Guidance

Technical note: A comparison of aviation emissions methodologies

Updated 23 October 2024

Technical terms

Definitions of technical terms can be found in the glossary.

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We welcome feedback from users on this methodology document and on future needs for the reporting and transparency of aviation emissions data. You can provide your comments by completing this survey which covers the following themes:

• whether there is a need for a regular report on these estimates
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• how the methodology comparison could be improved
• what factors would emissions estimates need to consider in the future

Alternatively, you can provide feedback by contacting us at [email protected].

Overview

Data sources

This technical note compares the three main sources of publicly available UK aviation emissions estimates. These are:

• the Department for Energy Security and Net Zero (DESNZ)
• the Organisation for Economic Co-operation and Development (OECD)
• the European Organisation for the Safety of Air Navigation (EUROCONTROL)

Summary of UK carbon (CO2) emissions from aviation in 2022, by data source

• DESNZ estimates of the total UK aviation emissions (domestic and international) were 29.3 million tonnes carbon dioxide (MtCO2)
• OECD and EUROCONTROL estimates were 26.1 MtCO2 and 27.7 MtCO2 respectively
• Compared to estimates from DESNZ, estimates from OECD were 11% lower, and estimates from EUROCONTROL were 6% lower

Summary of methodologies

DESNZ provide official estimates that set the benchmark for UK emissions and meets strict requirements in line with UNFCCC reporting. However, these estimates come with a significant time lag.

EUROCONTROL and OECD are based on aircraft activity rather than UK fuel usage, and so typically underestimate emissions. These are still a useful source of more timely data and are also easier to use for international comparisons.

The importance of accurate emissions estimates

Decarbonising aviation is a challenging but important goal for the UK government to realise its net zero ambitions. Monitoring and reporting emissions in an accurate and timely manner so policy makers can develop effective strategies to manage and reduce emissions from the sector is essential.

According to official statistics, aviation formed 23% of all transport emissions in 2019, making it the second largest transport emitter when both domestic and international emissions in the UK are considered. A projected increase in passenger numbers, and the need for global coordination, means that decarbonisation will require a consistent, long-term effort from government and industry, both in the UK and internationally.

This document aims to provide greater transparency on the reporting of aviation emissions estimates to the public. It explains the approaches used to estimate aviation emissions, before comparing how the methodologies used differ and the implications of this for the estimates produced. To compare approaches on a consistent basis, this note focuses on carbon dioxide (CO2) emissions, but will also specify where methodologies are able to estimate other types of emissions.

Commercial data sources also exist but are not covered by this methodology note which focuses on public sources only. This note sets out the 3 main publicly available sources for estimating direct emissions from aircraft combustion engines, otherwise known as Scope 1 emissions from aviation, and the methodological differences between them.

1. DESNZ publish final estimates of UK greenhouse gas emissions annually every February. These are official statistics used to monitor progress against the UK’s emissions reduction targets. Since 2023 they have published provisional data containing timelier estimates for international aviation, reducing the lag in official statistics on international aviation emissions by nearly a year.
2. The OECD publish monthly CO2 estimates for the UK on the OECD data explorer. More details on the methodology for calculating these estimates is available in the report OECD Statistics Working Paper CO2 emissions from air transport.
3. EUROCONTROL, the European Organisation for the Safety of Air Navigation, is the central organisation for coordination and planning of air traffic control for all of Europe and provides data to the EEA (European Environment Agency). They publish CO2 emissions for all EUROCONTROL member states, including the UK on the EUROCONTROL Aviation Sustainability Unit webpage.

General approach

The aviation emissions under consideration in this document refer to territorial emissions, rather than ‘residency’ based emissions for consistency with United Nations Framework Convention on Climate Change (UNFCCC) guidelines. This assigns all emissions from flights leaving airports in the country of departure to that country.

