Recommendations for calculation of the global warming potential of aviation including the radiative forcing index

Abstract

Purpose
There are specific effects of emissions in high altitude, which lead to a higher contribution of aviation to the problem of climate change than just the emission of CO2 from burning fuels. The exact relevance is subject to scientific debate, but there is a consensus that aircrafts have an impact that is higher than just their contribution due to the direct CO2 emissions. The gap between this scientific knowledge on the one side and the missing of applicable GWP (global warming potential) factors for relevant emissions on the other side are an important shortcoming for life cycle assessment (LCA) or carbon footprint (CF) studies which aim to cover all relevant environmental impacts of the transport services investigated.

Methods
In this paper, the state of the art concerning the accounting for the specific effects of aircraft emissions in LCA and CF studies is discussed. Therefore, the relevant literature was evaluated, and practitioners were asked for the approaches used by them.

Results and discussion
Five major approaches are identified ranging from an RFI (radiative forcing index) factor of 1 (no factor at all) to a factor 2.7 for the total aircraft CO2 emissions. If only emissions in the higher atmosphere are considered, RFI factors between 1 and 8.5 are used or proposed in practice.

Conclusions
For the time being, an RFI of 2 on total aircraft CO2 (or 5.2 for the CO2 emissions in the higher atmosphere if using present models in ecoinvent) is recommended to be used in LCA and CF studies because it is based on the latest scientific publications; this basic literature cannot be misinterpreted. Furthermore, it is also recommended by some political institutions. These factors can be multiplied by the direct CO2 emissions of the aircraft to estimate the total global warming potential.

Full text

ESU-services Ltd. Vorstadt 14 CH-8200 Schaffhausen
Niels Jungbluth jungbluth@esu-services.ch T +41 44 940 61 32
Christoph Meili meili@esu-services.ch T +41 44 940 61 35
www.esu-services.ch F +41 44 940 67 94
2018
Int J LCA
Recommendations for calculation of the
global warming potential of aviation
including the radiative forcing index

© ESU-services Ltd. - i -
Recommendations for calculation of the
global warming potential of aviation includ-
ing the radiative forcing index
Article accepted for publication in the International Journal of
LCA (online first 19.11.2018)
DOI: 10.1007/s11367-018-1556-3
https://link.springer.com/article/10.1007/s11367-018-1556-3
Authors
Niels Jungbluth;Christoph Meili
ESU-services Ltd.
Vorstadt 14
CH-8200 Schaffhausen
jungbluth@esu-services.ch
www.esu-services.ch
Tel. +41 44 940 61 32

© ESU-services Ltd. - ii -
Contents
ABSTRACT IV
KURZFASSUNG IV
RÉSUMÉ V
KEYWORDS V
RECOMMENDATIONS FOR CALCULATION OF THE GLOBAL WARMING POTENTIAL
OF AVIATION INCLUDING THE RADIATIVE FORCING INDEX VI
SUMMARY VI
1 INTRODUCTION 1
2 OVERVIEW ON APPROACHES USED IN LIFE CYCLE ASSESSMENT AND
CARBON FOOTPRINTING 5
3 RECOMMENDATIONS FOR CALCULATING THE GLOBAL WARMING POTENTIAL
OF AVIATION IN LCA 7
4 RESULTS 8
5 OUTLOOK 9
6 REFERENCES 9
7 ANNEXE 11

