Has the time has come to include
radiative forcing in the calculation of aviation emissions to more accurately
reflect climate impact?
Whether you are a scientist, in
business, an activist or just a plain member of the public, we all have one
thing in common; reducing our carbon footprint to help achieve the Intergovernmental
Panel on Climate Change (IPCC) 1.5oC objective.(1) The focus of action, or rather the
blame game, seems to veer from not eating meat to electric cars. We are
encouraged to use less, reuse or at best recycle. Yet there is an elephant in the room that we
are all too scared to at look in the face, and this is aviation and the real contribution
of our constant flying is making to climate change.
The fact that direct greenhouse gas
(GHG) emissions from aviation represents just 2% of global emissions, tends to
underestimate its importance in emission reduction. Taken in a different context, if aviation was
considered as a country, it would rank in the top 10 emitters,(2) with flying a significant and easily
manageable portion of personal, family and business carbon footprints. A
study in Ireland found personal air travel represented on average 20% of the
Irish primary household footprint, equivalent to 1.15 tonnes CO2 per
capita per year (t CO2 ca-1y-1) of an average
personal footprint of 5.70 t CO2 ca-1y-1.(3)
A person flying Dublin to New York (5100 km) and back generates 1.05 t CO2
ca-1. So, each flight we take is a significant contributor to our
total annual emissions. Aviation was
included in the European Union Emissions Trading System (EU ETS) in 2012 making
it the largest sectoral emitter of greenhouse gases after electricity
generation. This was followed in 2016 by
the International Civil Aviation Organization (ICAO) introducing The Carbon Offsetting and Reduction
Scheme for International Aviation (CORSIA) which aims to stabilise
emissions at 2020 levels while requiring airlines to offset emissions in excess
of this agreed level.(4) Although
these measures will help make the industry more efficient, they are unlikely to
affect growth, with aviation emissions rising by 70% between 2006 and 2018, and
predicted to rise substantially over the next 30 years (i.e. 300-700%). Scheduled airlines in 2006 carried passengers
equivalent to almost 4 trillion revenue passenger kilometres (RPK- i.e. a
passenger kilometre is equivalent to one passenger transported one kilometre)
which had risen to 7.8 trillion by 2017.(5) Passenger (RPK) traffic
is expected to continue to increase by 4.6% per annum over the period up to
2012 to 2032.(6)
Emission
factors for aviation are based on the distance flown, with passengers and cargo
considered separately. In Europe these are calculated by the EUROCONTROL small emitters tool.(7) This software uses the fuel efficiency
of each aircraft type and also the average number of passengers carried per
flight (i.e. the load factor), providing either CO2 emissions per
passenger kilometre (pKm-1) travelled on specific aircraft, or more usefully as
average emission values for all aircraft used for domestic, short-haul
(<3700km) or long-haul (>3700km) flights. Passenger kilometres are
transformed to CO2 emissions using conversion factors based on a range of variables with longer
flights tending to be more efficient in terms of CO2 emitted per km
due to the landing and take-off (LTO) cycle requiring a more intense fuel burn than cruising at a constant altitude (Table 1).(8)
Table 1 Calculation of CO2
emissions factors for passenger flights adjusted for average passenger numbers
carried (i.e. load factor), and weight of the passengers in relation to any
cargo carried, expressed as a percentage of the tonnes carried per kilometre (t
km-1). Emissions are
expressed in grams of CO2 emitted per passenger kilometre flown (g CO2 pkm-1). These
values do not include radiative forcing.(8)
Type of flight
|
Load factor
(%)
|
Passenger
only
|
Combined
passenger and cargo
|
||
Passenger
t km-1
(% of total)
|
emissions
g CO2 pkm-1
|
Passenger
t km-1
(% of total)
|
emissions
g CO2 pkm-1
|
||
Domestic
|
73.7
|
100
|
144.9
|
99.77
|
144.6
|
Short-haul
|
79.9
|
100
|
80.2
|
98.70
|
78.7
|
Long-haul
|
74.0
|
100
|
122.1
|
85.13
|
103.1
|
Emissions from carrying cargo by air
is normally considered separately, with values for dedicated cargo aircraft
being 2.9, 0.9 and 0.8 kg CO2 tonne kilometre (t km-1)
for domestic, short- and long-haul flights respectively. However, long-haul
passenger aircraft carry up to eight times more cargo in total than long-haul
cargo aircraft alone. So, like passenger load factors, this affects how the
emissions per passenger is calculated (Table 1).
