7. Airtravel Emissions Calculators (last changed for revision 1.3)

Calculators have to fulfill three requirements: they have to educate the consumer, be user friendly and accurate. Nine of the offset companies do not provide their customers with detailed information about the complexities of calculating air travel emissions. The four companies that do are: atmosfair, climate friendly, NativeEnergy and myclimate. These companies also link to the IPCC’s and other reports on this topic. Climate Care features a link to a paper they commissioned from Oxford University (1). The other nine companies do not discuss these complex issues on their webpage.

Since this study has been done CarbonCounter.org has overhauled its website. For updated information, see footnote (5).

7.1 User Friendliness of Calculator (last edited for version 1.3)
There are three basic ways customers can calculate emissions from their air travel:

A. Entering the total miles flown.
CarbonCounter.org, and Carbonfund.org require customers to enter the mileage they would like to offset. It is assumed that users can find this information for themselves. There are other websites (such as http://www.webflyer.com) which compute the distance between major airports. None of the offset companies that ask for mileage have links to such sites. Adding such links would increase the user friendliness of these sites.

B. Entering origin and destination of the trip.
Atmosfair, Climate Care, The CarbonNeutral Company, climate friendly, and Offsetters have calculators that let customers enter their airport of origin and their destination. Multiple flights may be calculated using this method and then offset simultaneously. NativeEnergy and myclimate offer a choice between entering mileage and entering the origin and destination of the flight.

C. Offsetting a fixed amount without calculating the precise emissions.
A number of the offset companies offer a simpler alternative to calculating emissions.
The CarbonNeutral Company offer in addition to their point to point calculator the option of choosing a short, medium and long haul flight, instead of calculating the precise emissions.

Better World Club’s system is not based on a careful calculation. Instead, they use a loose approximation of one ton per flight, for which they donate $11 to the Tides Foundation as an offset.

Solar Electric Light Fund (SELF) does not offer calculators on its website, but has links to calculators to determine the amount of carbon emitted per flight and offers a program (SELF’s Carbon Neutral Club) where people can donate $10 per ton of CO2 they emit.

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7.2 Calculator Accuracy
Measuring greenhouse gas emissions from aircrafts is a complicated issue as a number of effects must be considered such as contrails, cirrus clouds and additional greenhouse gases (Bows, 2005; IPCC, 1999) .

7.2.1 Radiative Forcing (last changed for revision 1.3)
Calculating the CO2 emissions from jet fuel burned on flights is relatively simple but the overall warming impacts of air travel are much more complex and difficult to calculate. Therefore, and to allow comparisons of varying types of emissions, the concept of radiative forcing is used. Radiative forcing 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 (IPCC, 1999).

The main greenhouse gases emitted from aircraft are carbon dioxide (CO2), water vapor, nitrogen oxides (NOx), and methane (CH4). Aircraft travel at altitudes of 9 to 13 kilometers (approximately 5.6 to 8 miles). At these altitudes, the effect of the emitted gases is considerably different than on the ground level and in many cases still incompletely understood (2). Aircraft also emit water vapor during flight. When emitted in the stratosphere, H2O 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 (William, Noland and Toumi, 2002; IPCC, 1999).

The IPCC has estimated total radiative forcing of air travel to be 1-5 times larger in the stratosphere than in the troposphere and calculated the average for full radiative forcing to be a factor of approximately 2.7 (IPCC, 1999.) Therefore to estimate the impact of an airplane trip a multiplier should be used on the CO2 emissions from jet fuel to account for full radiative forcing.

Unless the growth of the air travel industry is slowed (6), it is estimated that by 2050 air travel will be contributing at least 6% of the total radiative forcing from human activities (RCEP, 2003; Bows, 2005).