© ESU-services Ltd. - iii -
Imprint
Citation
Niels Jungbluth;Christoph Meili (2018) Recommendations for calculation of the global warming
potential of aviation including the radiative forcing index. Int J LCA (accepted), DOI:
10.1007/s11367-018-1556-3, Schaffhausen, Switzerland, www.esu-services.ch/de/publications/
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jungbluth@esu-services.ch
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ing and research in the fields of life cycle assessment (LCA), greenhouse gas footprints, water
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© ESU-services Ltd. - iv -
Abstract
Aircrafts contribute more to global warming than can be expected from their CO2 emissions
alone. The relevant scientific evidence is available. However, suitable GWP factors (Global
Warming Potential) for relevant emissions are lacking. This is a shortcoming in the calculation
of the CO2 footprint (CF). In an article accepted by the Int J LCA, the state-of-the-art for ac-
counting for such impacts is presented. Approaches found for the so-called RFI (radiative-forc-
ing-index) factor are ranging from 1 to 2.7. This RFI factor can be multiplied with the direct
CO2 emissions of aircrafts to calculate the total global warming potential of aviation services.
An RFI factor of 2 on total aircraft CO2 emissions is recommended in this article because it is
based on the correct interpretation of the most recent scientific publications. If detailed data on
the share of emissions in the higher atmosphere are available, calculations will be more accurate
if the CO2 emissions in the higher atmosphere are multiplied by a factor of 5.2. In this way, in
typical assessments, this leads to a relevant increase in the GWP impacts due to aviation ser-
vices.
The proposed method can be applied in carbon footprint and life cycle assessment studies. It is
recommended to use this factor at least in a sensitivity analysis if impacts of aviation transport
play a relevant role in a life cycle. The factor also needs to be considered for aircrafts using
biofuels.
Kurzfassung
Flugzeuge tragen mehr zur globalen Erwärmung, als allein auf Grund ihrer direkten CO2-Emis-
sionen zu erwarten ist. Die entsprechenden wissenschaftlichen Erkenntnisse liegen vor. Es feh-
len jedoch geeignete GWP-Faktoren (Global Warming Potential) für die für diesen Effekt rele-
vante Emissionen. Dies ist ein Mangel bei der Berechnung des CO2-Fußabdrucks (CF). In ei-
nem Artikel, der vom Int J LCA zur Publikation angenommen wurde, wird der Stand der Wis-
senschaft zur Berücksichtigung solcher Auswirkungen in Ökobilanzen aufgezeigt. Die Ansätze
für RFI-Faktoren (radiative Forcing-Index) reichen von 1 bis 2.7. Dieser RFI Faktor wird dann
mit den direkten CO2-Emissionen von Flugzeugen multipliziert, um das gesamte Treibhauspo-
tenzial von Flugverkehrsdiensten zu berechnen.
Gemäss der Analyse im Artikel wird ein RFI-Faktor von 2 für die gesamten CO2-Emissionen
von Flugzeugen empfohlen, da er auf der richtigen Interpretation der neuesten wissenschaftli-
chen Veröffentlichungen basiert. Wenn detaillierte Daten über den Anteil der Emissionen in
der höheren Atmosphäre vorliegen, sind die Berechnungen genauer, wenn ein Faktor von 5.2
direkt mit der Menge der CO2-Emissionen in der höheren Atmosphäre multipliziert wird. Auf
diese Weise führt dies in der Bewertung zu einer relevanten Erhöhung des Treibhauspotenzials
durch Luftverkehrsdienstleistungen.
Der vorgeschlagene Ansatz und Faktor kann für die Berechnung von CO2-Fussabdrücken und
Ökobilanzen verwendet werden. Es wird empfohlen diesen Faktor in alle Studien, zumindest in
einer Sensitivitätsanalyse, einzusetzen, in denen ein relevanter Beitrag von CO2 Emissionen aus
dem Flugverkehr gegeben ist. Der Faktor muss auch dann berücksichtigt werden, wenn Bio-
treibstoffe für Flugzeuge eingesetzt werden.

© ESU-services Ltd. - v -
Résumé
La contribution des avions au réchauffement de la planète est supérieure à ce que l'on peut
attendre seul de leurs émissions de CO2. Les preuves scientifiques pertinentes sont disponibles.
Toutefois, les facteurs de PRP (potentiel de réchauffement de la planète) appropriés pour les
émissions pertinentes font défaut. Il s'agit d'une lacune dans le calcul de l'empreinte CO2 (FC).
Dans ce article accepté par l'Int J LCA, l'état de l'art en matière de comptabilisation de ces
impacts est présenté. Les approches vont de facteurs RFI (radiatif-forcing-index) de 1 à 2,7 qui
peuvent être multipliés par les émissions directes de CO2 des avions pour calculer le potentiel
de réchauffement global total des services aériens.
Un facteur de RFI de 2 sur les émissions totales de CO2 des avions est recommandé dans cet
article car il est basé sur l'interprétation correcte des publications scientifiques les plus récentes.
Si des données détaillées sur la part des émissions dans la haute atmosphère sont disponibles,
les calculs seront plus précis si un facteur RFI de 5,2 est multiplié par ces émissions de CO2
dans la haute atmosphère. De cette façon, dans des évaluations typiques, cela conduit à une
augmentation significative des impacts du PRG dus aux services aériens.
La méthode proposée peut être appliquée dans les études d'empreinte carbone et d'analyse du
cycle de vie. Il est recommandé de l'utiliser au moins comme analyse de sensibilité si les inci-
dences du transport aérien jouent un rôle important dans un cycle de vie. Ce facteur doit égale-
ment être pris en compte pour les avions utilisant des biocarburants.
Keywords
global warming potential, aviation, radiative forcing index, climate change, aircraft, transport
services

