Calculating Airline Emissions and Offsets
Consumers who are concerned about the impact of their flights on climate change, but are unable to avoid flying completely, may wish to indirectly reduce their impact on the atmosphere by purchasing carbon offsets. Currently, there are two main options:
• Airlines have begun to offer offsetting at an extra fee.
• Carbon offset providers will calculate and offset these emissions for a price.
This webpage looks at issues associated with calculating airline emissions.
• Calculation Issues
• Additional Information For Calculators
• Links
Calculation of airline emissions is complex and often varies between different calculators. This is because a wide variety of information is needed to calculate CO2 emissions on a per person basis. The variation in the results of different calculators has been interpreted as a lack of rigor in the calculation of air travel and undermines carbon offsetting as an approach to fight climate change.
Stockholm Environment Institute (SEI) recently examined the key factors that have to be taken into account when calculating air travel emissions for the purpose of carbon offsetting. (See http://www.co2offsetresearch.org/aviation/index.html) Their paper contrasts three particular calculators used in the market and discusses the following aspects of emissions calculation:
1. Type of Aircraft
Aircraft fuel efficiency, has steadily improved over the last few decades through weight reduction and progress in aerodynamic and engine design, yet there is a considerable lag time between technology development and implementation.
2. Flight Profile and Flight Distance
Flight distance is an essential factor and the farther the route, the more fuel burned.
3. Cargo on Passenger Flights
The weight of people, baggage, flight crew, steward’s supplies, cargo etc. are all considered part of aircraft weight.
4. Seat Occupancy Rate (Load Factor)
Seat occupancy rate is the ratio of passengers to available seats on board a given flight.
5. Seat Class
Seat class is another factor in determining the emissions an individual is accountable for on a given flight.
6. Radiative Forcing Index (RFI)
Radiative forcing measures the rate at which a given atmospheric gas alters radiation that is entering the atmosphere.
ADDITIONAL INFORMATION FOR CALCULATORS
Calculators are a powerful tool, not only for their footprinting but also importantly for their capability to educate consumers. Calculators need to be accompanied by contextual information to realise this potential. Relevant information includes:
- Underlying assumptions
- Guidelines and protocols used including statements of their strengths and limitations
- data sources and up-dating frequency
- accuracy statements
In order to improve and maximise accuracy, calculators need to be updated regularly to keep up with the dynamics of the air travel industry and to integrate new research results and updated data sets. Finally, calculators should also be accompanied by information around comprehensive carbon management. This enables consumers to make informed decisions that reduce travel footprints, such as combining trips, avoiding upper-class seats, and opting for alternatives to travel such as video conferencing.
For further information please refer to the following helpful sources from which this information was derived.
IPCCIntergovernmental Panel on Climate Change is an international scientific panel which informs the UNFCCC approximately every 5 years with the latest scientific, technical and socio-economic evidence on climate change. With representatives from 130 nations it is the world's pre-eminent scientific advisory body on climate change. (1999) Aviation and the Global Atmosphere:6.2.3. Alternative Indexing of Aviation’s Climate Impact-RF Index (online) at http://www.grida.no/Climate/ipcc/aviation/index.htm
International Civil Aviation Organisation (ICAO) Carbon Emissions Calculator http://www2.icao.int/public/cfmapps/carbonoffset/carbon_calculator.cfm
TRX Travel Analytics Airline Carbon Emissions Calculator (recommended by SEI in 2008 report) (online) at http://carbon.trx.com
Transparent Noise Information Package (TNIP) Carbon Counter located at http://www.infrastructure.gov.au/aviation/environmental/transparent_noise/tnip_CC.aspx
NCOSThe Commonwealth Government’s National Carbon Offset Standard (NCOS) came into effect on 1 July 2010 coinciding with the cessation of the Government’s Greenhouse Friendly™ program. It is intended to ensure that consumers have confidence in the voluntary carbon offset market and the integrity of the carbon offset and carbon neutral products they purchase. It provides guidance to businesses who wish to make their organisation carbon neutral or develop carbon neutral products in a way that achieves emissions reductions, through the purchase and cancellation of eligible carbon offsets. More Information Carbon Neutral ProgramThe Carbon Neutral Program utilising the National Carbon Offset Standard officially commences on 1 July 2010. This Program is the successor to the Australian Government’s Greenhouse Friendly™ initiative (2001 – 2010). The Carbon Neutral Program is a voluntary scheme which certifies products or business operations as carbon neutral. The National Carbon Offset Standard, developed by the Australian government, underpins the integrity of the Program. Guidelines Annex 1 - Emission Fcators for Scope 3 emission sources located at http://www.lowcarbonaustralia.com.au/sites/default/files/upload/NCOS%20Carbon%20Neutral%20Program%20Guidelines%20-%20v2%201%20-%2027-6-2011.pdf.
