International aviation emissions to 2025: Can emissions be stabilised without restricting demand?

Abstract

International aviation is growing rapidly, resulting in rising aviation greenhouse gas emissions. Concerns about the growth trajectory of the industry and emissions have led to calls for market measures such as emissions trading and carbon levies to be introduced to restrict demand and prompt innovation. This paper provides an overview of the science on aviation's contribution to climate change, analyses key trends in the industry since 1990, projects international civil aviation emissions to 2025 and analyses the emission intensity improvements that are necessary to offset rising international demand. The findings suggest international aviation carbon dioxide (CO₂) emissions will increase by more than 110 per cent between 2005 and 2025 (from 416 Mt to between 876 and 1013 Mt) and that it is unlikely emissions could be stabilised at levels consistent with risk averse climate targets without restricting demand.

1. Introduction

Sixty years ago, civil aviation was an infant industry that was responsible for a tiny proportion of the transport task. Today it is an integral part of the world economy, accounting for approximately nine per cent of global GDP and carrying more than two billion passengers and 41 million tonnes of freight and mail each year (Airbus, 2007; International Civil Aviation Organization (ICAO), 2007a; United Nations Statistics Division (UNSD), 2008). Total world revenue traffic (international and domestic, passenger and cargo) on scheduled airlines in 2006 exceeded 510 billion revenue tonne kilometres (RTK),1 which included almost four trillion revenue passenger kilometres (RPK).2 In the same year, total revenues of the scheduled airlines of the parties to the Convention on International Civil Aviation of December 1944 (Chicago Convention) topped US$450 billion, more than 120 per cent above 1990 levels (ICAO, 2007a).

The success of the aviation industry is poised to continue over the coming decades. Industry forecasts predict worldwide RPK will grow at an average rate of approximately five per cent per annum over the next 20 years (Airbus, 2007; Boeing, 2007). Cargo traffic, as measured in RTK, is expected to grow at around six per cent over the same period. If realised, these forecasts would see passenger (RPK) and cargo (RTK) traffic increase by 180 per cent and 220 per cent, respectively, between 2006 and 2026.

Continuing rapid growth in aviation would provide economic benefits and allow greater mobility amongst the world's population. However, these benefits would come at a cost, most notably a significant increase in aviation greenhouse gas emissions. While aviation is not currently one of the main drivers of global warming, the growth trajectory of the industry suggests it could become a significant factor over the coming decades. A report prepared by the Intergovernmental Panel on Climate Change (IPCC) in 1999 at the request of the ICAO found that civil aviation carbon dioxide (CO₂) emissions could rise by between 60 per cent and over 1000 per cent between 1992 and 2050 (IPCC, 1999). More recent research suggests that if strong global economic growth continues, aviation CO₂ emissions are likely to experience a greater than three-fold increase between 2000 and 2050 (Berghof et al., 2005; Horton, 2006). Concerns about rapid growth in the industry and the associated threat to the climate system have prompted debate about the future of aviation.

The international market has been the focal point of a considerable amount of the discussion about aviation and climate change. This is partly due to the fact that international aviation is responsible for over 60 per cent of total aviation emissions and is the fastest growing segment of the market. Measures to address international aviation emissions must also deal with several important legal and policy issues that do not arise in the context of domestic aviation.

Concerns about emissions growth have led to calls for additional market measures to be introduced to restrict demand and prompt innovation in international aviation. These proposals have met resistance. Many governments have been concerned about the potential for such measures to adversely affect the aviation sector and associated industries such as tourism. Governments opposed to these initiatives have also raised questions about the legality of international aviation emission levies and measures that seek to unilaterally impose carbon prices on the extraterritorial emissions of foreign aircraft. The aviation industry has voiced similar concerns. Like many governments, the industry's current position is that it is opposed to emission levies, but supports emissions trading subject to certain conditions (e.g. it is introduced on the basis of mutual consent between participating nations and that the schemes are open and allow for permits to be traded with other sectors) (International Air Transport Association (IATA), 2008a, International Air Transport Association (IATA), 2008b).

This paper aims to add to the growing body of literature on aviation and climate change by providing an overview of the science on aviation's contribution to climate change, analysing key trends in the industry since 1990, projecting international civil aviation CO₂ emissions to 2025 and determining the emission intensity improvements that are necessary to offset rising international demand. The objective is to highlight that it is unlikely international aviation emissions could be stabilised at levels consistent with risk averse climate targets (i.e. keeping the increase in the global average surface temperature to ∼2 °C above pre-industrial levels) without restricting demand.