© ESU-services Ltd. - vi -
Recommendations for calculation of the global
warming potential of aviation including the radiative
forcing index
Niels Jungbluth;Christoph Meili
Article accepted for publication in the International Journal of LCA
DOI: 10.1007/s11367-018-1556-3
https://link.springer.com/article/10.1007/s11367-018-1556-3
Summary
Purpose
There are specific effects of emissions in high altitude, which lead to a higher contribution of
aviation to the problem of climate change than just the emission of CO2 from burning fuels.
The exact relevance is subject to scientific debate, but there is a consensus that aircrafts have
an impact that is higher than just their contribution due to the direct CO2 emissions. The gap
between this scientific knowledge on the one side and the missing of applicable GWP (global
warming potential) factors for relevant emissions on the other side is an important shortcoming
for life cycle assessment (LCA) or carbon footprint (CF) studies which aim to cover all relevant
environmental impacts of the transport services investigated.
Methods
In this paper, the state of the art concerning the accounting for the specific effects of aircraft
emissions in LCA and CF studies is discussed. Therefore, the relevant literature was evaluated,
and practitioners were asked for the approaches used by them.
Results
Five major approaches are identified ranging from an RFI (radiative forcing index) factor of 1
(no factor at all) to a factor 2.7 for the total aircraft CO2 emissions. If only emissions in the
higher atmosphere are considered, RFI factors between 1 and 8.5 are used or proposed in prac-
tice.
Conclusions
For the time being, an RFI of 2 on total aircraft CO2 (or 5.2 for the CO2 emissions in the higher
atmosphere if using present models in ecoinvent) is recommended to be used in LCA and CF
studies because it is based on the latest scientific publications; this basic literature cannot be
misinterpreted. Furthermore, it is also recommended by some political institutions. These fac-
tors can be multiplied by the direct CO2 emissions of the aircraft to estimate the total global
warming potential.

Introduction
© ESU-services Ltd. - 1 -
1 Introduction
Climate change is one of the environmental impacts addressed in nearly every life cycle assess-
ment (LCA) and it is in the focus of carbon footprinting (CF). The metrics commonly used for
the assessment is the global warming potential (GWP). This is expressed in most cases in the
unit of kilogram of carbon dioxide equivalents per functional unit (kg CO2-eq). The character-
ization factors allow assessing the relative impact of different greenhouse gases to the problem
of climate change. Different greenhouse gases such as methane (CH4) or dinitrogen monoxide
(N2O) are expressed as carbon dioxide (CO2), equivalents. Most LCA studies use the most re-
cent characterization factors published by the Intergovernmental Panel on Climate Change
(IPCC) with the reference year 2013 (IPCC 2013) or sometimes the older version with the ref-
erence year 2006 (Solomon et al. 2007).
These characterization factors did not change much in the past, based on more recent measure-
ments. The impact of such updates on calculated results was typically in the range of ± 5%.
1
Between 2013 and 2018 no indications on more relevant changes in these characterization fac-
tors were found within the LCA community.
However, there is one specific issue in this context, for which so far, no standardized method-
ology is available. There are several specific effects of emissions by aircrafts in the higher at-
mosphere which lead to a comparable higher contribution of aviation to the problem of climate
change than just the emission of CO2 (and other greenhouse gases) from burning the aviation
fuels. The following pathways are discussed (Penner et al. 2000; UBA 2012):
• Nitrogen oxide (NOx) emissions leading to ozone (O3) formation and methane (CH4) deg-
radation
• Stratospheric water
• Contrails
• Sulfate aerosols reflecting sunlight
• Soot aerosols absorbing sunlight
Nevertheless, it is difficult or impossible to provide global warming potential (GWP) charac-
terization factors for the different emissions that contribute to the problem and Penner et al.
(2000) states:
“GWP has provided a convenient measure for policymakers to compare the relative climate
impacts of two different emissions. However, the basic definition of GWP has flaws that make
its use questionable, in particular, for aircraft emissions. For example, impacts such as con-
trails may not be directly related to emissions of a particular greenhouse gas. Also, indirect RF
(radiative forcing) from ozone produced by NOx emissions is not linearly proportional to the
amount of NOx emitted but depends also on location and season. Essentially, the build-up and
radiative impact of short-lived gases and aerosols will depend on the location and even the
timing of their emissions. Furthermore, the GWP does not account for an evolving atmosphere
wherein the RF from a 1-ppm increase in CO2 is larger today than in 2050 and the efficiency of
NOx at producing tropospheric O3 depends on concurrent pollution of the troposphere. In sum-
mary, GWPs were meant to compare emissions of long-lived, well-mixed gases such as CO2,
CH4, N2O, and hydrofluorocarbons (HFC) for the current atmosphere; they are not adequate
to describe the climate impacts of aviation. In view of all these problems, we will not attempt
to derive GWP indices for aircraft emissions in this study. The history of radiative forcing (Fig-
ure 1), calculated for the changing atmosphere, is a far better index of anthropogenic climate
change from different gases and aerosols than is GWP.”
1
https://www.pre-sustainability.com/news/updated-carbon-footprint-calculation-factors, 15.08.2018

