Summary

The calculator assumes greenhouse gas emissions of ¼ tonne CO2 equivalent per hour flying.

Basis 1 (from fuel consumption per flight)

One way to calculate CO2 emissions is from fuel consumption per flight.

A Boeing 737-400 jet is typically used for short international flights.

For a distance of 926 km, the amount of fuel used is estimated to be 3.61 tonnes [38], including taxiing, take-off, cruising and landing.

Using a seating capacity of 164 [Wikipedia, viewed 28.2.08] and an average seat occupancy (or 'load factor') of 65% [14], this gives a fuel use of 36.6 g per passenger km.

CO2 emissions from aviation fuel are 3.15 grams per gram of fuel [38], which gives CO2 emissions from a Boeing 737-400 of 115 g per passenger km.

At a cruising speed of 780 km per hour [Wikipedia, 28.2.08], this is equivalent to 90 kg CO2 per hour.

The corresponding figures (same sources) for a Boeing 747-400 (used for long distance international flights) are:
Distance: 5556 km
Fuel used: 59.6 tonnes
Seats: 416
Seat occupancy: 80%
Fuel use: 32.2 g per passenger km
CO2 emissions: 101 g per passenger km
Cruising speed: 910 km per hour
CO2 emissions: 92 kg CO2 per hour

So for both aircraft, the emissions are around 90 kg CO2 per hour.

These CO2 emissions are generally into the high atmosphere, and this is thought to have a greater greenhouse effect than CO2 released at sea level. The emissions are therefore adjusted by multiplication by a factor of 2.00 (see 'Radiative forcing' below) to give 180 kg CO2 equivalent per hour.

Further allowance is needed for fossil fuel energy used in :
• extraction and transport of crude oil
• inefficiencies in refineries (around 7% [30])
• aircraft manufacture and maintenance, and staff training
• airport construction, maintenance, heating, lighting etc.

The CO2 emissions are therefore rounded up and the Carbon Independent calculator takes a values of 250 kg i.e. ¼ tonne CO2 equivalent per hour flying.

Basis 2 (from total UK fuel consumption)

The total sales of aviation fuels in the UK in 2006 was 12.7 million tonnes [BERR, 36]

CO2 emissions from aviation fuel are 3.15 grams per gram of fuel [38].

So the fuel used gave rise to 40.0 million tonnes CO2.

This estimate of CO2 emissions is from aircraft taking off from the UK, and there will be a similar quantity of CO2 generated on the return flights, giving a total of 80.0 million tonnes CO2.

The passengers flying on aircraft to and from the UK comprise UK residents travelling abroad and overseas residents travelling to the UK. The total numbers of visits (all modes of transport) were respectively 69.5 million and 32.7 million in 2006 [40]. Splitting the 80.0 million tonnes of CO2 in this proportion gives a total for UK residents of 54.4 million tonnes CO2 (0.90 tonnes per person).

UK residents took a total of 56.5 million flights abroad in 2006 [40] and these flights took an average of 3.99 hours each way (from [40] and standard journey times ).
This gives an average of 121 kg CO2 per hour, which on multiplying by 2.00 (see 'Radiative forcing' below) gives 242 kg CO2 equivalent per hour.

This estimate needs to be adjusted upwards to allow for
• extraction and transport of crude oil
• inefficiencies in refineries (around 7% [30])
• aircraft manufacture and maintenance, and staff training
• airport construction, maintenance, heating, lighting etc.
and to be adjusted downwards to allow for some of the aviation fuel being used for
• private aircraft
• military aircraft
• air freight
• flights within the UK.

A reasonable estimate for aviation CO2 emissions is therefore 250 kg i.e. ¼ tonne CO2 equivalent per hour flying, i.e. the same figure as obtained by basis 1 above. The agreement between the two methodologies gives some confidence in the figure, although it has to be said that the greatest uncertainty is in the allowance for 'radiative forcing'. It is to be hoped that this uncertainty will be resolved soon.

After allowance for 'radiative forcing', the CO2 emissions from aviation is 1.80 tonnes CO2 equivalent per UK resident.

This is consistent with the UK total of 0.7 tonnes CO2 per person per year given by source [7] which does not include allowances for 'radiative forcing' or for the proportion of UK residents on flights leaving the UK being over 50%.

Other calculators

• The UK Department for Transport journey planner assumes 0.158 kg CO2 / km [16, giving UK DfT as the source], which is equivalent to 134 kg CO2 per hour for a plane flying at 850 km per hour (this excludes 'radiative forcing')
• The National Energy Foundation [2] gives 0.29 kg CO2 / mile, which is equivalent to 150 kg CO2 per hour for a plane flying at 850 km per hour
• The Quaker Green Action calculator [1] assumes 350 kg CO2 equivalent per hour flying (using a multiplier of 3 [personal communication]).

The Defra source [14] points out that "The total CO2 emissions by UK residents from personal domestic and international flights are more difficult to accurately estimate. CO2 from domestic flights is readily available in the UK GHGI. However, data on CO2 emissions for international flights is only available in the GHGI resulting from aviation bunkers, which are based on the supply of aviation fuel to aircraft in the UK. This therefore represents only the fuel supplied to aircraft on the first leg of their outward flights from the UK and not the return flights. Such flights will obviously also include passengers who are non-UK residents. An indicative estimate of the CO2 from personal flights for UK residents was estimated from the aviation bunkers using detailed data on flight destinations and purpose from the international air passenger survey.(their ref 20) At the moment the current estimate used in the calculator may be a slight under-estimate..."
(Their ref 20 is http://www.statistics.gov.uk/ssd/surveys/international_passenger_survey.asp)

So the CarbonIndependent values of 250 kg CO2 equivalent per hour is within the range of other published values.