The paper is set out as follows. Section 2 provides background on the reasons for the separation of international and domestic aviation in policy processes. Section 3 discusses aviation's climate impacts. Section 4 analyses recent trends in the aviation industry, focusing on the international market. Section 5 projects international civil aviation emissions over the period 2005–2025 using ICAO traffic data. Section 6 analyses the emission intensity improvements that are necessary to offset the projected increases in demand. Section 7 provides a conclusion.

2. The division between international and domestic aviation

Civil aviation is split into international and domestic components for the purposes of greenhouse gas emissions accounting and climate governance. The IPCC Guidelines for National Greenhouse Gas Inventories, which are used for the purposes of reporting under the United Nations Framework Convention on Climate Change (UNFCCC), defines international civil aviation emissions as ‘emissions from flights that depart in one country and arrive in a different country’, including take-offs and landings for the relevant flight stages (IPCC, 2006, p. 3.58). Domestic aviation emissions are defined as ‘emissions from civil domestic passenger and freight traffic that departs and arrives in the same country’ (IPCC, 2006, p. 3.58). Military aviation emissions are reported separately from civil aviation, or not reported at all.

Under the UNFCCC, domestic aviation emissions are required to be included in national emission totals and are intended to be addressed at the national level within the UNFCCC/Kyoto Protocol regime. In contrast, international civil aviation emissions are excluded from national emission totals and are only reported in UNFCCC national inventory reports as a memo item. Further, Article 2(2) of the Kyoto Protocol vests responsibility for international aviation emissions in ICAO, a United Nations agency established under the Chicago Convention. As a consequence, international aviation emissions are intended to be addressed within the regime established under the Chicago Convention rather than through the UNFCCC.

The reasons for the separation of international and domestic aviation emissions relate to the nature of international transport, the UNFCCC greenhouse accounting framework and international aviation law. The emission accounting rules under the UNFCCC dictate that emissions are only attributable to a country if they result directly from activities within its territory. International transport involves movements between countries, creating a problem for the standard accounting system. Further, under international aviation law, each country has complete and exclusive sovereignty over the airspace above their territory. Beyond their territorial airspace, the capacity of a country to regulate aircraft is limited and is subject to the body of law associated with the Chicago Convention and related bilateral air service agreements. Due to these issues, ICAO was seen as the appropriate body for addressing international aviation emissions.

To date, ICAO has had little success in curbing the growth in international aviation emissions or progressing effective policy solutions. Its failures have been a product of a collection of factors. Most countries have been reluctant to undertake significant emissions abatement and obstruct growth in the aviation industry. As discussed, there is also an ongoing dispute about the legality of emission levies and the capacity of a country to unilaterally impose carbon prices on the extraterritorial emissions of foreign aircraft. In addition, there are debates about how international aviation emissions should be allocated between countries and the most appropriate design of an emissions trading scheme that applies to the sector. The political problems that have hindered progress on international aviation emissions are not unique. However, the legal, accounting and policy design issues that arise in this context set it apart from most other areas, including domestic aviation. International shipping is the only other sector that is confronted with the same types of problems.

3. Greenhouse gas emissions from aviation

Aircraft emit a range of gases and particles which affect the atmosphere. In the context of climate change, the major emissions are CO₂, nitrogen oxides (NO_x),3 water vapour (H₂O), sulphur oxides (SO_x) (which form sulphate particles) and soot (IPCC, 1999).4 A summary of the atmospheric impacts of these agents and estimates of their associated radiative forcings in 2000 is provided in Table 1 . Radiative forcing is a measure of the impact of an agent on the energy balance of the earth's atmosphere. It is technically defined as the change in net irradiance at the tropopause (i.e. the boundary between the troposphere and stratosphere) and is measured in watts per square metre (W/m²) (IPCC, 2001). A positive number indicates the agent has a warming effect, a negative number indicates a cooling agent.

Table 1.