Introduction
© ESU-services Ltd. - 2 -
Figure 1 Radiative forcing from aircraft movements in 2005 and quality of assessments (Lee et al.
2009)
2
The newer publications of the IPCC do not provide as much details for the contribution of
aviation anymore as shown in Figure 2.
2
https://www.icao.int/Meetings/EGAP/Presentations/E-GAP_Session%20I_David%20Fahey.Avia-
tion%20Climate.final.pdf

Introduction
© ESU-services Ltd. - 3 -
Figure 2 Radiative forcing estimates in 2011 (IPCC 2013:30)
The exact relevance of the emissions from aviation is still the subject of scientific debate. Some
of the relevant emissions have a short life time. Thus, the concept of GWP, which has been
developed for long-lived emissions, is not applicable. Calculations for the contribution of NOx
to these effects show a high variation. The effect of aircraft emissions depends also considerably
on the exact location and timing of the emission due to the nonlinear chemistry, which is an
important difference compared to the effects caused by “normal” greenhouse gases (see Solo-
mon et al. 2007, chapter 2, paragraph 2.10.3.4 for further references). Several studies have ad-
dressed the direct impact of contrails, but the indirect effect of contrails has not yet been inves-
tigated in detail (Penner et al. 2000:3.6).
Another study shows that contrail cirrus gives the largest warming contribution in the short
term but remains important at about 15% of the CO2 impact in several regions even after 100-
years. Results in this paper also illustrate both the short- and long-term impacts of CO2: while
CO2 becomes dominant on longer timescales, it also gives a notable warming contribution al-
ready 20-years after the emission (Lund et al. 2017).
On the other side, there is not much doubt that aircrafts have an impact on climate change that
is higher than just its direct contribution due to the CO2 emissions from burning the aviation
fuels (e.g., UBA 2012). Even if the effects of aviation have a short-time effect and would

Introduction
© ESU-services Ltd. - 4 -
diminish soon after stopping this technology, this does not seem to be a realistic scenario for
the time frame of decisions made today with LCA and CF studies. Furthermore, one should
consider the exponential growing importance of aviation today (Bows-Larkin et al. 2016).
3
The
authors of one article investigating these developments summarize this with the headline con-
clusion “the aviation industry’s current projections of the sector’s growth are incompatible
with the international community’s commitment to avoiding the 2 ◦C characterization of dan-
gerous climate change” (Bows-Larkin et al. 2016).
The application of only the GWP for greenhouse gases thus leads to an underestimation of
radiative forcing effects caused by aircrafts. The gap between this scientific knowledge on the
one side and the missing of applicable GWP factors on the other side is an important shortcom-
ing for LCA or CF studies which aim to compare all relevant environmental impacts of transport
services.
Different publications calculate so-called radiative forcing index (RFI) factor that can be mul-
tiplied by the direct CO2 emissions from burning aviation fuels in order to account for all cli-
mate change effects of aviation. Estimations for the RFI factor are ranging from 1.9 to 5 (e.g.,
Grassl & Brockhagen 2007; IPCC 2001, 2007; Penner et al. 2000). But, so far, there is no clear
recommendation, e.g., by the IPCC on a specific RFI factor to be used as customary practice.
The RFI factor is based on the observation of the present impacts that can be attributed to the
total aircraft emissions within one reference year. It is assumed that the amount of emissions
will be in a steady state to estimate their contribution to climate change. So far, it is not related
to a specific time frame of observation while GWP can be calculated for 20-, 100- or 500-year
time horizons.
The total RF of aviation is estimated with 0.078Wm2 in 2005 and represents approximately
4.9% of total RF from all human activities (Fahey & Lee 2016).
Based on different publications, IPCC 2013 assesses the combined contrail and contrail-in-
duced cirrus effective radiative forcing for 2011 to be + 0.05 (+ 0.02 to + 0.15) W/m² take into
account uncertainties on spreading rate, optical depth, ice particle shape, and radiative transfer
and the ongoing increase in air traffic (IPCC 2013:610). A low confidence is attached to this
estimate.
Since the assessment of the IPCC for 2005, not much new insights have been gained concerning
the relevance of aviation (Fahey & Lee 2016). Thus, some researchers recommend neglecting
these effects in global assessments (e.g., Brasseur 2008:38).
It is not possible to calculate easily characterization factors for the emissions caused by aircrafts
which lead to this specific problem and thus the concept of GWP cannot be applied directly.
There is a lively debate within the scientific community if it makes sense to develop some type
of metrics for the emissions due to aviation that is comparable to the GWP used for other green-
house gases (e.g., Fuglestvedt et al. 2010). This article presents also a literature review for GWP
developed for all types of transport-related emissions.
The variability of approaches can also be found in practical applications. So far, there are many
approaches used by different carbon footprint calculators and LCA practitioners to deal with
this issue. A discussion of the approaches used in practice is the focus of this article. For un-
derstanding the different calculation practices, some key questions must be answered:
• Which RFI factor is used by the practitioners in the calculation?
• Is the RFI factor multiplied by the total CO2 emissions during the operation of the aircraft
or just with the part of emissions in the higher atmosphere?
3
http://data.worldbank.org and ICAO sustainability report 2016 (https://www.icao.int/environmental-
protection/Documents/ICAO%20Environmental%20Report%202016.pdf), online 11.06.2018.

