The elephant in the room. Dealing realistically with emissions from aviation

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) 1.5^oC 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 1.5^oC 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 CO_2e ca^-1), with economy passengers each emitting 1.5 t CO_2e ca^-1, business class 4.4 t CO_2e ca^-1 and first class 6.1 t CO_2e 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 1.5^oC 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 1.5^oC. 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)

https://www.europarl.europa.eu/RegData/etudes/STUD/2015/569964/IPOL_STU(2015)569964_EN.pdf

 (13) IPCC, Summary for Policymakers. In: S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, H.L. Miller, Eds., Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge. (2007)

 (14) M. Berners-Lee, D.C. Howard, J. Moss, K. Kaivanto, W.A. Scott, Greenhouse gas footprinting for small businesses - the use of input-output data.  Science of the Total Environment 409: 883-91. (2011)

 (15) N. Jungbluth, C. Meil, Aviation and Climate Change: Best practice for calculation of the global warming potential. ESU-services Ltd. commissioned by ESU-services Ltd., Schaffhausen, Switzerland, (2018) www.esu-services.ch/de/publications/