Radiative forcing from aviation emissions in 2000

Agent	Comment	Radiative forcing (mW/m2)a	Scientific understanding
CO2	CO2 is a well-mixed, long-lived (5–200 years) greenhouse gas and its warming effects on the climate are relatively well understood	+25	Good
Nox	NOx emissions have warming and cooling effects. The warming arises because aviation NOx emissions result in the production of ozone (O3) (a greenhouse gas) in the troposphere. The cooling effects arise because chemical reactions associated with NOx remove methane (CH4) (a greenhouse gas) from the atmosphere
O3		+22	Fair
CH4		−10	Fair
H2O	Aviation H2O emissions have three main impacts. H2O emitted from aircraft builds up in the lower stratosphere and traps infrared radiation, leading to warming. Aviation H2O emissions create contrails, which act like clouds and trap heat. Contrails created by aircraft can lead to the creation of cirrus clouds, which also leads to warming
H2O		+2	Fair
Contrails		+10	Fair
Cirrus		+30 (range +10 to +80)	Poor
SOx	SOx emissions lead to the formation of sulphur aerosols, which reflect solar radiation and thereby cool the atmosphere. They can also have indirect effects by altering cloud formation. The radiative forcing estimates only consider the direct effects	−3.5	Fair
Soot	Soot can act as a warming agent by trapping and radiating heat. It can also have indirect warming effects by altering the albedo (reflectivity) of the earth's surface. The radiative forcing estimates only consider the direct effects	+2.5	Fair
Total (without cirrus cloud effects)		+48

Source: Sausen et al. (2005).

^amW/m² means milliwatt or 10⁻³ W per square metre.

The IPCC has estimated that the total radiative forcing associated with anthropogenic agents in 2005 was 1.6 W/m² (0.6–2.4 W/m²). This suggests aviation's contribution excluding cirrus cloud effects is in the order of three per cent. When cirrus cloud effects are included, aviation's contribution could possibly be as high as six to eight per cent. Hence, while aviation is not currently one of the main drivers of global warming, its effects are significant and should not be overlooked in policy processes.

A technical difficulty facing policy makers seeking to address aviation emissions is how to account for non-CO₂ emissions. The nature of most non-CO₂ emissions precludes the use of standard global warming potentials (GWPs) to convert non-CO₂ emissions into carbon dioxide equivalents (CO₂-e). GWPs are a measure of the cumulative radiative forcing associated with a unit mass of the relevant direct greenhouse gas when compared to the same mass of CO₂ over a specified period (typically 100 years) (IPCC, 2001). They are not calculated for H₂O and indirect greenhouse gases like CO and NO_x because of the variable nature of the relevant climate impacts (Forster et al., 2006; IPCC, 2001). The absence of GWPs for H₂O, NO_x and CO emissions means it is very difficult to compare the impacts of a unit mass of these gases to those associated with a unit mass of direct greenhouse gases like CO₂.

One proposed solution is to use a radiative forcing index (RFI) (i.e. the ratio between the total radiative forcing from aviation and the radiative forcing associated with aviation CO₂ emissions) to devise a metric (often called an ‘uplift factor’), which is used to estimate aviation CO₂-e emissions. To do so, CO₂ emissions from aviation are multiplied by the uplift factor to provide an estimate of the total impact of aviation emissions in CO₂-e. There is debate about the appropriateness of uplift factors and their use in policy processes. Questions have also been raised about what uplift factor should be applied. Recent studies indicate that the appropriate uplift factor could range between 1.7 and 5.1 times aviation CO₂ emissions depending on the timescales of the analysis (Forster et al., 2006; Sausen et al., 2005). If the purpose is to provide an approximation of CO₂-e using a 100-year timeframe, an uplift factor of 1.7 appears to be the best estimate, although it is subject to considerable uncertainty.

4. Recent trends in aviation traffic and emissions

The aviation industry has grown rapidly in recent decades. Between 1990 and 2006, total scheduled world RTK increased by 119 per cent, with scheduled passenger (RPK) and cargo (RTK) traffic rising by 108 and 140 per cent, respectively. The major driver of global traffic growth since 1990 has been international services. Between 1990 and 2005, total international RTK increased by 141 per cent. In comparison, total domestic RTK rose by only 54 per cent (see Fig. 1 ).

Fig. 1.

Total international and domestic revenue tonne kilometres (RTK), 1990–2005. Source: Author estimates based on ICAO (1991–2007) and IATA (2000–2007).

The increase in aviation traffic over the past 20 years has been accompanied by dramatic improvements in emission intensity. Data from the IATA suggest that system-wide fuel efficiency of scheduled IATA member services (i.e. international and domestic, passenger and cargo) improved by 23 per cent between 1994 and 2006; falling from 51.3 to 39.4 litres per RTK (L/RTK) (International Air Transport Association (IATA), 2000–2007, International Air Transport Association (IATA), 2007).5 Airbus has published data indicating that the fuel efficiency of the world passenger fleet improved by approximately 35 per cent between 1990 and 2007, falling from 7.3 to 4.7 L/100 RPK (Airbus, 2007).