Overview on approaches used in life cycle assessment and carbon footprinting
© ESU-services Ltd. - 5 -
• If the latter approach is used, how has the share of emissions in the higher atmosphere been
calculated?
The focus of this paper is to evaluate the state of the art of accounting for the specific effects of
aircraft emissions in LCA and CF studies. Therefore, LCA and CF experts were asked directly
and via different email discussion lists. Furthermore, relevant literature and internet investiga-
tion have been used to find further examples on this issue. It is not an aim of this article to
provide further knowledge or insights in the complicated matter as such. But the article should
help practitioners to interpret and understand the different approaches correctly and apply them
according to the goal and scope of their studies. Therefore, recommendations are provided how
to tackle this issue in practice. A first version as a working paper has been published in 2013
(Jungbluth 2013)and was then updated and extended in view of presentations at conferences in
2018.
2 Overview on approaches used in life cycle
assessment and carbon footprinting
Five major approaches for the interpretation of available literature, which are used in practice,
have been identified during the intensive literature research over the last seven years. All ap-
proaches identified in this research are shown in Table 1. They range from an RFI factor of 1
(no factor at all) to an RFI factor 2.7 applied on all aircraft CO2 emissions.
In life cycle inventory (LCI) analysis, information about the specific amount of aircraft CO2
emissions is difficult to extract (e.g., ecoinvent Centre 2010; European Commission 2010; His-
chier et al. 2001). But, in some databases such as ecoinvent CO2 emissions in the stratosphere
are accounted for as an emission in a specific sub-compartment (Frischknecht et al. 2007a;
Spielmann et al. 2007). This does allow to assign a specific GWP characterization factor for
this sub-category of CO2 emissions in the life cycle impact assessment.
In ecoinvent data v2.2 for average passenger transports by aircraft, the share of CO2 emissions
in the lower stratosphere and upper troposphere is 23.9% of the total aircraft CO2 emissions
(corrected data
4
from Spielmann et al. 2007). Thus, it is possible to recalculate the RFI factor
for this specific share of emissions in the higher atmosphere according to the following equita-
tion (1):
𝐶𝐹 𝐶𝑂2, 𝑠𝑡𝑟𝑎𝑡𝑜𝑠𝑝ℎ𝑒𝑟𝑒 =𝑅𝐹𝐼 𝑎𝑙𝑙 − (1 − 𝑆ℎ𝑎𝑟𝑒 𝐶𝑂2, 𝑠𝑡𝑟𝑎𝑡𝑜𝑠𝑝ℎ𝑒𝑟𝑒)
𝑆ℎ𝑎𝑟𝑒 𝐶𝑂2, 𝑠𝑡𝑟𝑎𝑡𝑜𝑠𝑝ℎ𝑒𝑟𝑒
(1)
where,
CF CO2,stratosphere = characterization factor for emissions of CO2 in the stratosphere
RFI all = RFI proposed for the total CO2 emissions of aircrafts
Share CO2,stratosphere = share of CO2 emissions in the stratosphere according to LCI data
The above mentioned RFI factor of 1 to 2.7 corresponds then to a characterization factor of 1
to 8.5 that can be applied on the CO2 emissions in the lower stratosphere and upper troposphere.
The column in Table 1 showing these figures is labeled as “RFI, fully on CO2, stratosphere” in
Table 1. These basic assumptions are also still valid for ecoinvent data v3.4 (ecoinvent Centre
2017):
4
An error in ecoinvent data has been discovered while elaborating this working paper and has been
corrected. The calculation of average contributions by Spielmann (2007:Table 7-7) was erroneous
and has been corrected with the shares of mode of operation provided by Spielmann (2007:Table
7-10).

