth Information
through the economic and technological consequences of changing operational
practices for environmental purposes until studies are scientifically defensible and
robust (Lee et al, 2000).
A more radical proposition is the development of a fleet of liquid hydrogen-
fuelled aircraft. Experimental aircraft have been built and a major European Commis-
sion research programme, CRYOPLANE, is examining the technological feasibility
and potential environmental impacts. A first study has been made of a hypothetical
fleet of cryoplanes by Marquart et al (2001), who found that contrail formation could
be a significant factor but were unable to give an overall prognosis of the environmen-
tal benefits or otherwise.
Contrails and cirrus cloudiness remain an important issue. More research is cur-
rently being committed to understanding the basic physics of contrail formation
through measurement programmes such as the European Union (EU) project INCA
and the German project PAZI.
3
The formation of particles in the plume and the engine
itself are only incompletely understood and the EU project PARTEMIS (coordinated
by QinetiQ) deals specifically with this issue. Since the IPCC (1999) report, a fur-
ther assessment of the radiative forcing of line-shaped contrails has been performed
using a global climate model (GCM), rather than an off-line radiative transfer
model (Ponater et al, 2002). This study showed a much smaller radiative forcing of
0.004Wm
-2
, compared with the estimate of Minnis et al (1999), who derived a fig-
ure of 0.02Wm
-2
. The reasons for the differences were difficult to ascribe compre-
hensively; but, in effect, the uncertainty on contrail radiative forcing has increased.
Finally, we return to one of the issues that initiated much of the research into
the effects of subsonic aviation: NO
x
emissions and O
3
production. The fallacy of
O
3
positive radiative forcing 'cancellation' by negative CH
4
forcing from aircraft
NO
x
emissions has already been referred to. Radiative forcing by aircraft-induced
O
3
tends to occur strongly in the Northern Hemisphere and the resulting negative
forcing (ie a reduction in overall forcing) from CH
4
is spread across the hemispheres.
The real climate impact of opposing homogeneous and inhomogeneous forcings is
not known. Remembering that radiative forcing is a
metric
(and proxy) for climate
change, it is necessary to understand the limits of its applicability. The IPCC (1999)
report broached the issue of the adequacy of radiative forcing as a metric for aircraft
climate impacts. There are two issues relevant to this discussion: firstly, the tempera-
ture response to inhomogeneous forcings; and, secondly, the temperature response
to O
3
forcings at different altitudes.
Central to the metric of radiative forcing is the assumption that the climate sen-
sitivity parameter in equation (1),
λ
, is a constant. While
λ
is expected to be model
dependent, it is implicitly assumed that it is constant in the model. However, in a
number of GCM experiments, for the same increase of O
3
at different model layers,
significant variation in
λ
has been found (eg Hansen et al, 1997). The parameter
λ
was found to vary significantly for different instantaneous radiative forcings for
small aircraft O
3
responses by Ponater et al (1999). Stuber et al (2001) have subse-
quently found that surface temperature was more sensitive to aircraft-induced O
3
perturbations than an equivalent CO
2
perturbation using the stratosphere-adjusted
radiative forcing, a forcing judged to be more reliable than the instantaneous forc-
ing. There are some indications that inhomogeneous forcings produce a stronger cli-
mate effect in terms of surface temperature than equivalent homogeneous forcings