The fuel efficiency and emission intensity of international aviation has followed the global trend. Over the period 1990–2005, the emission intensity of international aviation improved by 40 per cent (191–113 kg CO₂/100 RTK).6 These changes are attributable to three factors: beneficial changes in air traffic management (ATM) (ICAO Secretariat, 2007); improvements in aircraft and engine design7 ; and a significant increase in load factors (i.e. aircraft are using more of their capacity). As shown in Fig. 2 , the greatest advances in the emission intensity of international aviation were experienced in the 1990s when the average annual rate of improvement was almost 4.5 per cent. Since 2000, the rate of improvement has fallen to 1.2 per cent, suggesting the opportunities for gains are diminishing.

Fig. 2.

Emission intensity of total international aviation, 1990–2005, kg CO₂/100 RTK. Source: Author estimates based on IEA (2007a), ICAO (1991–2007) and IATA (2000–2007).

Despite the improvements in emission intensity, aviation CO₂ emissions have increased considerably since the early 1990s. Although there are differences in the estimates, the weight of evidence suggests that total global civil aviation CO₂ emissions rose from approximately 400 Mt in 1990 to at least 650 Mt in 2004 (International Energy Agency (IEA), 2007b, International Energy Agency (IEA), 2007c; IPCC, 1999; Kim et al., 2005). Data published by the IEA indicate that international aviation CO₂ emissions increased by 33 per cent over this period, from 292 to 390 Mt (IEA, 2007a).8 Global fossil CO₂ emissions (i.e. emissions from fossil fuel combustion, cement manufacture and gas flaring) in 2004 were approximately 29,029 Mt (7910 Mt C) (Marland et al., 2007), suggesting that aviation constituted approximately 2.2 per cent of the world total, with international aviation emissions comprising 1.3 per cent.

Aviation demand is set to experience rapid growth over the next 20 years. ICAO's most recent forecast suggests that the total scheduled passenger task (RPK) will grow at an average rate of 4.6 per annum between 2005 and 2025, with scheduled international RPK growing at 5.3 per cent per annum over the same period. Scheduled freight services are also expected to increase considerably, with overall freight RTK growing at 6.6 per cent per annum and international freight RTK growing at 6.9 per cent per annum. Boeing and Airbus have forecast similar rates of traffic growth (Airbus, 2007; Boeing, 2007).

An expectation of strong economic growth is the main underlying driver of the aviation traffic projections. Aviation traffic is usually highly responsive to economic growth rates; with increases in economic activity prompting greater than proportional increases in traffic (i.e. demand is income elastic).9 Where this relationship does not hold, the cause can typically be traced to unforeseen market shocks. These market dynamics are evident in the trends in international aviation since 1990 (see Fig. 3 ). International traffic growth rates have generally been responsive to global GDP over this period. Significant divergences between the two were witnessed in response to a number of external shocks, including the East Asian Financial Crisis in 1997–98, the September 11 terrorist attacks in 2001 (which coincided with the global economic slowdown in 2001–02) and the severe acute respiratory syndrome (SARS) outbreak that began in November 2002.

Fig. 3.

Annual growth rate of international aviation traffic (RTK) versus annual real growth rate of global GDP (at PPP), 1990–2005. Source: Author estimates based on Datastream International Database (2008), ICAO (1991–2007) and IATA (2000–2007).

Over the coming decades, strong global economic growth is expected to lead to considerable growth in aviation demand. To offset the emission increases caused by rising demand, there would have to be radical improvements in emission intensity. The available evidence suggests this is unlikely. The IPCC's special report on aviation (IPCC, 1999) included projections based on estimates of traffic demand prepared by ICAO's Forecasting and Economic Analysis Subgroup (FESG) and economic growth rate data from the IPCC's IS92a, IS92e and IS92c emission scenarios. The scenarios based on technological assumptions that are consistent with the current design philosophy suggest global civil aviation CO₂ emissions will rise from approximately 415 Mt in 1992 to between 1440 and 2302 Mt in 2050 (excluding the less likely, low growth Fc1 scenario). Several subsequent reports have produced similar findings. Details of selected results from the IPCC report and two subsequent papers are provided in Table 2 .

Table 2.

