Measuring up: A Comparison of Air Emissions Calculation Methodologies

If you shop carbon offset providers on the internet, you may notice that your air travel-based greenhouse gas emissions can vary significantly from site-to-site. That’s in large part due to the methods used to calculate those emissions.

To help illustrate how calculations can differ, we compared air travel emissions calculated using three leading methodologies, including Clean Air – Cool Planet, World Resources Institute, and the U.S. Environmental Protection Agency for a medium-distance flight from Raleigh-Durham to Orlando (about 645 miles) and a long-distance flight from Raleigh-Durham to San Francisco (about 2,390 miles).  The calculators differ primarily based on the emissions factors they apply to each mile traveled, the radiative forcing index employed by each (which takes into account the effect emissions have when released at high altitudes), and the specific greenhouse gases they include in their calculations.  

Duke University uses Clean Air-Cool Planet’s (CACP) Campus Carbon Calculator, which applies a high emissions factor and conservative radiative forcing index.  CACP’s Campus Carbon Calculator is the methodology followed by the majority of signatories to the American College and University Presidents’ Climate Commitment.  Duke University became a signatory to the ACUPCC in June 2007, thereby making a voluntary commitment to climate neutrality.  In addition to carbon dioxide (CO2), the CACP calculator accounts for emissions of nitrous oxide (N2O) and methane (CH4).

View the comparison of all three air emissions calculators.

Below are some definitions you may find helpful:

Emissions factor is the unique value used to scale emissions to activity data in terms of a standard rate of emissions per unit of activity (e.g., grams of carbon dioxide emitted per barrel of fossil fuel consumed).

Radiative forcing refers to how much a particular activity contributes directly to the atmospheric chemical reactions that cause climate change. A higher radiative forcing index (RFI) means an activity has a greater contribution to chemical changes in the atmosphere.  

Duke University uses the Clean Air-Cool Planet’s (CACP) Campus Carbon Calculator, which applies a high emissions factor and conservative radiative forcing index. The CACP calculator also includes nitrous oxide (N2O) and methane (CH4) in the calculator in addition to carbon dioxide (CO2), as does EPA.  WRI only counts CO2 emissions.

1. Intergovernmental Panel on Climate Change (IPCC), Aviation and the Global Atmosphere, 1999.
2. See U.S. Energy Information Agency, Voluntary Reporting of Greenhouse Gases Program, Glossary of Terms.
3. See Bonneville Environmental Foundation, FAQ Category, Calculator. See also Intergovernmental Panel on Climate Change (IPCC), Third Assessment Report - Climate Change 2001, Appendix I Glossary.

Return to the DukeEngage Carbon Offsets Pilot Project website.