Air Travel GHG Emissions Information and Education Program

The Zilkha Center recently completed the first full 12-months of the Air Travel GHG Emissions Information and Education Program. This mouthful of a name refers to the mostly automated monthly emails that go out to air travelers and those who book flights on behalf of travelers. These notifications provide information about the flights’ estimated greenhouse gas emissions, estimates of the associated damages using two values for the social cost of carbon (SCC), and information about ways to try and reduce air miles and emissions. The goal is to raise awareness of the climate impacts of air travel (800 million metric tons of CO2 or 2% of global energy-related CO2 emissions in 2022 according to the EIA) and suggest ways for reducing them.

With this blog post, I want to share information about (i) how the program works in practice, (ii) how we did in terms of air travel GHG emissions, and (iii) what might be next for the program this year.

How does the program work?

Estimating the GHG emissions associated with a person’s flight accurately and precisely requires multiple, detailed pieces of information, including the exact flight path, the aircraft type, age and seat configuration. We don’t have all of this information, but instead make an approximation based on the itinerary and emissions factors for CO2e emissions per passenger-mile. We compared our estimates with those of several online flight emissions calculators and found no substantial or systematic differences.

The itinerary information comes from the Business Office to the Zilkha Center director using purchasing card and reimbursement data. It typically includes the airports of origin, destination, and any stop-overs, although further assumptions are sometimes required because the locations provided do not correspond to an airport.

Following manual data preparation and cleaning, the files are processed in R: We calculate the miles flown between the airports in the itinerary (as the crow flies) and multiply the result with a CO2e emissions factor from the UK’s Department for Environment, Food and Rural Affairs (DEFRA), distinguishing between short-haul flights (up to 2,300 miles) and long-haul flights (over 2,300 miles). DEFRA updates the factors annually and current emissions factors might, therefore, differ slightly from earlier years. The estimated emissions are then multiplied with two Social Cost of Carbon values, the lower of which is the Obama Administration’s $51 per metric ton CO2e and the higher one is the more recent $185 per metric ton CO2e published by a consortium of university researchers and Resources for the Future.

Finally, we aggregate these statistics for all flights undertaken or booked and package them into the monthly email template. We also include some information on the program, emissions for some representative flights, as well as suggestions for reducing emissions

How did air travel emissions evolve over the past 12 months?

When we started the program with October 2022 emissions, we were slowly emerging from the Covid19 pandemic. Since then, global air travel has returned to pre-pandemic levels and we were curious about the trend at Williams.

Not surprisingly, college-sponsored air travel also picked up again, but our estimated emissions remain substantially below those recorded in the years prior to the start of the pandemic. For example, air travel emissions in FY19—the last full year not impacted by the onset of the pandemic—amounted to an estimated 4,799 metric tons of carbon dioxide equivalent (MTCO2e). In FY22, emissions reached 1,481 MTCO2e and in FY23 they climbed to 2,429 MTCO2e. This represents a 64% increase year-over-year but is still nearly 50% below the pre-pandemic level.

Figure 1: Fiscal year air travel emissions from FY19 to FY23

Looking at the monthly data for FY23, we observed relatively little variation. The lowest emissions months were July 2022 and February and March of 2023.

Figure 2: Monthly and cumulative air travel CO2e emissions for FY23.

While we are still in the middle of FY24, we see signs that emissions might continue to rise as emissions since cumulative July have been higher than they were in FY23.


Figure 3: Monthly and cumulative air travel CO2e emissions for FY24 to date.

We, nonetheless, hope that emissions might stabilize at a level below pre-pandemic emissions as a reflection of permanently changed travel patterns due to, among other things, the increased availability and quality of virtual meetings and conferencing, and increased awareness of the climate impacts of flying.

You can always view the latest monthly statistics on the program’s webpage.

What’s next for the program?

We want to continue improving how we communicate this information with you. For example, we know from your feedback that some travel preparers feel unsure or dissatisfied with the emails they receive since they are not the ones flying and might not have the freedom or ability to share the information with the travelers.

We also want to keep the monthly emails fresh and engaging. We are encouraged by the continued high opening rate of the emails (>85%) but we assume there are still opportunities for improvement.

If there are other aspects of the program that we could improve, please reach out to [email protected] or directly at [email protected]. Thank you for your support and partnership!

Tanja Srebotnjak

Director, Zilkha Center for Environmental Initiatives