Civil aviation projections to 2050

Authors	Comment	Base year estimate (Mt CO2)	2050 estimate (Mt CO2)
IPCC (1999)	The IPCC special report on aviation included details of several scenarios. The range here relates to the mid (Fa1) and high (Fe1) growth scenarios	415 (year 1992)	1440–2302
Berghof et al. (2005)	The European Union's ‘CONSAVE 2050’ project estimated global aviation emissions using assumptions similar to those in the IPCC's A1, A2 and B1 scenarios. The range here relates to the three most likely scenarios (ULS, RPP and FW)	470 (year 2000)	955–2442
Owen and Lee (2006)	Projections calculated using the FAST model based on FESG data to 2020 then IPCC growth data from the A1 and B2 scenarios for 2020 to 2050. The data cover scheduled traffic only, which partially explains the low historical emission estimates	482 (year 2005)	1996–2971

Aviation's contribution to global warming should not be exaggerated. Compared to activities such as electricity generation and agriculture, it is a minor contributor. However, sharp increases in emissions from any sector could compromise attempts to achieve risk averse climate targets and arguably the objective of avoiding dangerous climate change.

Article 2 of the UNFCCC states that the objective of the convention is the ‘stabilisation of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system’. Based on the available science and informed by value judgments, many individuals and institutions have suggested that the threshold for dangerous climate change is an increase in the average surface temperature of between 1.7 and 2.5 °C above pre-industrial levels.10 To have a reasonable chance of preventing global warming exceeding a 2 °C threshold, which is the most commonly cited target, the atmospheric concentration of greenhouse gases would have to be stabilised at less than 450 ppm CO₂-e. Meeting this stabilisation target would require dramatic action. Global carbon emissions would have to be reduced by in excess of 60 per cent on 2000 levels by 2050 (IPCC, 2007). The latest generation of climate models that account for climate–carbon cycle feedbacks suggest the required emission reductions may be greater than previously believed and that significant emission reductions may be necessary by 2030 just to keep the atmospheric concentrations of CO₂ below 450 ppm (∼500–550 ppm CO₂-e) (Friedlingstein et al., 2006; Meehl et al., 2007).

Theoretically, an increase in aviation emissions could be accommodated within a risk averse policy framework if there was offsetting abatement in other sectors. However, the extent of abatement that is necessary to ensure the increase in the global average surface temperature does not exceed 2 °C, and the speed at which it would have to occur, leaves little room for significant increases in emissions from any sector. Consequently, if there is a desire on behalf of policy makers to pursue a temperature target of around 2 °C, it is likely that steps will need to be taken in the near future to curb aviation emissions. International aviation emissions are of particular concern because of the speed at which demand is increasing and the political, legal and policy issues hindering the introduction of effective abatement measures.

5. International aviation emission projections

5.1. Emission projections

The reason for the concern about international civil aviation can be demonstrated by projecting international emissions over the period 2005–2025. To do this, the following equation was used:

$$E_n={\mathrm{RTK}}_n\times {\mathrm{EI}}_n$$

where E_n=t CO₂ emissions in year n, RTK_n=projected RTK in year n, EI_n=emission intensity of international aviation in year n, measured in t CO₂ per RTK.

This is a simple emission intensity approach. It does not account for the interaction between demand and emission intensity and is only intended to provide a broad indication of possible outcomes.

Two scenarios were developed using the emission intensity equation: S1 and S2. Details of S1 and S2 are provided in Table 3 .

Table 3.

Traffic and emission intensity assumptions for S1 and S2

Scenario	Traffic growth assumptions	Emission intensity assumptions
S1	Scheduled passenger and freight RTK grow in accordance with ICAO forecasts for the period 2005–2025. The ICAO growth rates are: •scheduled passenger growth rate=5.3%/yr •scheduled freight growth rate=6.9%/yr	International fleet emission intensity improves in line with UK DTI assumptions: •1.3%/yr between 2006 and 2010; •1.0%/yr between 2011 and 2020; and •0.5%/yr between 2021 and 2025.a
	Scheduled mail RTK grows in accordance with the average over the previous decade (i.e. 2.2%/yr) for the period 2006–2015, then the rate falls to 2.0%/yr between 2016 and 2025
	Based on historic trends, non-scheduled passenger and cargo RTK is assumed to grow by 1.5%/yr over the period 2006–2015, and by 1.0%/yr between 2016 and 2025
S2	As for S1	International fleet emission intensity improves in line with the IATA target of a 25 per cent improvement between 2005 and 2020 (i.e. 1.9%/yr). Between 2021 and 2025, the fleet emission intensity improves by 1.0%/yr

^aThese emission intensity improvement rates are derived from the United Kingdom Department of Trade and Industry (UK DTI) fleet fuel efficiency improvement estimates discussed in IPCC (1999).