'Radiative forcing'

Aircraft are thought to have greater climate effects than just the CO2 emissions from burning the fuel. The additional effects include contributions from nitrous oxides and ozone. Because of this, the CO2 emissions from aviation should perhaps be multiplied by an appropriate factor. The size of the factor is often taken to be 2.7 [6].

The most authoritative available opinion on this seems to be that in the Defra document [14] pages 18 and 34. This is a statement from Professor David Lee, Director, Centre for Air Transport and the Environment (CATE), Manchester Metropolitan University, and is reproduced in its entirety here:

Aviation has effects on climate beyond that resulting from its CO2 emissions, including effects on tropospheric ozone and methane from its NOx emissions, water vapour, particle emissions and formation of contrails/enhanced cirrus cloudiness. This is usually calculated with the climate metric 'radiative forcing'. Aviation was shown by the IPCC (1999) to have a total radiative forcing of 2.7 times that of its CO2 radiative forcing for a 1992 fleet (the so-called Radiative Forcing Index, or RFI), excluding any effect from enhanced cirrus cloudiness which was too uncertain to be given a 'best estimate'.

More recently, the radiative forcing for the year 2000 fleet was evaluated by Sausen et al. (2005) which implies an RFI of 1.9, based upon better scientific understanding, which mostly reduced the contrail radiative forcing. Similarly to IPCC (1999), Sausen et al. (2005) excluded the effects of enhanced cirrus cloudiness but others (e.g. Stordal et al., 2005) have improved calculations over IPCC (1999), which indicates that this effect may be 10 and 80 mW/m2 (cf 0 to 40 mW/m2 of IPCC) but are still unable to give a 'best estimate' of radiative forcing.

Whilst it is incorrect to multiply CO2 emissions by the RFI, it is clear from the foregoing that aviation's effects are more than that of CO2. Currently, there is not a suitable climate metric to express the relationship between emissions and radiative effects from aviation in the same way that the global warming potential does but this is an active area of research. Nonetheless, it is clear that aviation imposes other effects on climate which are greater than that implied from simply considering its CO2 emissions alone.

REFERENCES

IPCC (1999) Aviation and the Global Atmosphere, J. E. Penner, D. H. Lister, D. J. Griggs, D. J. Dokken and M. McFarland (Eds). Special Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge.

Sausen R., Isaksen I., Grewe V., Hauglustaine D., Lee D. S., Myhre G., Kohler M. O., Pitari G., Schumann U., Stordal F. and Zerefos C. (2005) Aviation radiative forcing in 2000: and update on IPCC (1999). Meteorologische Zeitschrift 114, 555 * 561.

Stordal F., Myhre G., Stordal E. J. G., Rossow W. B., Lee D. S., Arlander D. W. And Svenby T. (2005) Is there a trend in cirrus cloud cover due to aircraft traffic? Atmospheric Chemistry and Physics 5, 2155 * 2162.

According to this statement, it is incorrect to multiply CO2 emissions by the RFI, but it is also incorrect to ignore them. For the purposes of a calculator, some decision must be taken until further evidence is available, and the Carbon Independent calculator will for the time being multiply aviation CO2 emissions by a factor of 2.0. The practical consequences are in fact minor as regards informing people on the easiest way to reduce their carbon footprint; whatever the size of the factor used, the easiest way for most people who fly to reduce their carbon footprint will be to cut back on flying.

The UK Government calculator (Act on CO2) does not include a Radiative Forcing factor, whereas the Government does actually use a factor of 2 when 'offsetting' ministerial and official flights [Defra, 14].

Miscellaneous points

Emissions calculated according to great-circle distances and emission factors per km need to to be adjusted by an additional 9% to allow for delays and indirect flight paths [Defra, 14].

There are several complexities in allocating responsibility for aviation greenhouse gas emissions.

One is the variation in passenger occupancy rates of aircraft, with some fully laden with charter passengers, and others flying with less than half the seats occupied (which applies more to scheduled flights). Those flying in partly empty aircraft should perhaps allow a higher rate of CO2 emissions. Similarly those flying Business Class or First Class are responsible for a higher share of CO2 emissions.

These points are related to the fallacy of "the plane is taking off anyway". Some people argue that because a plane is flying anyway, it makes no difference to the CO2 released whether or not they are on it (since their added weight is insignificant). And so (they argue) they will continue flying and make no effort to cut back. But what this ignores is that the decision made by an airline to start up flights on a particular route and to continue them (and to increase or decrease their frequency) depends upon the revenues received by the airline. So it not the travelling on the aircraft that causes the CO2 emisions, but buying a ticket which encourages the airline to persist with the route and so release greenhouse gases - and the more money someone pays the airline, the more likely it is to continue or even increase its flights on a particular route. One consequence of this line of argument is that those buying high-priced peak tickets are more responsible for the planes flying than those buying cheap off-peak seats - and this effect can be by at least a factor of 10 since seat prices can vary by this much. Carbon calculators could perhaps incorporate this but the practical difficulties would be hard to overcome. But people who are finding it hard to cut back on flying, can take the intermediate action of avoiding the peak-time highly-priced seats where the airlines are getting their largest income and making their greatest profits.

References

See Reference List