The impact of NOx, water, and hydrocarbons at high altitude are poorly understood. It is possible that the forcing of water vapor is being underestimated by as much as a factor of 10 (see: Workshop on the Impacts of Aviation on Climate Change June 7-9, 2006, Boston, MA) (7)

Although more research is needed to fully understand the chemical processes in the stratosphere, the research used by the IPCC is very robust. We therefore recommend using a calculator that includes a multiplier for the increased radiative forcing in its carbon calculations. Only five of the evaluated offset companies use a multiplier to account for radiative forcing: atmosfair, Climate Care, climate friendly, myclimate, and NativeEnergy. (CarbonCounter.org has recently added a multiplier to their calculator, see footnote (5))

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7.2.2 Flight Distance (last changed for revision 1.3)
The rate at which fuel is burned is proportional to the drag which is the force of resistance that must be countered by the force of the engine’s propulsion. During the take-off and landing, the engine is at full thrust and more fuel is consumed during take-off and climbing. Shorter flights therefore have a lower overall fuel efficiency; ie. use more fuel per mile than long-distance flights (RCEP, 2003). As the aircraft climbs and begins to cruise - that is, above the altitude of 3000 feet - drag and therefore rate of fuel use decreases (IPCC, 1999). On longer flights (those over approximately 994 miles) the amount of fuel used during take-off is less significant compared to the whole. This efficiency gain is partly offset on long distance flights by the added weight of the fuel that an airplane needs to carry on such long trips (RCEP, 2003.)

On the other hand, cirrus clouds from contrails only develop at higher altitude. On short-haul flights the percentage of time the plane will spend at high altitude is less than on long-distance flights. That means the increased warming effect from cirrus clouds is less strong on short haul flights. In other words, the factor to account for full radiative forcing is likely lower for short haul flights than for long haul flights.

To more accurately calculate emissions, some of the companies’ carbon offset calculators distinguish between short, medium or long flights. atmosfair, myclimate, The CarbonNeutral Company, and NativeEnergy account for fuel efficiency differences between long and short flights. NativeEnergy, for example, uses a calculator that asks for place of origin and destination or mileage flown to be entered. The data entry points are then divided into three categories: short, medium and long haul flights and CO2 emissions factor of 0.64, 0.44 or 0.40 lbs of CO2 per passenger mile, are applied respectively.

Often, airplanes do not take the most direct route and having to change airplanes is very common. This leads to additional inefficiencies. atmosfair accounts for route and layover.

7.2.3 Occupancy Efficiency
At full occupancy an aircraft will fly at maximum efficiency. Therefore a flight that is at maximum payload burns less fuel per passenger than a flight that is at less than its maximum payload. On average, international flights fly at 78% of maximum payload and domestic flights at around 65% (RCEP, 2003).

The atmosfair emissions calculator addresses the different seat occupancy rates by applying a common average of 80% for charter flights. For scheduled flights the seat occupancy rates are also differentiated according to the flight region: for Germany 60%, EU 62%, intercontinental traffic 75%. If the flight type is not known, an average of 75% is applied (3).

7.2.4 Business vs. Economy (last changed for revision 1.3)
Business and first class seats are larger and take up more room. Therefore, a passenger traveling in business or first class is responsible for more emissions because they have effectively excluded additional people from traveling on that same flight (IPCC, 1999). Further research has shown that first class travel on long haul flights could have an impact 6 times as large as an economy traveler. (See The Carbon Cost of Business And First Class Long Haul Flying (Word Document).)

atmosfair calculates that business class seats require 1.4-times as much space as the economy seats. For fuel consumption this means that economy passengers consume 10% less than the average for all seats, while business passengers consume 40% more.

myclimate allows customers to enter whether they fly business or economy class. A passenger traveling business class is charged 1.5 times the emissions of a traveler in economy class.

7.2.5 Type of Plane
Type of plane also effects efficiency. The size, number of seats, engine types and other characteristics all influence the emissions of a flight. In general, older airplanes are less efficient than newer models. Most calculators use an average based upon all planes or choose just one typical commercial plane (IPCC, 1999).

atmosfair allows customers to enter information about the type of airplane. Climate Care uses the fuel efficiency of 737s for short haul flights and 747s and the Airbus A340 for long distance flights.

7.2.6 Accuracy versus Ease of Use
The air travel emissions calculators do not vary widely in terms of overall ease of use. All that is required for any calculator is the entry of mileage or airports. Additional information, may be entered but is not required. Therefore a trade-off between accuracy and ease of use is not necessary.