The traffic data for 2005 and 2006 were obtained from ICAO (2007a) and International Air Transport Association (IATA), 2006, International Air Transport Association (IATA), 2007, with the exception of the non-scheduled cargo data for 2006, for which data were not available. The 2006 non-scheduled cargo estimate was derived in accordance with the assumptions discussed in Table 3. The emission intensity for the base year (2005) was derived using the IEA's estimate of international aviation emissions (IEA, 2007a). There is considerable uncertainty associated with international traffic, fuel and emission statistics, including those published by the IEA. The weaknesses in the available databases are well known and are discussed at length elsewhere.11 Key problems include the reporting of non-scheduled traffic and the allocation of fuel between domestic, international and military uses. These issues add further uncertainty to the emission projections.

The results for S1 and S2 are shown in Fig. 4 . Indices of projected CO₂ emissions at five-year intervals are provided in Table 4 . Under S1, international aviation CO₂ emissions rise from 416 in 2005 to 1013 Mt in 2025. Under S2, emissions increase to 876 Mt in 2025.

Fig. 4.

International civil aviation CO₂ emissions, 2005–2025.

Table 4.

International civil aviation CO₂ emissions, 1995–2025, indices of growth

	1995	2000	2005	2010	2015	2020	2025
S1	84	87	100a	122	152	190	244
S2	84	87	100a	119	141	168	211

^aBase year (2005) estimate=415.65 Mt CO₂.

The above projections of international civil aviation CO₂ emissions do not fully account for the impacts of aviation emissions on the climate system. As discussed in Section 3, aviation non-CO₂ emissions (particularly NO_x and H₂O) have important climate impacts and are believed to account for 50–80 per cent of the current warming associated with aviation. If an uplift factor of 1.7 is used to estimate total CO₂-e emissions, the emission intensity model suggests international aviation emissions will be between 1488–1722 Mt in 2025. While the use of uplift factors is controversial, these figures illustrate the potential magnitude of the total climate impacts associated with international aviation.

5.2. Comparison with previous projections

The results for S1 and S2 from the emission intensity model are at the higher end of previous international aviation emission projections. Olsthoorn (2001) projected that international aviation CO₂ emissions will be 86–144 per cent above 1995 levels by 2020 (or 383–503 Mt) and 187–513 per cent above 1995 levels by 2050 (591–1263 Mt) if carbon is not priced. These results suggest that by 2025, emissions will be approximately 100–190 per cent above 1995 levels (∼410–600 Mt CO₂). In comparison, the emission intensity model predicts that emissions will be 150–190 per cent above 1995 levels in 2025 (876–1013 Mt CO₂). The results from Olsthoorn's (2001) low and high emission growth scenarios (the Ecological and Schumpter scenarios (marked ‘OE’ and ‘OS’ respectively)) are shown in Fig. 5 , along side the results from the emission intensity model. Much of the difference in absolute numbers is attributable to Olsthoorn's (2001) lower estimate of emissions in 1995 (206 Mt CO₂ compared to the latest IEA estimate of 306 Mt CO₂). In relation to the proportional increases, the emission intensity model results overlap the upper end of the range in Olsthoorn (2001). The higher estimates from the emission intensity model are due mainly to differences in economic growth rates, which are a major determinant of aviation demand. The scenarios in Olsthoorn (2001) assumed annual global real GDP growth of 1.6–3.1 per cent. The ICAO traffic forecast used in the emission intensity model assumed real GDP growth of 3.5 per cent to 2025.

Fig. 5.

Comparison of results from emission intensity model to those from international aviation emission studies, 1995–2030, Mt CO₂.

Owen and Lee (2006) provide another source of comparison. The report, which was prepared for the UK Department of Environment, Food and Rural Affairs (DEFRA), projected scheduled domestic and international aviation CO₂ emissions over the period 2000–2050. The traffic forecasts prepared by the FESG for the sixth meeting of ICAO's Committee on Aviation Environmental Protection (CAEP/6) were used as the basis for the projections to 2020 (Wickrama et al., 2003). Traffic forecasts for 2020–2050 were developed using a non-linear Verhulst logistic model derived from the historical relationship between aviation traffic and GDP between 1960 and 1995. The model was applied using the GDP growth rates from the IPCC's A1 and B2 SRES scenarios. The results suggest that scheduled international aviation CO₂ emissions will increase from approximately 240 Mt in 2000, rising to 264 Mt in 2005, then 540 Mt in 2020 and to between 666 and 808 Mt in 2030. These results suggest scheduled international aviation CO₂ emissions will be approximately 600–675 Mt in 2025 (∼150–180 per cent above 2000 levels). The emission intensity model indicates that emissions will be 141–179 per cent above 2000 levels in 2025. The results from Owen and Lee's (2006) A1 and B1 scenarios are marked on Fig. 5 as ‘OL-A1’ and ‘OL-B1’, respectively. The differences between the Owen and Lee (2006) projections and those from the current study are primarily due to a lower base year fuel quantity estimate in Owen and Lee (2006) and differences in scope (i.e. Owen and Lee (2006) exclude non-scheduled traffic, which is likely to have reduced their estimates by approximately 10 per cent).