The estimation of aviation emissions is typically based on flight level data which is then aggregated. The general approach for estimating the emissions generated by an individual flight is to take the distance or duration of the flight, estimate how much fuel was burnt, and estimate emissions produced from burning that much fuel. There are several variables to consider for each of these 3 parts:

1. For flight distance or duration, different phases of a flight require different modes of engine thrust and therefore different fuel burn. Take-off requires the most thrust and taxiing around the airport requires much less. The distances or duration of parts of the flight vary by airport and can be particularly hard to calculate for short journeys where planes do not reach cruise altitudes. For the overall flight distance, several estimates are used including the shortest distance around the surface of the globe between the departure and arrival points (great circle distance), and more precise distances derived from aircraft surveillance equipment or flight plan details provided to airspace regulatory bodies.
2. For fuel burn, different aircraft burn fuel at different rates depending on their size, age and the fuel efficiency of their engines.
3. For emissions, the factors used for converting fuel burn into emissions, and the types of emissions covered, will affect the final estimates. Aviation emissions from all the relevant flights are aggregated to get a final figure for the UK, which is then typically reported in terms of MtCO2. The primary greenhouse gas produced by aircraft is CO2, but there are also smaller amounts of nitrous oxide (N2O) and methane (CH4) produced as well, as shown in DESNZ’s statistics. Where these different gases are reported together, they are expressed in terms of MtCO2e, a metric which weights non-carbon dioxide gases by their global warming potential relative to that of carbon dioxide.

The specifics of this general approach vary for each set of estimates. The rest of this document will detail the differences, which are summarised in Table 1 in the next section.

Comparison of methodologies

Department for Energy Security and Net Zero (DESNZ)

DESNZ estimates for UK aviation emissions can be found on the UK territorial greenhouse gas emissions page and more details on the methodology for calculating these estimates is available in section MS7 of the annual report submitted by the UK NAEI (National Atmospheric Emissions Inventory) to the UNFCCC. Methodology details for the provisional estimates can be found in the provisional 2022 UK greenhouse gas emissions statistics methodology summary.

The NAEI estimates are produced by a group of specialist consultants on behalf of DESNZ, the Department for Environment, Food and Rural Affairs (DEFRA) and the devolved administrations. DESNZ UK greenhouse gas emissions official statistics are used for monitoring decarbonisation across government due to their trustworthiness and quality. These separate domestic aviation emissions, as part of the UK’s emissions total which are reported to the United Nations Framework Convention on Climate Change (UNFCCC), and emissions from UK-based international aviation bunkers, which are reported to the UNFCCC as a memo item.

DESNZ publishes final annual statistics around a year after the end of the reported year (2022 data was published in February 2024). Provisional data is available a few months after year end (2023 provisional statistics were published in March 2024).

Flight distance or duration

DESNZ primarily use the Civil Aviation Authority’s (CAA) Airport Data, which covers around 50 UK airports (and UK Crown Dependencies), for its flight data input. To estimate the distance covered by the flights in this data, DESNZ uses the great circle distance between departure and arrival airport for each flight. This distance estimate is then directly converted to fuel burn.

Great circle distance: the shortest distance between two points on the surface of the earth. In the real world, the trajectory of a plane is impeded by other factors such as meteorological conditions and airspace restrictions^{[footnote 1]}. This is in addition to the inherent error from the assumption in great circle distance calculations that the earth is a perfect sphere. DESNZ recommend that in the absence of fuel burn data an uplift is applied, with an increase of 8% applied to the 2023 Government Greenhouse Gas Conversion Factors for Company Reporting. This is lower than the 9 to 10% suggested by the International Panel on Climate Change (IPCC) but has been agreed with DfT based on analysis as more appropriate for flights arriving and departing from the UK.

Fuel burn

Aircraft data comes from the EMEP/EEA Emission Inventory Guidebook, which gives a selection of generic aircraft types and their fuel burn over several standard flight distances. The CAA’s Airport Data contains a more detailed list of aircraft types than in the EMEP/EEA Emission Inventory Guidebook. Therefore, each specific aircraft type in the CAA data has been assigned to a generic type in the Guidebook.

The Inventory Guidebook is produced jointly by EMEP (European Monitoring and Evaluation Programme) and the EEA (European Environment Agency) and provides guidance on how to prepare national emissions inventories.

Emissions

DESNZ official statistics have the most sophisticated methodology for estimating emissions for phases of flight at lower altitudes: in particular, airport specific studies were conducted that calculated aircraft exhaust emissions in kilograms from each mode of operation in kilograms per second, and the duration of each phase including landing and take-off, taxi-in and landing. They also incorporated data on real-world operational efficiency and adjusted estimates for both airport and aircraft type in the study.