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7.3 Sample Calculations (last changed for revision 1.3)
To better compare how offset companies calculate and price their emissions offsets we have calculated for two sample flights:

- A short domestic flight: Boston - Washington, DC - Boston
- A long-distance, transatlantic flight: Boston - Frankfurt, Germany - Boston

The following three companies were not included:

Solar Electric Light Fund does not have its own calculator but has a link to http://www.earthfuture.com/climate/calculators/ which lists many available calculators. For air travel, the recommended calculator is http://chooseclimate.org/flying/ . The consumer has to click on an interactive map to choose departure and destination point. The consumer can also enter occupancy rate and choose between economy and business class. For the international flight, this calculator estimates a trip length of 4370 miles and 5.3 tons of CO2 emissions per person. This calculator seems to underestimate trip length. The calculated CO2 emissions are very high. The site gives detailed information about how the numbers are calculated. It would go beyond the purpose of this report to analyze this calculator in more detail but it seems that the CO2 emissions are possibly overestimated on this site.

Better World Club does not have a carbon calculator on its site but also has a link to http://www.earthfuture.com/climate/calculators/ . For each flight booked through BWC, BWC donates $11 to the Tides Foundation which administers the funds.

Cleanairpass focuses on offsets from vehicle emissions. The site offers no easy way to purchase carbon to offset air travel. It was therefore not included in this example.

Table 3: Domestic Flight: Boston - Washington, D.C. – Boston. Sorted by Emissions

Chart 3: Domestic Flight: Boston - Washington, D.C. – Boston. Sorted by Emissions

Notes:
Calculations and currency conversions made on 7/3/06 and on 10/31/06 06 (for myclimate), using online converter found at: http://www.xe.com/ucc/
Italic numbers indicate that the information was taken from a separate webpage:
1 These companies either required the customer to enter mileage flown or did not offer the distance after the calculation was made. Therefore the distance of 822 miles for the domestic flight and 7320 miles for the international flight were used. These estimates were provided by an airport mileage calculator found at: http://www.webflyer.com/travel/milemarker/
2 These companies only sell offsets on a per ton basis. Therefore the cost used is to offset one ton of carbon.
3 The US site for myclimate does not display the number of miles traveled.
4 This is the price for the TerraPass Puddle Jumper which offsets 2,500 lbs of CO2 emissions
5 This is the price for the TerraPass Intercontinental which offsets 7,500 lbs of CO2 emissions
6 The The CarbonNeutral Company reported to us that they do not only sell on a per ton basis (e-mail communication 3/2/07). They offer currently three different portfolios ranging from $2.79 to $3.66 for the short-haul flight and from $18.14 to $23.83 for the long distance flight. Their prices also include 17.5% UK VAT. The tables and charts were adjusted on 3/31/07 for the charts, the average prices were chosen.

Table 4: International Flight: Boston - Frankfurt – Boston. Sorted by Emissions

Notes: See above.

Chart 4: International Flight: Boston – Frankfurt -- Boston. Sorted by Emissions

Notes: See above.

The calculated distance between the place of origin and destination is relatively similar across the different calculators. The difference in tons of carbon emitted is more significant. For the domestic flight it ranges between 0.19 to 0.44 tons; for the international flight between 1.45 to 4.43 tons. The companies that calculate the highest emissions for the domestic and international flights all use a multiplier to account for full radiative forcing (atmosfair, climate friendly, NativeEnergy and myclimate). atmosfair, NativeEnergy and myclimate account for fuel efficiency differences depending on flight distance. These companies all use the airport method to calculate emissions (as opposed to the mileage method) Also, atmosfair and myclimate include additional factors in their calculations.

myclimate has two separate sites, one for its European customers and one for its American customers. These sites use separate calculators and the results are noticeably different in their carbon calculations as well as their pricing (4). In the evaluation of the calculators we have focused on the Swiss site.

Two of the companies with the lowest calculations (Offsetters and CarbonCounter see footnote (5)) do not clearly explain the assumptions under which their calculators operate. Interestingly, Climate Care accounts for radiative forcing, uses the airport method of calculation and accounts for airplane type and still has one of the lowest estimates for these flight examples. It is not clear why this calculator varies in this way.

7.4 Calculator Evaluation

Table 5: Comparison of Calculators

Because of all the variables and uncertainties involved in calculating the climate impacts of air travel it is difficult to say with certainty which is the most accurate calculation. Yet calculators that take into account a greater number of air travel factors seem to be the most accurate. For example, it is safe to assume that those calculators that do not include a multiplier for full radiative forcing are underestimating the impact of air travel (RCEP, 2003).