As discussed in Section 4, a number of long-range global aviation emission projections have been prepared. Vedantham and Oppenheimer (1998) projected global aviation emissions over the period 1990–2100 and provided interim results for 2025. Berghof et al. (2005) (the ‘CONSAVE’ project) conducted a similar exercise for the period 2000–2050, providing interim results for 2020. As part of the AERO2k project, Eyers et al. (2004) projected global aviation emissions between 2002 and 2025, while Horton (2006) projected global emissions between 2002 and 2030 using the AERO2k database and FESG traffic forecasts. Fig. 6 compares the results from the emission intensity model to selected adjusted results from Vedantham and Oppenheimer (1998), Eyers et al. (2004), Horton (2006) and Berghof et al. (2005). The international component of the global estimates from these studies was estimated by assuming that international civil aviation CO₂ emissions are proportional to the international share of global scheduled traffic. The international/domestic traffic shares were devised using the ICAO (2007b) traffic forecasts.12

Fig. 6.

Comparison of results from emission intensity model to those from global aviation emission studies, 1990–2030, Mt CO₂.

From Vedantham and Oppenheimer (1998), only the base-demand level projections for the IS92a (IPCC base case), IS92c (low growth) and IS92e (high growth) scenarios are shown (marked ‘IS92a’, etc.). Their high-demand projections were excluded because they currently appear to be less plausible than the base-demand projections. From Berghof et al. (2005), only the Unlimited Skies (ULS) and Regulatory Push and Pull (RPP) scenarios are shown (marked as ‘CONSAVE-ULS’ and ‘CONSAVE-RPP’). The other two scenarios from Berghof et al. (2005) (Fractured World (FW)13 and Down to Earth (DtE)14 ) were considered less likely. From Horton (2006), only the Case 3 (or base case) outcomes are shown (marked ‘AERO2k-H’) because Cases 1 and 2 were theoretical cases based on no change in fuel efficiency, while Cases 4 and 5 were dependent on the imposition of carbon prices. The results from Eyers et al. (2004) are marked as ‘AERO2k-E’.

As Fig. 6 shows, the results of the current study for 2025 align well with the base case (IS92a) and low growth (IS92c) scenarios from Vedantham and Oppenheimer (1998), while being noticeably higher than those from Eyers et al. (2004) and Berghof et al. (2005).15 The trajectory of S1 and S2 also suggests emissions will be above the Horton (2006) projections in 2030. The differences between the results of the current study and those from Eyers et al. (2004), Horton (2006) and Berghof et al. (2005) are attributable to a combination of a higher base year estimate in the current study, coverage (i.e. the current study includes scheduled and non-scheduled traffic) and higher traffic growth rates.

International aviation demand is growing faster than many previously forecast. ICAO's (2007) traffic analysis suggest the strong growth will continue over the next two decades. The rise in oil prices and economic slowdown experienced in 2008 may alter these forecasts. However, if the 2007 predictions are realised, international aviation emissions are likely to be at the upper end of current projections.

6. Can international emissions be stabilised without cutting demand?

The results from the emission intensity model indicate that international aviation CO₂ emissions are likely to be more than 110 per cent above 2005 levels by 2025. Given the ambitious nature of the IATA target of improving the fuel efficiency of the fleet by 25 per cent by 2020,16 the more probable outcome is an increase in international aviation emissions of greater than 140 per cent by 2025. If the international community agrees to pursue risk averse climate targets, new measures are likely to be necessary to control international aviation emissions.

Calls for the introduction of mandatory measures to address aviation emissions have met with industry resistance. This is not extraordinary; most emission-intensive industries have resisted mandatory measures to curb greenhouse gas emissions. In responding to proposals for mandatory measures, the industry has relied heavily on the improvements in emission intensity as a means of deflecting criticism. Yet history has shown that demand growth generally outstrips emission intensity gains. Whether this trend continues in the future is unknown. However, the available evidence suggests international aviation emissions are unlikely to be stabilised unless there is a radical shift in technology or demand is restricted.

