To try to understand where the different RF factors come from, I dug through all the calculators to see what they had to say about RF. Then I tried to follow any references they made. I brought all the information here. It's a long list - you can scroll through or click on the one you're interested in.
Go straight to calculator summary:
atmosfair RF=3 (>9km)
brighterplanet RF=2
carbonfootprint RF=1.9
carbonfund RF=2.7
carbonneutral RF=1.9
carbonindependent RF=2
chooseclimate RF=3
clevel RF=1.9
climatecare RF=1.9
climatepartner RF=3
co2nsensus RF=1
flygrn RF=1.9
greentripper RF=2
icao RF=1
myclimate RF=2
myclimate lufthansa/swiss RF=1
offsetters RF=1.9
omnicalculator flight-emissions RF=2
southpole RF=1.9
terrapass RF=1
Go straight to report/article summary:
IPCC (1999)
Sausen et al. (2005)
Kollmuss & Crimmins (2009)
Lee et al. (2010)
Jungbluth & Meili (2018)
UK government (2009-2019)
Calculators accessed 2019-08.
"In order to appropriately represent the climate impact of all flight emissions, the atmosfair emissions calculator multiplies the CO2 emissions emitted at altitudes over 9 km with the global average multiplicator of 3. This multiplicator results when the global warming potential of all non-CO2 effects is integrated and discounted over 100 years (UNFCC) and (David Lee et al., "Transport impacts on atmosphere and climate: Aviation", in atmospheric environment (44), 2010)."
"Aviation multiplier. A multiplier to account for the extra climate impact of greenhouse gas emissions high in the atmosphere. Use 2.0 after Kollmuss and Crimmins (2009)"
"Carbon emissions from planes at high altitude have an increased effect on global warming. Check the box if you would like to multiply aviation emissions by DEFRA's recommended Radiative Forcing factor of 1.891."
"Emissions from planes at high altitudes impact climate change more than if the emissions were released at ground level. To take account of this you have the option to factor up the CO2 emissions released by aviation Radiative Forcing Factor of 1.9. From 2013, DEFRA have include emissions factors to cover radiative forcing which have been used in the online calculators."
"At high altitudes, the effect of greenhouse gases is considerably different than on the ground level. Aircraft also emit water vapor during flight which 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 sunís rays back into space. However, contrails lead to a net warming factor, which is estimated to be 2.7 times the normal effect (IPCC, 1999)."
"An RFI of 1.9 is applied against CO2"
"What is Radiative Forcing Index (RFI)?: The IPCC (Intergovernmental Panel on Climate Change) has identified that aircraft emissions at altitude have a greater impact on the atmosphere due to other non-CO2 climate change effects from aviation (e.g. NOx, water vapour, contrails) and the RFI factors allows for these other impacts"
"Aircraft are thought to have greater climate effects than just the CO2 emissions from burning the fuel. The additional effects include contributions from nitrous oxides and ozone. Because of this, the CO2 emissions from aviation should perhaps be multiplied by an appropriate factor. The size of the factor is often taken to be 2.7"
(long quote)
"According to this statement, it is incorrect to multiply CO2 emissions by the RFI, but it is also incorrect to ignore them. For the purposes of a calculator, some decision must be taken until further evidence is available, and the Carbon Independent calculator will for the time being multiply aviation CO2 emissions by a factor of 2.0."
"And remember: that's just the CO2: the total warming effect of CO2 + H2O + NOx is about 3 times greater."
"The total warming effect of all emissions (CO2, H2O and NOx) put together, is in the range 2-5 times greater than that of CO2 alone. This range was confirmed by analysis in the "Special Report on Aviation" published by the "Intergovernmental Panel on Climate Change" (IPCC) in April 1999."
"The emission factors used in our calculations take radiative forcing into account, which increases total emissions by 90%. Radiative forcing is the increased damage caused by emissions released into the atmosphere at high altitude rather than those released at sea-level."
"Radiative Forcing Index (RFI) is a multiplier that takes account of the extra gases emitted to the atmosphere when you fly at altitude – which are greater than burning fossil fuels at ground level. Scientists estimate the impact is between 1 and 4 times greater and it is considered best practice by Defra/DECC to include a multiplier of 1.9 to take account of these impacts. ClimateCare includes an RFI multiplier in all our flight calculations."
