Radiative Forcing Index
The Radiative Forcing Index (RFI) has been used to quantify non-CO2 warming effects of air travel. RFI is the ratio of total radiative forcing (RF) of all GHGs to RF from CO2 emissions alone for aircraft emissions (IPCC, 1999).
RFI = RFtotal / RF CO2
Many air travel calculators use a dimensionless multiplier between 2 and 3 to account for non-CO2 warming effects. Usually these multipliers are based on the RFI calculated in the 1999 IPCC report on aviation. The RFI for aviation emissions was estimated by the IPCC to be 2.7 with an uncertainty of ±1.5 (IPCC, 1999). In other words, the IPCC estimated that total RF of aviation was 2.7 times that of just CO2 emissions from aviation. When the IPCC estimates were updated, RFI was calculated to be approximately 2 (Sausen et al., 2005).
Yet RFI is an inappropriate metric to use for personal air travel emissions calculators because RFI calculations are based on RF values for aviation emissions from the last approximately 50 years. RFI therefore includes warming responses from past air travel emissions. Furthermore, future warming due to long-lived greenhouse gases is not included in these calculations. RFI was never intended to be used to calculate the total effect of current aviation, and is therefore not appropriate for our purpose.
RFI is not the correct metric for determining total climate effects of current aircraft emissions in order to calculate climate footprints of air travel passengers. Or, as the IPCC states:
[T]he RF index (RFI) introduced by IPCC (1999), should not be used as an emission metric since it does not account for the different residence times of different forcing agents. (IPCC, 2007, WGI, section 2.10.4, p 215)