As first and business class
passengers take up considerably more space in the aircraft, between 3-6 times
more, than an economy passenger, the total number of passengers carried is
reduced thereby increasing the average CO2 emissions per
passenger. When these are reflected in the
emission factors then significant changes in emissions per passenger kilometre
travelled are seen (Table 2). This mean that on the return flight from Dublin
to New York with a weighted average emission of 1.05 tonnes of CO2
per passenger, the economy passenger is only responsible for 0.8 tonnes
compared to 2.3 tonnes for the business class passenger or 3.2 tonnes of CO2
emitted for each first-class passenger on the same flight.
There is overwhelming evidence that
the impact of aviation emissions on climate are far greater than are currently
being estimated using CO2 emissions alone. One of the problems in
considering GHG emissions from aviation in terms of carbon footprinting is the
diverse range of emissions that result from flying at high altitudes resulting in the impact of emissions being greater
than at ground level.(9) The radiative forcing Index (RFI) accounts for non-CO2 climate
change factors such as aerosols, water vapour and NOx, and is a multiplier that
adjusts CO2 production to estimate the full impact of aviation on
climate. Although there are
uncertainties associated with the RFI, especially as non-CO2 effects
are largely independent of actual CO2 production, its use remains
the best option available.(8) The best estimates of radiative forcing come from the IPCC report of 1999(10)
supported by subsequent researchers.(9)
But for over three decades atmospheric scientists have been unable
to agree the extent to which aviation impact on climate is being
underestimated. Since the IPCC report, no alternative RFI value other than 1.9
has been considered for adoption. So, with the IPCC, EU and the UK all
accepting a 1.9 RFI, based on best available scientific evidence, we need to
act now by including RFI in GHG emission accounting of aviation if we are to
achieve the IPCC 1.5oC target. (1, 8, 12, 13)
This raises the question of whether in life cycle analysis or in carbon
footprinting, emissions from aircraft
should be considered differently from land based emissions. Non-CO2 emissions are not always
taken into account in other sectors, but aviation poses a different problem due
to the complexity of its effects on global warming; so simply taking a fuel
derived footprint does not take into account the total impact which is much
higher than their CO2 emissions alone. Although many researchers have suggested
non-CO2 effects should not be included until the science is better
understood, the consensus is that a RFI
should be used in all GHG analyses to take into account the full climate impact
of aviation emissions.(14, 15) So, when RFI is added to the CO2
emissions from that return flight from Dublin to New York the weighted average
increases to 2.0 tonnes of carbon dioxide equivalent per capita (t CO2e
ca-1), with economy passengers each emitting 1.5 t CO2e
ca-1, business class 4.4 t CO2e ca-1 and first
class 6.1 t CO2e ca-1.
That is almost six times higher than the unweighted CO2 value
commonly used at present (Table 2).
Table 2 The CO2 emissions factors adjusted for cabin seating
class(8) with and without RFI.
Flight Type
|
Seating class
|
Load factor
(%)
|
% total
seating
|
Emissions
g CO2
pkm-1
|
Emissions
plus RFI
g CO2e pkm-1
|
Domestic
|
Weighted
average
|
73.7
|
100
|
144.6
|
231.4
|
Short-haul
|
Weighted
average
|
79.9
|
100
|
78.7
|
149.5
|
Economy
|
79.9
|
96.7
|
77.4
|
147.1
|
|
First/Business
|
79.9
|
3.3
|
116.2
|
220.8
|
|
Long-haul
|
Weighted
average
|
74.0
|
100
|
103.1
|
195.9
|
Economy
|
74.0
|
83.0
|
78.9
|
149.9
|
|
Economy plus
|
74.0
|
3.0
|
126.3
|
240.0
|
|
Business
|
74.0
|
11.9
|
228.9
|
434.9
|
|
First
|
74.0
|
2.0
|
315.7
|
599.8
|
The IPCC recommend a reduction in
personal emissions to 2.5 tonnes by 2030 and 0.7 tonnes by 2050 to keep within
the 1.5oC objective.(1) Many countries, including Ireland and
the UK set GHG emission reduction targets of 80% from 1990 levels by 2050. However, during 2019 countries began to
seriously consider a ‘net zero’ target reflecting the urgency to tackle climate
change. With total revenue passenger
kilometres flown increasing by 4.7% per year to 2032, any delay in
adopting RFI begs the question if we really are taking the effect of flying on
the climate seriously? If we are to hope
for any meaningful reduction in overall carbon emissions, then individuals and
households will have to start engaging far more in the managed reduction of
their primary carbon footprint which should include all personal travel
including flying.