Overview on approaches used in life cycle assessment and carbon footprinting
© ESU-services Ltd. - 6 -
1. The first group of approaches does not apply a specific RFI factor to aircraft CO2 emis-
sions. Thus, these approaches take a conservative interpretation of the available litera-
ture and only account for the GWP of greenhouse gases (IPCC 2007, 2013). The inter-
pretation that aircraft emissions do not have a specific higher impact is mainly made by
database developers (e.g. European Commission et al. 2011; Frischknecht et al. 2007b),
by software providers such as SimaPro (SimaPro 8.5.3), within life cycle impact assess-
ment methods (European Commission et al. 2011; Frischknecht et al. 2009; Goedkoop
& Spriensma 2000; Goedkoop et al. 2009; Huijbregts et al. 2017), and in several inter-
national standards related to LCA and carbon footprinting (e.g., Carbon Trust & DE-
FRA 2011; International Organization for Standardization (ISO) 2011; WBCSD & WRI
2011). Considering the broad range of literature confirming the surplus impacts of air-
crafts concerning climate change, these approaches are not considered to be appropriate
to be used in assessment.
2. The second group of approaches includes the GWP caused by contrails, water vapor,
and aviation-induced cirrus clouds. But, the contribution of clouds is neglected as the
estimate is considered to be too uncertain. Thus, this approach can be categorized as
minimum estimate of the possible effects (e.g., Ecoplan / Infras 2014:307; Frischknecht
et al. 2016). The approach can be used if generally a cautious perspective is taken on
uncertain environmental effects.
3. The third group of approaches applies a RFI factor of 2.7-3 only to the CO2 emissions
in the higher atmosphere (e.g., atmosfair 2008; Grießhammer & Hochfeld 2009; Knörr
2008). It seems as if it is not clear how the older IPCC publications have to be interpreted
and if the factor provided in these publications has to be applied to the total CO2 of the
aircraft or just the part in the higher atmosphere (Grassl & Brockhagen 2007; IPCC
2007; Penner et al. 2000). This approach was mainly found to be used in the German
language area. It seems to be based on a report and interpretation published by the Ger-
man Federal Environmental Agency (Mäder 2008). It is used by some companies for
calculations necessary to provide carbon offsetting for passenger flights (e.g., atmosfair
2008). As these approaches are based on partly outdated literature that is not easy to
interpret, they are not considered for providing recommendations in this article.
4. The fourth group of approaches applies a factor of 1.7 to 2 to all CO2 emissions from
aircrafts, which corresponds to a factor of about 3.9 to 5.2 for emissions in the higher
atmosphere. This approach is also used in more recent papers published in scientific
journals (Lee et al. 2009; Lee et al. 2010; Peters et al. 2011). These papers provide clear
recommendations how they applied and used the RFI factor. The Stockholm Environ-
ment Institute and the German Umweltbundesamt came also to these RFI figures based
on a more political discussion of different literature sources (Kollmuss & Crimmins
2009; UBA 2012). This RFI factor is used by at least one company providing carbon
offsetting services (myclimate 2009). A new but in the range similar calculation has
been made (Azar & Johansson 2012). They calculated emission weighting factors
(EWFs) for the CO2 from aircrafts with five different metrics (GWP, GTP, SGTP, and
two economic metrics, relative damage cost (RDC) and a cost-effective trade-off (CE-
TO)). The range found for the EWF was 1.3 to 2.9. They named 1.7 to be the best esti-
mate using the GWP metric. This group of approaches seems to be based on the most
recent literature. The range of results is confirmed by different independent researchers.
Thus, this group of approaches seems to be the most appropriate one for an estimation
of the effects.
5. The last group of approaches is based on the same original literature as the third one
(IPCC 2007), but interprets the factors 2.7 to 2.8 in a way that it has to be applied to the
total CO2 released by aircrafts (Frischknecht et al. 2007b; Gössling & Upham 2009).
This would correspond to an RFI factor of about 8.1 to 8.5 on the CO2 emissions in the
higher atmosphere. This approach is used by some companies providing carbon