Using the local airport studies and emissions factors from the EMEP/EEA Emissions Inventory Guidebook, annual fuel burn for each mode of engine thrust is obtained by summing contributions over all engines for all aircraft movements provided by the CAA in the year. This is also adjusted using estimates for the average times spent in each mode at specific airports from the local airport studies.

The emission factors used for carbon dioxide are from Fuels Industry UK and for other gases are a combination of national airport specific factors for landing and take-off (derived from local airport studies) and EMEP/EEA EUROCONTROL cruise factors for generic aircraft.

The unique aspect of DESNZ’s methodology is that it is ultimately calibrated to fuel sales, as opposed to deriving emissions from estimated fuel burn only. To do this, these emissions estimates have been normalised against the fuel consumption data in DESNZ’s Digest of UK Energy Statistics (DUKES). This is done to align with the UNFCCC’s reporting requirements, which mandate that emissions totals are based on fuel sold. Whether emissions from refuelling at UK-based international aviation bunkers can be used as an accurate estimate of UK international aviation emissions depends on what assumptions are being made about how to allocate international aviation emissions to different countries.

DESNZ obtain data on military aircraft fuel consumption directly from the Ministry of Defence, so they can subtract this from the total to obtain the civil aviation estimates which are discussed in this methodology note. Military aircraft emissions are reported separately within their official statistics publication.

The provisional estimates are calculated from provisional fuel consumption estimates in DESNZ’s monthly Energy Trends publication, by applying the percentage change in the annual total for aviation fuels to the emissions estimate from the latest year.

Organisation for Economic Co-operation and Development (OECD)

The Organisation for Economic Co-operation and Development (OECD) estimates use flight data compiled by the International Civil Aviation Organization (ICAO). For data up to 2018 this is based on scheduled flights and the great circle distance between airports. From 2019 onwards this covers actual flight distances from the Automatic Dependent Surveillance - Broadcast (ADS-B) system, a system which allows the monitoring of aircraft positions, providing comprehensive global coverage.

Flight distance or duration

Using actual flight distances, rather than estimating them based on great circle distance, provides a better account of emissions as the actual distance travelled by any given flight depends on airspace constraints and meteorological conditions on the day the flight occurs.

A key difference is that actual coverage for each flight is determined by the ability of ADS-B transmitters to pick up data over the entire course of a flight via air traffic control ground-based or satellite-based receivers, in the absence of schedule data. This issue is not unique to data collected by the OECD: ADS-B do not have uniform cover, and it is possible that some flights are incomplete because they operate to/from areas where there are few receivers. However, as most aircraft are equipped with ADS-B transponders, data from cargo aircraft, and some General Aviation are included^{[footnote 2]}. Not all General Aviation flights have ADS-B equipment fitted so coverage of General Aviation is limited. Additionally military aircraft are not included.

OECD have made further improvements to initial UK estimates by taking account of runway length at UK airports. Larger aircraft cannot take off or land at shorter runways, therefore it can be inferred that even if the last recorded ADS-B location of a large aircraft is a small airfield within the UK, it is likely that the aircraft has simply flown over the airfield en route to a larger airport. This assumption can then be used to assume a more feasible final destination.

Fuel burn

Fuel burnt is calculated using average fuel consumption data for the Landing/Take-Off (LTO) phase and various cruise phase lengths, for an array of representative aircraft categories.

Emissions

To convert flight data into emissions, the OECD use input data from the EMEP/EEA Emission Inventory Guidebook to estimate emissions, taking their standard flight distances and assuming a linear relationship between the distances to estimate emissions for actual distances, by aircraft type. Emissions factors are also taken from the EMEP/EEA Emission Inventory Guidebook.

The OECD provide breakdowns of emissions by flight type (passenger or freight) and down to a monthly level. They also provide emissions on a residential basis.

EUROCONTROL

EUROCONTROL began publishing regular monthly estimates by member state in 2023, covering January 2019 onwards with a 2-month lag on their CO2 emissions page. Earlier emissions estimates included an analysis on the impact of the coronavirus (COVID-19) pandemic on CO2 emissions and how this varied across Europe.

Flight distance or duration

EUROCONTROL data covers all flights operated under Instrument Flight Rules (IFR) passing through European airspace that they manage, including actual flown distances based on flight path information submitted to them. Using actual distances, rather than estimating distances using great circle distance, is expected to provide more precise estimates.

