Part 1 of this post focused on estimating emissions from commuting. This post focuses on emissions from air travel.

In order to calculate emissions from air travel, you will need to determine the total number of passenger air miles paid for by or through your institution.  The challenge is that institutions generally don't track this data in one place (if at all) so you may have to pull together data from several entities, including the business office, study abroad office, athletics department, and your institution's travel agent(s).

The business office is likely to maintain records on travel reimbursements, probably in the form of total expenditures on air travel.  You can then use statistics on the average price per passenger air mile from the Air Transport Association of America to convert the total air travel expenditures into passenger air miles.*  Unfortunately, these figures exclude taxes, and other ATA data indicate that taxes can comprise about anywhere from 10 to 50 percent of the total ticket price.  We recommend adjusting the cost per passenger mile up by 20 percent to take taxes into account.

If the available figures from your business office don't differentiate between air travel and other travel expenditures (like lodging, car rentals, food, etc), you can use a random sample of travel reimbursement requests to estimate the percentage of total travel expenditures that is used for air travel.

To ensure that you haven't left out any of the institution's air travel, you'll also want to check if data supplied by the business office includes air travel for study abroad programs and campus athletics.  If not, the study abroad and athletics departments should each be able to provide you with a list of destinations and the number of travelers on each trip.  You then calculate total passenger air miles for each trip by using one of the many online tools that show the round trip distance between the airport closest to your campus and each destination.

Here's an example to illustrate how this works in practice:

In 2006, Example College spent $1,000,000 on total travel expenses, not including travel paid for by the study abroad and athletics programs.  A random sample of travel reimbursement requests indicated that 80 percent of travel expenses went towards air travel. ATA statistics show that the price per passenger air mile in 2006 was 13 cents.  Analysis of data provided by the study abroad and athletic programs show that the study abroad program was responsible for 250,000 passenger air miles while the athletic program was responsible for another 500,000.  Total passenger air miles would then be calculated as follows: 250,000 + 500,000 + ($1,000,000*.8) / (.13*1.2) =  ~5,750,000 passenger air miles

This method isn't precise, but it does provide a good ball-park estimate of total passenger air miles that should be sufficient for purposes of a GHG inventory.

Adjusting for Radiative Forcing

Once you have an estimate of the total passenger air miles paid for or through your institution, you'll need to decide whether you want to estimate only the carbon emissions from air travel or whether you want to estimate the total contribution to climate change due to your institution's air travel.  According to the Intergovernmental Panel on Climate Change (IPCC) and others, air travel's actual contribution to climate change is potentially several times higher than would be captured by looking at carbon emissions alone.  This is because other effects of air travel, including upper atmosphere emissions of NOx and the formation of contrails, also contribute to climate change.

In an attempt to take the full impact of air travel into account, many institutions multiply their carbon emissions from air travel by what is known as a radiative forcing factor.  The Clean Air-Cool Planet Campus Carbon Calculator, for example, incorporates a radiative forcing factor of 2.8, which is derived from IPCC's best estimates of the ratio of total radiative forcing from air travel to that from CO2 emissions alone.  It may well be an underestimate since it doesn't incorporate air travel's impact on cirrus clouds, which is not well understood but believed to result in additional radiative forcing.

**We previously recommended that campuses use a figure of 25 cents per passenger mile to convert total air travel expenditures into passenger air miles.  We now believe the ATA's statistics referenced above to be more appropriate for this purpose, especially since they are updated annually.  Special thanks to Carol Dollard and Patrice Stafford at Colorado State University for bringing this data source to our attention.*