"In addition to the direct emissions caused by the combustion of kerosene, we include also indirect effects of air travel on the climate. Indirect effects result from the emission of particles at high altitudes, which may contribute to additional cloud formation and global warming (so-called radiative forcing effect). There is agreement that this effect is significant, but the amount of the correction factor to be applied (RFI factor) cannot be clearly determined scientifically. ClimatePartner is therefore guided by current practice, which places the RFI factor between 2 and 5 depending on the calculation model. ClimatePartner calculates with an RFI factor of 3, which is applied to direct emissions from air travel."
"CO2nnector adopts The ICAO Carbon Emissions methodology by The International Civil Aviation Organization for emission caused by your flights... we never add non-scientific additions to our calculations such as radiative forcing."
"We chose for the calculation factors of DEFRA"
"This estimate is in the same order as the UK Government (DEFRA) and FlyGRN uses (1.9)."
"Radiative forcing (RF) is caused by contrails, the white could stripes left behind planes. They have an additional greenhouse effect. Independent science (Bilan Carbone, ADEME) estimates that RF of aviation is 2 times that of just CO2 emissions from burning kerosene."
"Why does ICAO not take account of non-CO2 effects in the methodology? The ICAO Carbon Emissions Calculator is limited to the calculation of the CO2 amounts released into the atmosphere by the aircraft engines during a flight. Consequently, the ICAO Emissions Calculator does not quantify the climate change impact of aircraft emissions using the Radiative Forcing Index (RFI) or other such multipliers. The scientific community has not yet reached consensus on the use of the RFI or other such multipliers and therefore ICAO will only adopt a multiplier if and when the scientific community reaches a general agreement on this issue. ICAO is working in collaboration with IPCC on this subject and will adapt a multiplier methodology in due course accordingly."
"Aircrafts do not only emit CO2 but also other forcing agents that affect the Earth’s radiative balance and thus the climate. Amongst other factors, emissions from aviation also lead to short-term increases in tropospheric ozone as a consequence of nitrogen oxide (NOx) emissions, initiate condensation trails (contrails) and may affect the formation of cirrus clouds. The total radiative effects have thus been estimated to be two to four times larger than the direct CO2 radiative forcing. However, research is ongoing inorder to constrain the uncertainties. Furthermore, a comparison of CO2 and non-CO2 effects is particularly challenging as they act on different time scales. Still, ignoring these effects would not be a far-sighted scientific approach. Latest studies (Jungbluth & Meili 2018), based on the correct interpretation of the most recent scientific publications, recommend an RFI factor of 2 on total aircraft CO2 emissions. This substantiates the myclimate calculations, as myclimate already decided to multiply the estimated CO2 emissions by a factor of 2 (referred to as multiplier) to account for the warming effect due to non-CO2 aircraft emissions (Jungbluth & Meili 2018, Kollmuss & Crimmins 2009)."
"Besides CO2, are any other emissions included in the calculation of the offset amount? No, only CO2 emissions are included in the Lufthansa/SWISS calculator. This is because in science there is only a clear certainty with regard to the aviation sector about the climate impact of CO2 emissions. However, after calculating the CO2 emissions each customer has the option of varying the payment by clicking on "I'd rather donate a different amount instead"."
"In addition to CO2, air transport also produces further emissions (especially nitrogen oxides and water vapour) that are believed to contribute to climate change. Science has not yet, however, been able to determine the precise extent of their contribution here. In view of this, the emissions calculator that myclimate provides for SWISS and Lufthansa does not consider any other types of emission apart from carbon dioxide."
"We use emissions factors from the comprehensive list published by the UK Department for Business, Energy & Industrial Strategy (BEIS) in 2018 Government GHG Conversion Factors for Company Reporting. These emission factors include both a radiative forcing factor and an uplift factor for aviation, as recommended in the IPCC report, Aviation and the Global Atmosphere. Radiative forcing accounts for the additional climate impacts associated with aviation."
"Radiative forcing factor: finally, the emission takes place high in the atmosphere, and that's precisely where we don't want the CO2 to be because of its greenhouse effect. To account for that, we include a radiative forcing factor. We take its value to be 2, based again on the analysis from www.carbonindependent.org."
"Finally, South Pole’s methodology also takes into account indirect emissions such as ... and a Radiative Forcing Index (RFI) multiplier of 1.9 to account for non-CO2 climate change effects from air travel. This includes contrails, water vapour, NOx emissions, etc. UKBEIS recommends a multiplier of 1.9 as a central estimate, based on research from the European Assessment of Transport Impacts on Climate Change and Ozone Depletion (ATTICA, http://ssa-attica.eu) and analysis by Lee et al (2009) reported on by the Committee on Climate4Change (2009)."