Aircraft fuel efficiency, has steadily improved over the last few decades through weight reduction and progress in aerodynamic and engine design, yet there is a considerable lag time between technology development and implementation. Issues relevant to an accurate calculation of GHGGreenhouse Gases in the earth's atmosphere absorb and re-emit infrared radiation. The Kyoto Protocol lists six major greenhouse gases, which vary in their relative warming effect. The six gases are: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), HFCs (hydrofluorocarbons), PFCs (perfluorocarbons) and sulphur hexafluoride (SF6). for offsetting include:
• Aircraft Model
• Fuel burn rate (fuel consumption, fuel efficiency)
• Fuel consumption varies by aircraft model and engine type. While the latest technology can be used as a benchmark, it is not commonly used on the ground, due to time-lags of technology uptake, and long life spans of aircraft.
• Engine type (different engines fitted to the same type of aircraft can affect consumption)
The more accurate calculators reflect fuel burn and efficiencies associated with specific aircraft models. However, in circumstances when a passenger does not know the aircraft type, the more flexible calculators identify the aircraft for the user or clearly note that they are relying on average fuel burn.
Flight Profile and Flight Distance
Just as a car experiences different fuel efficiencies at different speeds and under different conditions, aircraft experience different burn rates in various flight profiles, including: taxi, takeoff, climb, cruise, landing approach, and landing.
The take off phase requires full engine thrust, and thus uses the most fuel. As the aircraft ascends to higher altitudes the drag decreases and so does the rate of fuel use. Flight distance and holding delays provide a challenge to emissions calculation.
Flight distance is an essential factor and the farther the route, the more fuel burned. Over very long distances the fuel use per mile increases because of the greater amount of fuel that has to be carried during the early stages of flight. The size of this increase varies depending on the aircraft.
Holding delays can result from congestion at airports, weather variations that cause route variations, regional differences, regular delays etc.
Most offset calculators allow for additional kilometres flown to reflect the holding delays described above, although some do not account for routing and delays. Realistically, carbon offset calculators may use an average multiplier to account for routing and delays, but the most accurate air travel calculators use specific information about total fuel consumption and flight distance.
The majority of a flight’s total weight is the aircraft itself and the fuel it carries. Flight crew, crew luggage, steward’s supplies, etc. are all considered part of aircraft weight. An industry-wide standard of 220 lbs (100 kgs) is assumed for each passenger and their luggage.
Passenger airplanes usually also carry additional cargo. ‘Cargo payload’ consists of freight and mail. Thus, cargo should be allocated some of the GHG emissions associated with the flight. The calculators may vary the way and degree to which they subtract the emissions associated with marginal increase in fuel consumption caused by the weight of the cargo. The additional cargo carried varies by country and by airline therefore the most accurate calculators should have specific data on the amount of cargo carried.
Seat Occupancy Rate (Load Factor)
Not all flights take off fully occupied. Seat occupancy rate (also called passenger load factor) is the ratio of passengers to available seats on board a given flight. However, the number of passengers on board has a small impact on total fuel consumption relative to the type of aircraft. Changes to pricing schedules have fuelled competition which has had the effect of steadily increasing occupancy rates on flights over the last two decades.
The most accurate air travel calculators take occupancy rates into account. Ideally occupancy data would be available by route and not just by air carrier.
Seat class is another factor in determining the emissions an individual is accountable for on a given flight. First and business class seats take up more space and fit fewer passengers. Weight is another consideration: the seats and entertainment systems for business and first class are larger and heavier than for economy seats. On the other hand, anecdotal evidence indicates that families traveling in economy often have more luggage.
Since higher-class passenger’s de-facto replace more economy passengers, emissions should be allocated by space (i.e. each upper-class passenger is allocated the emissions of the number of economy passengers that could have been seated in the same space). The most accurate air travel emissions calculators take seat class into account.
Radiative Forcing Index (RFI) in the context of airline emissions calculation, is usually referred to as the multiplier that expresses any extra warming effects that occurs as a result of the emissions occurring while planes are in the air. Radiative forcing is complex and difficult to calculate. It measures the rate at which a given atmospheric gas alters radiation that is entering the atmosphere. A positive value denotes warming; a negative number signifies cooling.
The main greenhouse gases emitted from aircraft are carbon dioxideA greenhouse gas that is produced as a by-product of oil and gas production, burning fossil fuels and biomass, as well as from all animals, plants, and a number of other natural sources. Carbon dioxide is the principal anthropogenic greenhouse gas that affects the earth’s temperature. (CO2), water vapor, nitrogen oxides (NOx), and methaneMethane (CH4) is a greenhouse gas with a GWP of 21. (CH4). Aircraft travel generally at altitudes of 9 to 13 kilometers. At these altitudes, the effect of the emitted gases is considerably different than they would be if the aircraft was at ground level and in many cases still incompletely understood.
Aircraft also emit water vapor during flight. When emitted in the stratosphere, this water vapour can cause the formation of ice clouds, called contrails. Where contrails persist, cirrus clouds begin to form which have an additional impact on global warming. Clouds can have a double effect on radiation: they warm the earth by reducing the amount of radiation from the earth that escapes into space but also cool the earth by reflecting the sun’s rays back into space. However, contrails lead to a net warming (IPCC, 1999).
The IPCC calculated in 1999 that the average for full radiative forcing to be a factor of approximately 2.7, although this number has been disputed since then. Therefore to estimate the impact of an airplane trip a multiplier should notionally be used on the CO2 emissions from jet fuel to account for full radiative forcing.