EUROCONTROL also supplement this with radar information and through other assumptions, for instance based on the last exit point in EUROCONTROL airspace, the remaining route may be deduced beyond where radar drops out. EUROCONTROL do not use ADS-B data to estimate distances, though it is used by them for specific purposes in the monitoring of airspace.

Differences between ADS-B (Automatic dependent surveillance – broadcast data) and IFR (Instrument Flight Rule) information

Both can provide data on the actual flight path of an aircraft.

IFR refers to the set of obligations and procedures required to meet international standards for flight, rather than a single tracking technology or source. IFR procedures mean the pilot no longer needs to rely on visual cues for navigation but can accurately check visibility and weather conditions via on-board equipment. As IFR procedures are safer, nearly all commercial flights use IFR flight rules.

Conversely, ADS-B is a specific aviation surveillance technology fitted to aircraft which periodically broadcasts to air traffic control ground based or satellite-based receivers. Aircraft transmit several types of data to comply with Instrument Flight Rules including but not limited to flight plan details, ADS-B data and Flight Data Recorder (FDR) data. While the share of aircraft with ADS-B varies globally, EUROCONTROL estimate that over 96% of aircraft operating under IFR in the EUROCONTROL Network Managed (NM) area are fitted with ADS-B^{[footnote 3]}.

Fuel burn

Fuel burn is calculated using EUROCONTROL’s Small Emitters Tool (SET) which is also used for estimating emissions from air traffic covered by the UK Emissions Trading Scheme (ETS). This uses estimates split by ICAO aircraft type and distance flown. The tool computes average values for fuel consumption and CO2 emitted. These may differ from those of a single flight in the real world, which can be influenced by many factors including the number of passengers, cargo, wind conditions or aircraft sub-type. Over large numbers of flights, the averages used by the SET generally provide a very accurate estimate, due to the law of large numbers. Additionally, EUROCONTROL incorporate some fuel burn samples from real-world flight operations into the SET to improve accuracy.

Emissions

As described above, emissions are automatically calculated along with fuel burn with the SET. One limitation of the SET is that the fuel burn is based on average weight for that aircraft type, as detailed above. This means that during coronavirus, when flights were operating with less passengers per flight, estimates for fuel burn are likely to be overestimates.

Coverage and differences compared to other sources

One key difference is that EUROCONTROL do not split aviation emissions into domestic and international categories. Moreover, estimates for the UK include emissions from Crown Dependencies such as Jersey, Guernsey and the Isle of Man, but exclude emissions from Overseas Territories which are outside UK airspace. This is slightly different to categorisation for official DESNZ emissions estimates for the UK. Though DESNZ also produce estimates for emissions from Crown Dependencies and certain Overseas Territories in line with its international reporting requirements, these are not included in the table below, and only make up around 1% of the UK aviation emissions total^{[footnote 4]}.

Flight plans

In the UK, a flight plan is filed by a pilot or flight dispatcher to EUROCONTROL prior to departure which indicates the plane’s planned routes or flight path. In the UK, this is then automatically communicated to National Air Traffic Services (NATS). This is distributed as needed to any relevant air traffic control bodies or operational partners to ensure safe and efficient air traffic management.

If you wish to fly within the EUROCONTROL regulated airspace you must submit a flight plan. Most commercial aircraft operate within this airspace and are sufficiently large that flight paths require forward planning. Additionally, such flights require both monitoring and assistance within the airspace for safety reasons, for example if they were required to change plans due to weather conditions.

Coverage of Instrument Flight Rules (IFR)

All standard commercial flights, and any other aircraft that fly in restricted airspace must operate under IFR for safety reasons. Small commercial flights that do not fly within restricted areas, for example those used for skydiving that operate close to the ground outside restricted areas, are not obligated to operate under IFR. Similarly, General Aviation flying outside restricted airspace and landing at quiet aerodromes are not required to operate under IFR therefore such flights are not covered in EUROCONTROL estimates.

While it is true that standard aircraft flying for military purposes will be included if they have filed a flight plan, specialised military aircraft are out of scope.

Summary comparison table

The table below summarises the main components of the methodologies used by the 3 organisations to calculate emissions estimates for the UK.