Recommendations for calculating the global warming potential of aviation in LCA
© ESU-services Ltd. - 7 -
offsetting services such as Primaklima
5
and greenmiles.
6
As this seems a misinterpreta-
tion and overestimation of the effects, this group of approaches is not considered for the
recommendations in this article.
The scenarios calculated by two groups of authors (Frischknecht et al. 2007b; Peters et al. 2011)
consider also the share of different types of emissions to the total. This would allow calculating
specific GWP factors for the contribution of single air emissions as described in the beginning
of this article. Nevertheless, these GWP factors depend on the actual total amount of emissions
contributing to these pathways and thus it would be more complicated to be updated.
Another approach to tackle this problem is the characterization of emissions like water,
NMVOCs, particulates, NOx, and SOx with single characterization factors for each type of
emission. Two publications have been found that suggest such factors (Fuglestvedt et al. 2010;
Lund et al. 2017). We tried to apply these factors in our LCA software SimaPro, but different
difficulties occurred in the interpretation of the published factors (e.g., they are not provided
per kg of emission or information concerning the share of emissions in higher and lower atmos-
phere were missing). Both approaches also still applied an additional RFI factor on the CO2.
Results of this calculation seem to be lower than the RFI factor recommended by us by a factor
of 5, but due to the uncertainty of the interpretation we refrain from publishing these results
here.
Due to these uncertainties, an approach to apply characterization factors on different single
emissions is thus not further followed up in this article because of the high uncertainties while
interpreting the available literature.
3 Recommendations for calculating the global
warming potential of aviation in LCA
This paper cannot solve all the scientific issues and difficulties behind calculating RFI or GWP
of aircraft emissions. Nevertheless, it seems to be necessary to better harmonize the approaches
used in LCA and CF calculations and to provide better guidance on this issue. In the moment,
different approaches come to quite different results and thus have an enormous influence on the
outcome of studies where emissions from aircrafts play a significant role.
Different approaches have been evaluated in depth in the previous chapter. The influence on
the results has been highlighted in Table 1 (supplementary material). Currently, a characteriza-
tion factor (CF) of 2 kg CO2-eq per total direct aircraft CO2 emissions (or 5.2 for the emissions
in the higher atmosphere if using ecoinvent v2.2, ecoinvent v3.4, or ESU 2018 data) is seen as
the most convincing approach for the following reasons. It is based on the different approaches
used in scientific publications (Azar & Johansson 2012; Lee et al. 2009; Lee et al. 2010; Peters
et al. 2011). This basic scientific literature cannot be misinterpreted (as it is the case for the
third and fifth group of approaches). The proposal does not neglect the proofed additional ef-
fects of aviation. It is based on a reasonable guess of the average effects in contrast to the second
approach which only makes a minimum assumption. Furthermore, it is also recommended by
some political institutions (Kollmuss & Crimmins 2009; UBA 2012).
It is recommended to apply the factor if possible in the LCA calculation tool only on the emis-
sions in the higher atmosphere because this allows for a better differentiation between short-
and long-distance flights. Based on the evaluations of the state of the art in this article, it is
recommended using this factor for the time being.
While using other databases, the average share of emissions in higher atmosphere must be con-
sidered in the calculations and the characterization factor for CO2 emission in the higher
5
www.prima-klima-weltweit.de
6
www.greenmiles.de

Results
© ESU-services Ltd. - 8 -
atmosphere can be calculated accordingly according to equation (1). For the applications with
ecoinvent data a characterisation factor of 5.2 is calculated in equation (2):
5.2 = 2 − (1 − 23.9%)
23.9%
(2)
A CSV file with an LCIA method for SimaPro is provided as supplementary material for this
article.
Depending on the goal and scope of their study, LCA practitioners might also apply other ap-
proaches as described in the previous chapter. This article can then help to provide arguments
in view of such a choice.
4 Results
Figure 3 shows the implications of this recommendation for the calculation of the GWP with a
100-year time horizon according to IPCC (2013) and expressed in carbon dioxide equivalents
(CO2-eq). Without applying an RFI factor, long- and short-distance flights show a carbon foot-
print between 118 and 230 g of CO2-eq per passenger-kilometer, respectively. Including addi-
tional impacts in the higher atmosphere rises this to 230 to 340 g of CO2-eq. Taking the RFI
factor into account, flying is clearly worse from a global warming point of view than other
means of passenger transportation compared in Figure 3. Without the application of an RFI
factor, short-distance flights would have about the same emissions as average passenger cars.
It must be noted that for a full environmental picture and comparison of different means of
transport, also other environmental indicators must be considered in an LCA. Thus, this figure
should only be read as an example regarding the influence of the impact assessment for the
global warming indicator, but not as a general statement regarding the pros and cons of different
transport devices.
Figure 3 Global warming potential 2013 of different means of passenger transports based on ESU
database 2018 (ESU 2018; LC-inventories 2018; Spielmann et al. 2007) considering the
recommended RFI factor of 5.2 for emissions in the higher atmosphere. RER – European
average, CH – Switzerland, DE – Germany, FR – France, IT - Italy

Outlook
© ESU-services Ltd. - 9 -
The results presented in this figure can also be directly compared with the results for an average
airplane calculated with all approaches investigated in this paper as shown in Table 1.
5 Outlook
This recommendation should be revised as soon as the IPCC provides clear recommendations
on this issue or if new scientific results are published leading to different conclusions.
This is an ongoing debate. Thus, comments by the LCA and CF community to this paper and
its usefulness are highly welcome.
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7 Annexe