As the table above illustrates, in our evaluation, atmosfair has the most detailed and best documented calculator. When we rated the calculators, we considered the following amounts most accurate: Domestic flight: minimum 0.35 tons per passenger; international flight: minimum 3 tons per passenger. We rated as acceptably accurate: Domestic flight: minimum 0.25 – 0.35 tons per passenger; international flight: 2.5 - 3 tons per passenger.

Table 6: Rating of Calculators

With the voluntary carbon market expected to grow, it would be helpful if emissions calculations for air travel would be standardized by an independent third party. Such standardized emissions parameters would streamline the calculations and lend greater credibility to the offset companies much in the same way verifications standards are used to guarantee the quality of offset projects.

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Notes

1 Jardine, Christian N. (2005). Part 1: Calculating the Environmental Impact of Aviation Emissions. Environmental Change Institute: Oxford University Centre for the Environment. Last accessed on 7/7/06.

2 For example, NOx emissions have a stronger global warming impact in the lower troposphere. When NOx is emitted in the troposphere, ozone levels increase, causing the earth to warm because ground level ozone is a greenhouse gas (IPCC, 1999).

3 The Atmosfair Emissions Calculator (pdf), accessed last on 10/31/06

4 According to an e-mail communication with R. Heuberger from myclimate on 2/14/06, the price difference is a result of different projects: US customers automatically fund projects of their ‘balance’ portfolio, whereas Swiss customers fund projects of their ‘sustainable’ portfolio, which is more expensive.

5 CarbonCounter.org has done a major overhaul of its webpage in early 2007 and now has much more detailed information about its calculator. It now also includes a factor of 2 to account for full radiative forcing. The customer can choose between an emissions estimate, by entering approximate number of hours flown, or calculate the exact emissions by entering the number of miles flown. CarbonCounter.org does not provide a link to an external site for customers to calculate the number of miles flown. The carbon emissions in our two examples would now be 0.36 /3.21 tons of CO2 respectively. That means that according to the criteria in this study, they no longer underestimate emissions from air travel. We did not update the numeric tables or graphs. We also did not update the pricing information. We did change the information in the final calculator evaluation graph and in the company profile in chapter 9

6 Increasing efficiency of new aircraft could potentially contribute to a slowing of emissions growth of air-travel.

CO2 production is directly related to aircraft efficiency, and to some degree how the aircraft is flown. Increasing aircraft efficiency has four components:
1. increasing the lift of the wing,
2. decreasing the drag,
3. increasing the thrust per pound of fuel burned (called thrust specific fuel consumption), and
4. decreasing the weight of the aircraft.

Aircraft are not as efficient during climb, but it has been shown that the overall greatest flight efficiency occurs when an aircraft climbs fast and spends more time in cruise. It might be that overall efficiency can be increased by sacrificing climb efficiency in order to spend more time in cruise.

There are new strategies being worked on to make descent more efficient. Currently during a conventional stairstep descent, the engine revs up and down – causing high emissions. So-called "continuous descent approach" that is much more environmentally friendly is currently being worked on. (e-mail communication, Prof. Rich Wlezien, Tufts University)

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7 The atmospheric chemistry of air plane emissions at high altitude is very complex. To delve into detail goes beyond this paper. We recommend the following list of readings:

Workshop on the Impacts of Aviation on Climate Change June 7-9, 2006, Boston, MA
http://web.mit.edu/aeroastro/partner/reports/climatewrksp-rpt-0806.pdf

IPCC (1999) Aviation and the Global Atmosphere: 6.2.3. Alternative Indexing of Aviation's Climate Impact-RF Index [online]. Available from: http://www.grida.no/climate/ipcc/aviation/071.htm#623

Sausen, R. et al. (2005) Aviation radiative forcing in 2000: An update on IPCC (1999) Meteorologische Zeitschrift 14: 555-561 - available from: http://folk.uio.no/gunnarmy/paper/sausen_mz05.pdf

Forster et al. (2006) It is premature to include non-CO2 effects of aviation in emission trading schemes. Atmospheric Environment 40:1117-1121

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