We need to use real emission values in
carbon management. The IPCC has shown that we each have a personal allowance as
well as reduction goals when it comes to emissions. How you use your allowance is up to you, but
we need urgent policy changes so we accurately account for emissions from aviation
by including radiative forcing in calculations.
Professor Nick Gray, Centre for the Environment, University of Dublin Trinity College, Dublin 2, Ireland.
References:
(1) IPCC, Global Warming of 1.5oC. Summary
for Policymakers. Intergovernmental Panel on Climate Change, Switzerland.
(2017) https://report.ipcc.ch/sr15/pdf/sr15_spm_final.pdf
Accessed 23 September 2019.
(2) EU, Reducing Emissions form Aviation. (2019)
https://ec.europa.eu/clima/policies/transport/aviation_en
Accessed 19 September 2019.
(3) T.
Kenny, N.F. Gray, A preliminary survey of household and personal carbon dioxide
emissions in Ireland. Environment International 35: 259-272. (2009) https://doi.org/10.1016/j.envint.2008.06.008
(4) ICAO, Doc 9501. Environmental Technical Manual:
Volume IV. Procedures for demonstrating compliance with
the Carbon Offsetting and Reduction Scheme for International Aviation. International Civil Aviation Authority,
Montreal, Canada. (2018) https://www.icao.int/environmental-protection/CORSIA/Pages/ETM-V-IV.aspx
(5) IATA Economic Performance of the Airline
Industry. (2018) https://www.iata.org/publications/economics/Reports/Industry-Econ-Performance/IATA-Economic-Performance-of-the-Industry-mid-year-2018-report-final-v1.pdf
(6) ICAO, ICAO Long-Term Traffic Forecasts: Passenger and cargo. (2016) https://www.icao.int/Meetings/aviationdataseminar/Documents/ICAO-Long-Term-Traffic-Forecasts-July-2016.pdf
(7) Eurocontrol, Small emitters tool (SET)- 2018: Emissions
calculator. (2018) https://www.eurocontrol.int/publication/small-emitters-tool-set-2018
(8) BEIS, 2018
Government GHG Conversion Factors for Company Reporting: Methodology Paper For Emission Factors -
Final Report. Department for Business, Energy and Industrial Strategy, UK
Government, London. (2018) https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/726911/2018_methodology_paper_FINAL_v01-00.pdf
(9) D.S. Lee, G. Pitari, T.
Berntsen, V. Grewe, K. Gierens, J.E. Penner, A. Petzold, M. Prather, U. Schumann, A. Bais, D. Iachetti, L.L. Lim, Transport impacts on atmosphere and climate: aviation. Atmospheric Environment 44: 4678-4734. (2010)
(10) IPCC, Aviation and the global atmosphere. J.E.
Penner, D.H. Lister, D.J. Griggs, D. Dokken, M. McFarland, Eds., Intergovernmental Panel on Climate
Change. Cambridge University Press, Cambridge, UK. (1999)
(11) IPCC, IPCC Guidelines for National Greenhouse Gas Inventories. Volume 2 —
Energy. Chapter 1 — Introduction. (2006)
http://www.ipcc-nggip.iges.or.jp/public/2006gl/pdf/2_Volume2/V2_1_Ch1_Introduction.pdfAccessed
26 September, 2019.
(12) EU,
Emission Reduction Targets for
International Aviation and Shipping. Directorate General for International
Policy. IP/A/ENVI/2015-11, European Parliament, Brussels. (2015)