Table 1: Differences in aviation emissions methodologies: DESNZ official UK statistics, EUROCONTROL and OECD

Feature	DESNZ	OECD	EUROCONTROL
Frequency of updates	Annual	Monthly	Monthly
Time lag	13 months	3 months	2 months
Breakdown of domestic and international emissions available?	Yes. Provisional figures for international flights available with 3 month lag	Yes	No
Source of flight level data	CAA Airport Data	ICAO: schedules up to 2018, actual ADS-B flight data from 2019 onwards	EUROCONTROL flight data of flights operated under Instrument Flight Rules
Airports covered	Around 50 UK airports	All UK, Crown Dependency and Overseas Territory airports covered by ADS-B	All UK and Crown Dependencies airports where IFR flights operate
Treatment of Crown Dependency and Overseas Territory data	Crown Dependency airports and certain Overseas Territories airports estimates are reported separately	Crown Dependency and Overseas Territories are included	Crown Dependency data but not Overseas Territory data are included
Types of flights covered	All commercial passenger, cargo and General Aviation flights which are recorded by the CAA	All commercial passenger and cargo flights, and (from 2019) some General Aviation flights equipped with ADS-B	All commercial passenger and cargo flights, some General Aviation flights that fly under IFR rules
Types of flights excluded	Military emissions are reported separately	Military aircraft are out of scope	Military aircraft are out of scope
Distance or duration calculations	Estimated with great circle distance	Estimated with great circle distance up to 2018, using actual distances from 2019 onwards	Uses actual distance flown (full trajectory from origin to destination)
Source of fuel burn for different engine phases[footnote 5] for modelling	CAA Airport studies	EMEP/EEA Emission Inventory Guidebook	The Small Emitters Tool estimates emissions for the entire flight only, based on averages
Source of aircraft data on fuel burn	EMEP/EEA Emission Inventory Guidebook	EMEP/EEA Emission Inventory Guidebook	The Small Emitters Tool, with additional real-world flight burn data
Source of emissions factors	National airport specific factors, Fuels Industry UK data on the carbon content of aviation fuels, and EMEP/EEA Emission Inventory Guidebook	EMEP/EEA Emission Inventory Guidebook	The Small Emitters Tool
Types of emissions included in estimates	CO2, N2O, CH4	CO2	CO2
Calibrated against UK aviation fuel deliveries	Yes	No	No
International comparability	Yes, 43 more countries available on the UNFCCC website, as well as other countries reporting less frequently	Yes, 186 countries worldwide	Yes, currently available for 43 European countries
Principle for aggregation	Territory only	Territory and residence	Territory only

Comparison of estimates

The table below is a comparison of the emissions from UK aviation (both international and domestic) between all sources since 2013 where available. Under the territory principle, estimates are only focusing on departing flights to avoid double-counting emissions for the same international flights between countries, and for the same domestic flights within the UK.

While the exact geographical coverage of the UK, and how each methodology attributes a flight to the UK differs, care has been taken to make the closest comparisons from data available. For example, DESNZ official statistics for UNFCCC reporting requires that emissions are also submitted for Crown Dependencies and specific Overseas Territories, though the estimates discussed in this note are for the UK only.

The figures below are for CO2 emissions for civil aircraft only, and exclude emissions estimates from military aircraft and aircraft support vehicles. DESNZ estimates for international flights are based on fuel combustion from UK international aviation bunkers.

Table 2: CO2 emissions from domestic and international aviation in million tonnes, United Kingdom: 2013 to 2023

Emissions source	Notes	2013	2014	2015	2016	2017	2018	2019	2020	2021	2022	2023
DESNZ Domestic	None	1.7	1.5	1.6	1.4	1.5	1.5	1.4	0.6	0.7	1.1	NA
DESNZ International	Note 1	32.4	32.6	33.1	33.3	35.9	36.3	36.4	14.7	13.9	28.2	32.6
DESNZ Total	None	34.0	34.2	34.7	34.8	37.4	37.7	37.8	15.3	14.6	29.3	NA
EUROCONTROL Domestic	Note 2	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA
EUROCONTROL International	Note 2	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA
EUROCONTROL Total	None	31.8	32.9	33.7	35.7	36.4	37.0	37.3	15.0	13.8	27.7	33.5
OECD Domestic	Note 3	1.6	1.5	1.5	1.5	1.5	1.5	1.3	0.6	0.8	1.2	1.5
OECD International	Note 3	29.1	29.7	30.7	32.4	33.5	34.2	34.9	13.6	11.8	24.8	30.3
OECD Total	Note 3	30.7	31.2	32.3	33.9	35.0	35.7	36.1	14.2	12.6	26.1	31.8

Figures are based on data downloaded in September 2024 and are subject to change pending any improvements made to methodology at a later date. Emissions are based on the territory principle and presented in million tonnes, MtCO2. There are slight variations in both coverage and methodology as summarised in Table 1.

