Environmental Engineering Reference
In-Depth Information
view with regard to the principal potentials available for limiting emissions. In gen-
eral, direct passenger and freight transport GHG emissions can be explained and
estimated by the following equation, which also identifies the main potentials for
emission reduction (Schipper et al, 2000, p10):
GASI F
=
×
×
×
In this identity, (G) denotes the GHG emissions from transport. (A) is total passen-
ger or freight transport activity (in passenger kilometres or tonne kilometres respec-
tively), (S) is a measure of the shares of the transport modes used, (I) is a measure of
the energy intensities of the respective modes, and (F) identifies the volumes of fuels
burned by those modes during operation. Further to this, emissions may indirectly
result from the construction, maintenance and scrapping of vehicles and infrastruc-
ture and the production of fuels.
This general approach for modelling and estimating the total emissions from
transport operation may translate into the following equation if GHG emissions
from civil aviation are considered:
GAI F
=
×
×
Activity (A) can be explained by the variables 'frequency of travel' and 'average dis-
tance travelled for passenger transport' or 'volume and average distance transported
for freight transport' respectively. The energy intensity variable (I) can be subdivided
into a technical determinant that depends upon aircraft technology and an opera-
tional determinant that comprises operation-oriented design specifications of indi-
vidual aircraft (distance range), flight operation, air traffic management and utilization
of aircraft capacity. The variable (F) for the fuel used is of limited value for reducing
aviation's GHG emissions as long as no fuels other than kerosene are considered.
Another important variable for aviation's impact upon the climate that is related to
flight operation and is independent from the total volume of emissions is flight alti-
tude for radiative forcing caused by water (H
2
O) and nitrogen oxide (NO
x
) emissions.
Demand has so far been the most important single factor influencing fuel use and
related carbon emissions from aviation. This can be expected to continue in the future
in a business-as-usual scenario owing to the potential savings in travel times offered
by air travel. Average travel time budgets have been found to be fairly constant over
time and across different cultures (Schafer and Victor, 2000, p174-175). Thus, growth
in overall travel is primarily dependent upon increases in travel speed, prices for trans-
port services and available income. In many developed countries, car use no longer
allows significantly increased travel speed, and high-speed rail also offers limited poten-
tial for travelling faster. Therefore, aviation has the greatest potential for growth in
personal high-speed, long-distance travel. Growth in demand for air travel is influenced
by a variety of technological, economic, political, social and psychological factors
(Nielsen, 2001). Many of those are important only owing to the advantages in travel
time that air travel offers and the fact that it has become affordable to an ever-growing
number of humans. However, in dealing with growing demand, policy must address
all relevant determinants as it cannot be expected that historic trends of increasing
speed and affordability of air travel will be reversed.