Annexe
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Table 1 Overview on approaches used for the calculation of greenhouse gas emissions related to aviation. If not provided in the publication, the “RFI, fully on CO2,
stratosphere” has been calculated based on the share of this type of emissions in ESU database 2018.
Group
Application
RFI, CO2
stratosphere
RFI, other
airplane
CO2
RFI, fully on
CO2, strato-
sphere
calculated
GWP per
pkm
Interpretation
Scientific background paper
1
Ecoinvent
1
1
1.0
0.168
Frischknecht et al. 2007b
IPCC 2007
SimaPro
1
1
1.0
0.168
SimaPro 8.5.3
IPCC 2007
PAS 2050:2011
1
1
1.0
0.168
Separate reporting of aircraft CO2 is
necessary
Carbon Trust & DEFRA 2011
ISO/CD 14067.3:2011
1
1
1.0
0.168
CO2 from aircrafts should be reported
separately, no recommendation for
assessment
International Organization for
Standardization (ISO) 2011
Product Accounting & Reporting
Standard
?
?
?
?
For air travel emission factors, multi-
pliers or other corrections to account
for radiative forcing may be applied to
the GWP of emissions arising from
aircraft transport. If applied compa-
nies should disclose the specific fac-
tor used.
WBCSD & WRI 2011
ILCD Handbook
1
1
1.0
0.168
Not mentioned as a specific issue
Hauschild et al. 2011
2
Frischknecht et al. 2016
http://www.lcaforum.ch/por-
tals/0/df66/DF66-
02_Frischknecht.pdf
1.35
1.35
1.50
0.210
Additional GWP caused by contrails,
water vapor and aviation induced cir-
rus clouds. Contribution of clouds ne-
glected as to uncertain, 70% of CO2
in stratosphere
Ecoplan / Infras 2014:307, Lee et
al. 2010
Forster et al. 2006, 2007, without
cirrus
1.2
1.2
1.8
0.192
Gössling & Upham 2009
Cited as Forster et al. (2006,
2007), http://www.sciencedi-
rect.com/science/arti-
cle/pii/S1352231005010587$
3
PCF - Germany
2.7
1
2.7
0.216
Grießhammer & Hochfeld 2009
IPCC 2007; Penner et al. 2000
Atmosfair
3
1
3.0
0.225
atmosfair 2008
Grassl & Brockhagen 2007 based
on IPCC 2007
EcoPassenger
3
1
3.0
0.225
Based on (atmosfair 2008), calculated
range of total RFI of 1.27 to 2.5 based
on travel distances.
Knörr 2008

Annexe
© ESU-services Ltd. - 13 -
Group
Application
RFI, CO2
stratosphere
RFI, other
airplane
CO2
RFI, fully on
CO2, strato-
sphere
calculated
GWP per
pkm
Interpretation
Scientific background paper
CO2OL, www.co2ol.de
1.27-2.7
1.27-2.7
3.0
0.225
Depending on travel distance. Own
assumption based on (Grießhammer
& Hochfeld 2009; Knörr 2008).
Knörr 2008
ESU-services, scenario, 2010
2.99
1
3.0
0.224
geometric mean of RFI 1.9 to 4.7, at-
mosfair concerning application only to
CO2, stratosphere
Grassl & Brockhagen 2007 based
on IPCC 2007
4
Stockholm Environment Institute
2
2
5.2
0.286
Kollmuss & Crimmins 2009
IPCC 2007
myclimate
2
2
5.2
0.286
myclimate 2009
Kollmuss & Crimmins 2009
Lee et al. 2009
2
2
5.2
0.286
N. Jungbluth
Lee et al. 2009; Lee et al. 2010
Klima-Allianz Schweiz
2
2
5.2
0.286
Klima-Allianz Schweiz 2016
Lee et al. 2009; Lee et al. 2010
Peters et al. 2011
1.9
1.9
4.9
0.280
N. Jungbluth, Soli: I think, but don’t re-
member 100% sure, that the share of
air emissions occurring in higher alti-
tudes were adapted by the cicero
people to reflect the aviation industry
average, but that the fuel use data
from the air process given in the re-
port, were used.
Peters et al. 2011
Azar 2012
1.7
1.7
3.9
0.251
This study
2
2
5.2
0.287
Recommendation for best-practice
This paper
5
Forster et al. 2006, 2007, with
max. cirrus
2.8
2.8
8.5
0.381
Gössling & Upham 2009
Cited as Forster et al. (2006, 2007)
ecoinvent, scenario
2.72
2.72
8.2
0.372
Frischknecht et al. 2007b
IPCC 2007
Primaklima
2.7
2.7
8.1
0.369
http://www.prima-klima-welt-
weit.de/co2/kompens-berechnen.php
IPCC 2007
greenmiles
2.7
2.7
8.1
0.369
Personal communication with Dr.
Sven Bode (Greenmiles GmbH)
IPCC 2007