NA: Data is not available.

Note 1: DESNZ estimate for 2023 is provisional.

Note 2: EUROCONTROL provide total UK aviation emissions only.

Note 3: Due to the nature of ADS-B data used in OECD estimates from 2019 onwards, the signal can drop out outside national airspace, reducing coverage and impacting how the data is allocated as domestic and international for that country. As UK domestic emissions form such a small share of total emissions, this may cause larger percentage differences, relative to changes seen in international figures. The OECD database may be periodically revised due to methodology updates.

Chart 1: Total UK aviation emissions, million tonnes, MtCO2, 2010 to 2022

Estimates from OECD are consistently lower than those from DESNZ for total emissions, by an average of 2.3 MtCO2 (7%) a year between 2013 and 2019. EUROCONTROL estimates were also lower than DESNZ over the same period except for 2016, when they were 0.9 MtCO2 higher. Overall, EUROCONTROL were a closer match to the DESNZ estimates, at an average of 1.1 MtCO2 (3%) lower a year when excluding 2016.

It is worth noting that the DESNZ estimates discussed here include all civil aviation emissions, from both commercial aircraft and General Aviation, even though coverage of the activity from General Aviation in CAA data is limited.

Both OECD and EUROCONTROL estimates are expected to be lower than DESNZ estimates, as the latter are the only estimates normalised by fuel sales, given 100% coverage with activity data alone is not possible. This normalisation by fuel sales includes all aviation fuel: standard kerosene jet fuel, aviation spirit and more recently sustainable aviation fuel. DESNZ data covers fuel distributed to all aerodromes, airports and military airfields within the UK therefore they are able to fully account for all types of aircraft emissions with this normalisation.

Chart 2: Percentage differences from UK official aviation CO2 emissions statistics: EUROCONTROL and OECD, 2013 to 2022

For example, OECD aircraft coverage depends on aircraft being fitted with ADS-B equipment. Some General Aviation aircraft operates without ADS-B equipment, therefore wouldn’t be included in their estimates but would be accounted for in DESNZ estimates given all fuel use is captured in their normalisation process.

Other methodological and definitional differences (as shown in the summary comparison table) may also be a factor. For example, DESNZ estimates used here exclude flights departing from Crown Dependencies or Overseas Territories, whereas OECD includes both and does not separate them. EUROCONTROL estimates are based on airspace around the UK which includes Crown Dependencies but excludes Overseas Territories.

The differences are consistent with the OECD analysis comparing their estimates to all UNFCCC inventories for 2019. It showed that OECD estimates tend to be lower than UNFCCC, however this gap narrowed when using actual flight data rather than schedules. Full details of the comparison across countries can be found in their methodology paper.

The fact that EUROCONTROL’s data is closer to DESNZ estimates indicates that they have achieved greater coverage of aircraft activity with the use of IFR data.

Emissions savings from offsetting or SAF usage

Estimates are currently looking at tailpipe emissions, that is emissions emitted directly through the plane’s tailpipe, as opposed to considering the lifecycle carbon footprint of jet fuel. Any indirect emissions, and associated savings of indirect emissions that occur in the downstream supply chain such as those from SAF use or carbon offsetting are classified as Scope 3 emissions.

It is important to note that the chemistry of SAF is very similar to that of standard kerosene jet fuel, however Scope 3 emissions savings are possible as it is produced from sustainable feedstocks resulting in a reduction in carbon emissions over the lifecycle of the fuel. Lifecycle emissions of a fuel include all emissions from extraction or production through to its final use in an engine. SAF creates a net carbon saving either by using feedstock that would have otherwise been burnt as waste such as used cooking oil, or by more modern methods such as using carbon capture and electricity from renewable sources.

