seating and leased modern aircraft can reduce
fuel consumption to about 3 l of kerosene per
passenger and 100 km, while a net carrier with
first and business class seats, some old aircraft
and rather empty airplanes on certain dates for
certain city pairs cannot reach higher ranks.
Conclusions and Outlook
Mobility has a growing share in changing climate
in comparison to other human activities like
industrial production and heating or cooling of
houses. Fortunately, the European Union’s major
research project named QUANTIFY was able to
compare all modes of transport and very recently
quantified the different modes’ contributions.
Taking from their conclusions and focusing here
on the contribution of aviation (see QUANTIFY
[30] for all modes of transport) we find:
1. The impact of transport, in particular of aviation and shipping, on climate grows faster than
the impact from other sectors of human activity. Hence, aviation has to excel concerning
efficiency measures in comparison to all other
human activities.
2. The effects of non-CO 2 emissions on climate
caused by aviation and shipping (e.g., through
nitrogen oxide emissions and induced changes
in cloudiness) are particularly large in comparison to other modes of transport.
3. A NOx molecule from aviation is responsible
for five times more ozone than a molecule from
road transport.
4. A NOx molecule from shipping is four times
more efficient in reducing methane concentration in the atmosphere than a NOx molecule
from aviation.
5. Aircraft-induced clouds warm the atmosphere.
Their radiative forcing (RF) is of similar magnitude than RF from aviation CO 2 . Hence, a
multiplication factor of at least around 2 (including air chemistry effects) seems justified when
dealing with climate effects of aviation.
6. The relative weights of the non-CO 2 effects of
different modes of transport strongly depend
on the metric chosen and on the time horizon.
7. A temperature-based climate metric leads to a
very different result than one based on the
radiative forcing and thus only on the greenhouse warming potential.
8. The per person and per kilometer climate
impact of passenger transport is much lower
for railways compared to road transport and
aviation, and the per person per kilometer climate impact of passenger air travel approaches
that of cars on time scale of decades. Despite
all the progress achieved so far many open
questions remain [30].
Most of the quantifications can only be given
with partly substantial uncertainty, sometimes
even effecting the sign of an effect. In particular,
the climate impact of indirect aerosol effects on
clouds is highly uncertain. We still do not know
whether soot cirrus really exists. The net effect of
NOx emission from air traffic might cause a net
negative radiative forcing, if a recently proposed
additional chemical reaction is confirmed. Therefore, it is presently difficult to include some
non-CO 2 effects in climate policies related to aviation or to develop suitable mitigation strategies
for all non-CO 2 effects caused by the transport
sector. Furthermore, many of the emitted species
responsible for the non-CO 2 effects of air traffic
emissions also impact air quality.
By applying a hierarchy of models (from process models, cloud resolving models to chemical
transport models, global general atmospheric circulation models, and coupled atmosphere/ocean/
land-models) and by analyzing measurements
from satellites and dedicated field campaigns,
the main uncertainties with respect to short-lived
non-CO 2 effects on the atmospheric composition
and on climate should be reduced:
1. The importance and magnitude of trade-offs in
the NOx effect (short-term increase in ozone
versus longer-term decrease in methane and
the associated longer-term ozone decrease)
should be explicitly simulated by comprehensive models covering the long response time of
methane.
2. The climate impact of contrail cirrus should be
simulated by climate models including
Aviation and Atmosphere
343
fuel consumption to about 3 l of kerosene per
passenger and 100 km, while a net carrier with
first and business class seats, some old aircraft
and rather empty airplanes on certain dates for
certain city pairs cannot reach higher ranks.
Conclusions and Outlook
Mobility has a growing share in changing climate
in comparison to other human activities like
industrial production and heating or cooling of
houses. Fortunately, the European Union’s major
research project named QUANTIFY was able to
compare all modes of transport and very recently
quantified the different modes’ contributions.
Taking from their conclusions and focusing here
on the contribution of aviation (see QUANTIFY
[30] for all modes of transport) we find:
1. The impact of transport, in particular of aviation and shipping, on climate grows faster than
the impact from other sectors of human activity. Hence, aviation has to excel concerning
efficiency measures in comparison to all other
human activities.
2. The effects of non-CO 2 emissions on climate
caused by aviation and shipping (e.g., through
nitrogen oxide emissions and induced changes
in cloudiness) are particularly large in comparison to other modes of transport.
3. A NOx molecule from aviation is responsible
for five times more ozone than a molecule from
road transport.
4. A NOx molecule from shipping is four times
more efficient in reducing methane concentration in the atmosphere than a NOx molecule
from aviation.
5. Aircraft-induced clouds warm the atmosphere.
Their radiative forcing (RF) is of similar magnitude than RF from aviation CO 2 . Hence, a
multiplication factor of at least around 2 (including air chemistry effects) seems justified when
dealing with climate effects of aviation.
6. The relative weights of the non-CO 2 effects of
different modes of transport strongly depend
on the metric chosen and on the time horizon.
7. A temperature-based climate metric leads to a
very different result than one based on the
radiative forcing and thus only on the greenhouse warming potential.
8. The per person and per kilometer climate
impact of passenger transport is much lower
for railways compared to road transport and
aviation, and the per person per kilometer climate impact of passenger air travel approaches
that of cars on time scale of decades. Despite
all the progress achieved so far many open
questions remain [30].
Most of the quantifications can only be given
with partly substantial uncertainty, sometimes
even effecting the sign of an effect. In particular,
the climate impact of indirect aerosol effects on
clouds is highly uncertain. We still do not know
whether soot cirrus really exists. The net effect of
NOx emission from air traffic might cause a net
negative radiative forcing, if a recently proposed
additional chemical reaction is confirmed. Therefore, it is presently difficult to include some
non-CO 2 effects in climate policies related to aviation or to develop suitable mitigation strategies
for all non-CO 2 effects caused by the transport
sector. Furthermore, many of the emitted species
responsible for the non-CO 2 effects of air traffic
emissions also impact air quality.
By applying a hierarchy of models (from process models, cloud resolving models to chemical
transport models, global general atmospheric circulation models, and coupled atmosphere/ocean/
land-models) and by analyzing measurements
from satellites and dedicated field campaigns,
the main uncertainties with respect to short-lived
non-CO 2 effects on the atmospheric composition
and on climate should be reduced:
1. The importance and magnitude of trade-offs in
the NOx effect (short-term increase in ozone
versus longer-term decrease in methane and
the associated longer-term ozone decrease)
should be explicitly simulated by comprehensive models covering the long response time of
methane.
2. The climate impact of contrail cirrus should be
simulated by climate models including
Aviation and Atmosphere
343