To demonstrate this, two hypothetical scenarios (S3 and S4) were developed using the traffic forecasts outlined in Table 3.

• •S3 sought to answer the question: if demand is not restricted, by how much will the emission intensity of international aviation have to improve in order to keep CO₂ emissions below 831 Mt in 2025 (i.e. 100 per cent above 2005 levels).
• •S4 was the same as S3, only the object was to determine by how much the emission intensity of international aviation would have to improve in order to stabilise CO₂ emissions at their 2005 levels by 2025 (i.e. 416 Mt).

The results are shown in Table 5 and are compared to those from S1 and S2. Fig. 7 shows the emission intensities under S1–S4 and compares these to the trends over the last decade (1996–2005).

Table 5.

Emission intensity improvements to keep international aviation emissions below targets, 2005–2025

	Emission intensity in 2005 (kg CO2/100 RTK)	Emission intensity 2025 (kg CO2/100 RTK)	Average annual improvement (%)	Total improvement 2005–2025 (%)
S1	113	94	1.0	18
S2	113	81	1.7	29
S3	113	77	1.9	32
S4	113	39	5.2	65

Fig. 7.

Historic and projected emission intensity improvements 1996–2025.

As Table 5 and Fig. 7 illustrate, the task faced by the aviation industry if it proposes stabilising emissions without restricting demand is daunting. Without the imposition of carbon prices on international flights to suppress demand, it seems unlikely that international aviation emissions will be able to be kept below 850 Mt CO₂ by 2025 (i.e. more than 100 per cent above 2005 levels). International aviation CO₂ emissions are only likely to stay below this level if one or more of the following events occur: a prolonged global economic slowdown; an international shock, such as a major international conflict, pandemic or a substantial increase in the price of oil; or the emergence and rapid deployment of new emission saving technology. While more efficient aircraft are currently being rolled out (e.g. A380 and B787), the fuel efficiency gains associated with the latest generation of aircraft are unlikely to be sufficient to offset the increases in international demand. Moreover, international aircraft have relatively long commercial lifetimes (typically 15–35 years) (ICAO, 2007c). Without aggressive policy measures to promote the deployment of state-of-the-art technology, the slow rate of turnover in the fleet will hinder progress on curbing emissions growth.

7. Conclusion

The international aviation industry is facing increasing pressure to curb growth in its greenhouse gas emissions. To date, it has been able to deflect calls for the imposition of effective abatement measures, successfully arguing that it is only a minor cause of global warming and that it has achieved admirable advances in emission intensity since the early 1990s. The cause of the industry has been aided by a lack of political will and legal and policy disputes about how best to address emissions from international aviation.

Frustrated at the lack of action, the European Union (EU) has proposed extending its emissions trading scheme to international aviation in 2012 (Council of the European Union (CEU), 2007; European Commission (EC), 2006). This proposal has been met with near universal opposition from non-EU countries. At this point, it is unclear whether the EU will go ahead with its scheme and defy the international community. However, its willingness to propose the scheme is a sign of the intensity of the pressure for more action to be taken to contain the growth in emissions.

The basis for the EU's concerns has been highlighted in this article. Projections based on ICAO's (2007) traffic forecast suggest international aviation CO₂ emissions will increase by between 111 and 144 per cent between 2005 and 2025 (i.e. from 416 Mt to between 876 and 1013 Mt). These projections are at the upper end of previous estimates, primarily because of an increase in forecast economic growth, higher base year emission estimates and broader coverage. It should also be emphasised that they are confined to CO₂ emissions and do not account for emissions of other gases and particles like H₂O and NO_x. If an uplift factor of 1.7 is used to estimate total CO₂-e emissions, the emission intensity model suggests international aviation emissions will be between 1488 and 1722 Mt in 2025.

Stabilising international aviation emissions at levels consistent with risk averse climate targets without restricting demand will be extremely difficult. To prevent emissions from increasing by more than 100 per cent between 2005 and 2025, the emission intensity of international aviation would have to fall by 32 per cent, requiring an average annual decrease of 1.9 per cent. To stabilise emissions at 2005 levels, the emission intensity would have to be improved by 65 per cent, necessitating an average annual decrease of 5.2 per cent. With current technology, emission intensity improvements of this magnitude appear unlikely.

Further action on international aviation emissions is likely to be necessary if the international community wants to keep the increase in the global average surface temperature to less than 2 °C. Under current policy settings, emissions are likely to increase significantly unless there is a major global economic downturn or other shock to the aviation market.