Data on the volume of SAF will be recorded from fuel suppliers under the Renewable Transport Fuel Obligation (RTFO) and the forthcoming SAF Mandate in 2025. This is published by the Department for Transport on the renewable fuel statistics page. These estimates are likely to develop over time as evidence on SAF being produced at large scale emerges and SAF itself is used more widely in the aviation industry as a fuel component. Data from the UK Emissions Trading Scheme and the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) are also collected by the Environment Agency to monitor both schemes. However, DESNZ Official emissions statistics follow strict definitions in accordance with national inventory guidelines. Therefore, they represent Scope 1 emissions which estimate direct emissions from aircraft combustion engines only. Carbon savings, such as those generated from the production of synthetic fuel from greenhouse gas removals (GGRs) would be accounted for elsewhere in the inventory, at the point of capture. Reductions in life-cycle emissions would be reflected in the part of the economy where they are observed, which would be unlikely to be reflected in aviation sector emissions and could be outside of the UK. Whilst this is not a consideration in current estimates, going forward more consideration will need to be given to SAF as a component in constructing emissions estimates as uptake grows.

Conclusion and feedback

DESNZ estimates remain the lead indicator to monitor progress against national emissions targets due to their high quality, particularly because they cover non-CO2 greenhouse gases. They are valuable as they meet specific guidelines and are under constant review by specialist producers with access to supplementary UK-specific data.

However, the other indicators also add value as they can be used for a range of purposes, in particular:

• Estimates from OECD and EUROCONTROL enable more timely monitoring at monthly level that can be used by individual member states
• International comparisons with a wider group of countries are possible

They reflect the global nature of aviation and international emissions monitoring, and the increased interest in decarbonisation.

There will continue to be need for transparency as methodologies are further developed, including to explain any differences in accounting for aviation emissions technological developments in the future.

Future improvements could include:

• using actual 4D flight paths rather than modelling flight phases
• adjustments to account for offsetting
• coherent reporting of scope 1, scope 2 and scope 3 emissions including data on sustainable aviation fuel (SAF)

We welcome feedback from users on this methodology document and on future needs for the reporting and transparency of aviation emissions data. You can provide your comments by completing this survey, which covers the following themes:

• whether there is a need for a regular report on these estimates
• for which purpose(s) and at which frequency this would be required
• how the methodology comparison could be improved
• what factors would emissions estimates need to consider in the future

Alternatively, you can provide feedback by contacting us at [email protected].

Glossary of terms

ADS-B (Automatic dependent surveillance – broadcast data): A type of aviation surveillance technology fitted to aircraft which periodically broadcasts to air traffic control ground based or satellite-based receivers for the purposes of airspace safety and management.

Aviation gasoline, aviation spirit: Light hydrocarbon oils intended for use in aviation piston-engine power units typically found in vintage or classic planes, and light aircraft favoured by private pilots. More details on this definition are published by DESNZ in the Crude oil and petroleum products: methodology note.

Aviation turbine fuel, jet fuel: A specialised type of aviation fuel based on kerosene used to power jet engines such as those in commercial airliners, typically derived from petroleum crude oil through refining processes. More details on this definition are published by DESNZ in the Crude oil an Aviation gasoline, aviation spirit: Light hydrocarbon oils intended for use in aviation piston-engine powd petroleum products: methodology note.

Carbon budget: A carbon budget places a restriction on the total amount of greenhouse gases the UK can emit over a 5-year period. The UK is the first country to set legally binding carbon budgets.

CH4 (Methane): Chemical formula for methane, a potent greenhouse gas emitted from various natural and anthropogenic sources, including livestock farming, fossil fuel extraction, and waste management.

Civil aviation: Any aviation, both private and commercial, that excludes military flights and other flights undertaken for government operations.

CO2 (Carbon Dioxide): A colourless, odourless gas, a trace gas in the Earth’s atmosphere and also produced by the combustion of carbon-containing fuels in the context of aviation (but also a naturally occurring gas), primarily responsible for anthropogenic climate change and global warming.

Emission Factor (EF): A parameter indicating the amount of a specific pollutant emitted per unit of activity, such as fuel consumption, energy production, or vehicle mileage.

Engine thrust: Engine thrust is the force produced by an engine, particularly in aircraft, that propels it forward. In aircraft, this is typically from a jet engine or propeller.