"For a variety of reasons, greenhouse gas emissions created high in the atmosphere are thought by scientists to be worse than greenhouse gas emissions at ground level. This is sometimes referred to as Radiative Forcing. Unfortunately, no consensus yet exists as to how much worse, so presently terrapass doesn’t take this effect into account. If and when a scientific consensus on radiative forcing develops, we will update our calculations accordingly."
Summary for policymakers 4.8. What are the Overall Climate Effects of Subsonic Aircraft?
"the overall radiative forcing by aircraft (excluding that from changes in cirrus clouds)
for all scenarios in this report is a factor of 2 to 4 larger than the forcing
by aircraft carbon dioxide alone"
Chapter 6 Executive summary
"The Radiative Forcing Index (RFI)-the ratio of total radiative forcing to that from
CO2 emissions alone-is a measure of the importance of aircraft-induced climate
change other than that from the release of fossil carbon alone. In 1992, the RFI for aircraft
is 2.7; it evolves to 2.6 in 2050 for the Fa1 scenario. This index ranges from 2.2 to 3.4 for
the year 2050 for various E- and F-type scenarios for subsonic aviation and technical options
considered here. The RFI increases from 2.6 to 3.4 with the addition of HSCTs (scenario Fa1H),
primarily as a result of the effects of stratospheric water vapor. Thus, aircraft-induced climate
change with RFI > 1 points to the need for a more thorough climate assessment for this sector.
By comparison, in the IS92a scenario the RFI for all human activities is about 1, although for
greenhouse gases alone it is about 1.5, and it is even higher for sectors emitting CH4
and N2O without significant fossil fuel use."
6.6.5. Climate Change
"In 1992, the RFI for aircraft was about 2.7, with an uncertainty of at least ±1.5.
The RFI changes to 3.0 by 2015 then drops to 2.6 for the Fa1 scenario (see Table 6-1). This
index ranges from 2.2 to 3.4 in the year 2050 for the various E- and F-type scenarios for subsonic
aviation and technical options considered here. The RFI increases from 2.6 to 3.4 with the addition
of HSCT aircraft (scenario Fa1H), as a result of the effects of stratospheric water vapor."
Quoting the DEFRA interpretation of this paper:
"Estimates for scaling CO2 emissions to account for Radiative Forcing
impacts are not quoted directly in the table, but are derived as follows:
IPCC (1999) = 48.5/18.0 = 2.69 ≈ 2.7
TRADEOFF = 47.8/25.3 = 1.89 ≈ 1.9"
"a multiplier of 2 or greater should be used for air travel emissions calculators to account for non-CO2 warming effects"
"Emission-based metrics such as the GWP and GTP have been reviewed and formulated into CO2-equivalent emissions. The value of these depends on the time-horizon selected. Emissions weighting factors that represent the total effect of aviation as a ratio to that of CO2 have been calculated and presented for GWPs and GTP metrics and different time-horizons. For GWPs, these range from 4.3 to 4.8 (GWP20) and 1.9–2.0 (GWP100) for 2005, including AIC; excluding AIC they are 2.1–2.6 (GWP20) and 1.3–1.4 (GWP100). In the context of these calculations, AIC has a similar uncertainty to aviation NOx GWPs, given that the few different estimates change in sign."
"An RFI factor of 2 on total aircraft CO2 emissions is recommended
in this article because it is based on the correct interpretation of the most recent
scientific publications. If detailed data on the share of emissions in the higher
atmosphere are available, calculations will be more accurate if the CO2
emissions in the higher atmosphere are multiplied by a factor of 5.2."
Where "higher atmosphere" seems to be the stratosphere. The height that the stratosphere
starts depends on where you are. From wikipedia: "Near the equator, the stratosphere
starts at as high as 20 km, around 10 km at midlatitudes, and at about 7 km at the poles."
Every year the UK government publishes emission conversion factors for greenhouse gas company reporting. Depending on the year, the department can be called BEIS (2016-2019), DECC (2014-2015) or DEFRA (2009-2013). I looked back as far as possible and picked out the relevant bits:
2014-2019
"A multiplier of 1.9 is recommended as a central estimate"
"It is important to note that the value of this 1.9 multiplier is subject to significant uncertainty"
Reference Sausen et al. (2005)
2010-2013
"A multiplier of 1.9 is recommended as a central estimate"
Reference Sausen et al. (2005)
2009
"The appropriate factor to apply is subject to uncertainty but was estimated by the
IPCC in 1999 to be in the range 2-4, with current best scientific
evidence suggesting a factor of 1.9."