Great Circle flight methodology

This methodology represents greenhouse gas emissions associated with the air transport according to Great Circle distances between two locations. The data and calculation methodology for plane-associated greenhouse gas emissions originates from the UK government department DEFRA, as published in their most recent greenhouse gas (GHG) conversion factors documentation. 'Great Circle' distances are calculated using the haversine formula. Emissions relating to both passenger and freight transport are available herein.

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The methodology

Emissions model

Emissions are calculated based on the DEFRA methodologies for passenger and freight transport. These methodologies contain emissions factors which describe the rate at which greenhouse gas emissions are produced in relation to distance travelled and - in the case of freight - the quantity of goods freighted.

Emissions - expressed in terms of mass (e.g. kg) - are calculated by multiplying these rates (e.g. mass emitted per distance; e.g. kg CO₂ per km) by the great circle distance between two locations (e.g km) travelled (as well as the quantity freighted, in the case of freight).

Great Circle distance calculations

Flight distances are calculated as the 'Great Circle' distance between two locations, which represents the shortest distance between two points on a sphere (an approximation of the Earth) and is derived from departure and destination locations using the haversine formula. Departure and destination locations can be defined on the basis of latitude and longitude or the IATA codes representing the respective departure and destination airports, with distances between airports are calculated by reference to longitude and latitude data for a comprehensive range of global airports.

Model data

The DEFRA methodology provides passenger- and freight-based emissions factors representing typical 'domestic', 'short haul' and 'long haul' flights. The selection of the appropriate flight type is determined - following DEFRA methodology - by the relationship between departure and destination locations:

• domestic flights are those which originate and terminate within the UK
• short-haul flights are international flights (originating and terminating in distinct countries) extending no further than 3700 km
• long-haul flights are international flights extending beyond 3700 km

Each case (passenger, freight; domestic, short- or long-haul) is represented in the DEFRA methodology by six distinct emissions factors which differentiate greenhouse gas emissions into the following types:

• direct CO₂ emissions: carbon dioxide emissions produced during flight, i.e. fuel combustion
• direct CH₄ emissions: methane emissions produced during flight, i.e. fuel combustion
• direct N₂O emissions: nitrous oxide emissions produced during flight, i.e. fuel combustion
• total direct emissions: all direct emissions, i.e. CO₂ + CH₄ + N₂O
• indirect emissions: emissions associated with stages in the fuel production chain such as raw material extraction and fuel delivery
• total or 'life cycle': the total of direct and indirect emissions

Each of these emissions factors are expressed in terms of kg CO₂e per tonne km.

Similar methodologies

Alternative DEFRA methodologies are available representing passenger and freight transport in which emissions are calculated on the basis of absolute specified distances rather than departure and destination locations.

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Using this methodology

Choosing a specific activity type

There are no drill down choices associated with this category.

Activity data required

The activity data which is required varies depending on whether passenger-based or freight-based emissions are required.

In all cases departure and destination locations must be specified using either of the following combinations of profile item values:

• lat1, long1, lat2, long2 (i.e. a pair of latitude and longitude coordinates)
• IATACode1, IATACode2 (i.e. a pair of airport IATA codes)

Longitude must be in the range -180 to 180 with negative numbers representing eastern longitudes. Latitudes must be in the range -90 to 90 with negative numbers representing southern latitudes.

For freight-based emissions only: If requiring an emissions calculation associated with freight, the quantity (mass) of goods freighted must be specified using the mass profile item value. If multiple instances of the same quantity of freighting are under consideration these can be specified using the journeys profile item value.

For passenger-based emissions: If passenger-based emissions are required a number of options can be specified in order to make the most accurate calculation. Multiple journeys and passengers can be specified by setting the journeys and passengers profile item values respecitively. Return flights can be specified by setting the isReturn profile item value to 'true'. The specific passenger class can also be specified using the passengerClass profile item value with one of the following valid values:

• 'average': use if passenger class is unknown (default)
• 'economy': for economy class
• 'premium economy': for premium economy class or 'economy plus'
• 'business': for business class
• 'first': for first class

Following DEFRA methodology, all passenger class specifications revert to 'average' for domestic flights, while 'premium economy' reverts to 'economy' and 'first' reverts to 'business' for short-haul flights.

In addition the effects of radiative forcing can be incorporated by setting the useRFI profile item value to 'true' (default value is 'false'). The value of the the radiative forcing index can be set using the radiativeForcingIndex profile item value, but defaults to 1.9 otherwise. For an explanation of the radiative forcing index (RFI) see here.

Users can choose to specify their activity data in a comprehensive range of appropriate units.

Calculation and result

Great Circle distances are calcuated using the haversine formula and inflated by 9% to account for indirecting routing and congestion - as specified by DEFRA methodology. Emissions calculations are made based on the calculated Great Circle distance and the appropriate emissions factors for either passenger or freight. Passenger flights are assumed by default; freighting is indicated by the specification of a mass quantity.

The returned emissions quantities for this methodology are inclusive of CO₂, CH₄ and N₂O. The following discrete amounts are returned:

• CO2: CO₂ emissions
• methaneCO2e: CH₄ (methane) emissions
• nitrousOxideCO2e: N₂O (nitrous oxide) emissions
• totalDirectCO2e: total direct emissions
• indirectCO2e: Indirect emissions
• lifeCycleCO2e: Full life cycle emissions

Users should note that all quantities (including the separated CH₄ and N₂O emissions) are expressed in terms of CO₂e - the quantity of CO₂ which would exert the same atmospheric warming effect and the emissions quantity.