General Aviation (GA): is defined by the International Civil Aviation Organisation (ICAO) as all civil aircraft operations except for commercial air transport or aerial work. Aerial work includes all non-military aircraft employed for alternative purposes such as agriculture, search and rescue or surveillance. This typically refers to private transport and recreational flying.

Greenhouse gases: Gases in the Earth’s atmosphere that trap heat. They let sunlight in bit prevent some of that heat from leaving the atmosphere resulting in net warming known as the greenhouse effect.

In-flight phase: The period during which an aircraft is airborne and operating between take-off and landing, encompassing various flight phases, including climb, cruise, descent, and landing.

International bunker fuels: Emissions from fuel used for international aviation and maritime transport.

Instrument Flight Rules (IFR): The set of obligations and procedures required meet international standards for flight. IFR procedures mean the pilot no longer needs to rely on visual cues for navigation but can accurately check visibility and weather conditions via on-board equipment. As IFR procedures are safer, nearly all commercial flights use IFR flight rules.

The National Atmospheric Emissions Inventory (NAEI): The NAEI provides a range of data necessary to support reporting associated with international commitments to the United Nations and other national legislation aimed at reducing emissions of air pollutants and greenhouse gases. For greenhouse gases, these include an annual report for submission under the Framework Convention on Climate Change.

N2O (Nitrous oxide): A potent greenhouse gas produced by natural and anthropogenic sources, including agricultural activities, industrial processes, and combustion, contributing to global warming and stratospheric ozone depletion.

Residence principle: focussing on emissions from UK residents and UK-registered businesses, in this case airlines, regardless of where they occur.

Scope 1 emissions: direct emissions from owned or directly controlled sources, in this case from aircraft combustion engines.

Scope 2 emissions: indirect emissions from the generation of purchased energy, for example energy required to heat and run airport buildings.

Scope 3 emissions: all other indirect emissions that occur to deliver the service of flight, encompassing the full supply chain. This includes but is not limited to the production and transportation of aircraft and aviation fuel, as well as emissions from passengers travelling to the airport (known as surface access).

Sustainable Aviation Fuel (SAF): Renewable or low-carbon alternatives to conventional fossil-based aviation fuels, produced from biomass, waste, or synthetic processes, with the potential to reduce greenhouse gas emissions and environmental impacts from aviation.

Synthetic SAF, e-SAF: A type of sustainable aviation fuel produced from renewable resources, such as biomass, waste, or carbon capture and utilisation technologies, offering a low-carbon alternative to conventional jet fuel.

Territory principle: focussing on emissions which occur within the UK’s borders as determined by the UN panel on climate change, the Intergovernmental Panel on Climate Change (IPCC). This assigns all flights leaving UK airports to the UK emissions inventory.

Related statistics

DfT publishes wider Transport energy and environment statistics. This includes:

• Transport and environment statistics: an annual compendium of statistics on the impact of transport on the environment, including journey comparisons of greenhouse gas emissions for different transport modes available on an interactive dashboard
• ENV02: data tables on greenhouse gas emissions by transport modes Energy and environment: data tables(ENV)
• Renewable fuel statistics - GOV.UK (www.gov.uk) The supply of renewable fuel (commonly referred to as biofuel) is measured annually as required under the Renewable Transport Fuel Obligation (RTFO). Renewable fuel statistics

1. Frédéric Dobruszkes, Why do planes not fly the shortest routes? A review, Applied Geography. ↩

2. Filippone A, Parkes B, Bojdo N, Kelly T. Prediction of aircraft engine emissions using ADS-B flight data. The Aeronautical Journal. 2021;125(1288):988-1012. https://doi.org/10.1017/aer.2021.2 ↩

3. EUROCONTROL ADS-B equipage monitoring, Automatic dependent surveillance - broadcast airborne equipage monitoring (ADS-B EQUIPAGE). The EUROCONTROL NM area includes the 27 EU Member States, Iceland, Norway, Switzerland and the UK. ↩

4. 2022 UK Greenhouse Gas Emissions, Final Figures (publishing.service.gov.uk) Only overseas territories that are party to the UK ratification of the United Nations Framework Convention on Climate Change are included. These are the Cayman Islands, Bermuda, the Falkland Islands and Gibraltar. ↩

5. Distinct engine phases include LTO (landing and take-off), cruise, taxi-in and hold and are described in the National Emissions Inventory Report. The level of thrust and fuel burn varies for each stage. ↩