Recommendations for Use of Multipliers

Read the Explanations of Our Recommendations
For more details & references download Carbon Offsetting & Air Travel Part 2: Non-CO2 Emissions Calculations (pdf)

Multipliers or any other type of metric that tries to express the full climate impacts of aviation have to be chosen carefully depending on  their specific intended purpose.

Our recommendation is specifically intended for calculating the non-CO2 portion of the climate footprint resulting from an individual’s or a company’s air travel. It is NOT a general recommendation for calculating the impacts of all emissions from air travel.

For example, certain aviation policy choices might lead to a decrease of short-lived GHGs but an increase in fuel consumption and therefore (long-lived) CO2 emissions. In this case, a long time frame would need to be used to evaluate the most climate-friendly policy, because long-term climate effects would likely outweigh short-term benefits from reducing shorter-lived GHGs. In this example, one policy option is being weighed against another. But for our purposes of calculating total climate footprint, CO2 effects are already accounted for and the question we are answering is: how much should we add (expressed as a multiplier) to account for non-CO2 effects?

For of the reasons we elaborate on below, we advocate that a multiplier of at least 2 be used for air travel emissions calculators to account for non-CO2 warming effects.

We emphasize that our recommendation is not solely based on scientific arguments, but on ethical ones as well. It is based on our best understanding of the current knowledge of aviation emissions in particular and climate science in general. As the science progresses, the models become more sophisticated, and the ethical and political debates on climate change develop in the coming years, our recommendation should be revisited and refined.

(Coincidentally, the RFI figure of 2.7 frequently used and cited in air emissions calculators fits with our own  recommendation of a multiplier of at least two. Although the numeric figures happen to be very similar, they are based on different metrics. We can therefore say that the results of calculators that do use the RFI figure of 2.7 are not necessarily wrong, despite the fact that they are based on an inappropriate choice of metric.)


Explanations of Our Recommendations

There is no single metric, no single multiplier and no single answer to the question of how the effect on climate change from air travel should be calculated so that an individual or a company can accurately calculate the climate footprint of their current air travel. Metrics and underlying assumptions have to be chosen according to the questions we are trying to answer and the goals we are trying to achieve. For example, if the main concern is the near-term impacts of climate change, a shorter time horizon is more appropriate. Nevertheless, we would like to make the following final observations and recommendations:

The Science:

There is still uncertainty related to quantifications of the climate impacts of non-CO2 air travel emissions. Clearly, more research and more sophisticated models are needed.  Although there is no simple answer to what the overall impact of aviation is, it is clear that total contribution to climate change is greater than that of CO2 alone.

It seems therefore less defensible to exclude non-CO2 effects than to choose a multiplier that is greater than 1.

Time Frame:

As we have illustrated, the chosen time frame greatly influences the results. For example, if we choose temperature change after 100 years for the evaluation, the effects of short-lived GHGs are de-emphasized, and changes of temperature in between the time of emission and the evaluation year are not captured (Fuglestvedt et al., 2008). Yet including these short-lived regional impacts into a climate metric is important because they might trigger feedback mechanisms (Berntsen and Fuglestvedt, 2008). For example, researchers have recently found that short-lived emissions in the Arctic result in an equal or greater climatic response than long-lived emissions due to the positive feedback mechanisms associated with ice albedo.. Reducing these short-lived but high impact non-CO2 emissions in the critical near-term may be more effective in slowing Arctic warming and preventing a “tipping point” for ice disintegration than emphasizing long-term efforts to reduce long-lived GHGs such as CO2 (Quinn et al., 2008).

For these reasons, we advocate a short time horizon (e.g. 20 years) that includes short-lived effects be used. (This is a value-based choice and only applies to calculating effect on climate change from air travel in order to best estimate the footprint of an individual or a company due to their current air travel,  see discussion at the end of this section.)

The Climate Challenge:

It is becoming increasingly clear that climate change is happening faster than was expected (e.g. loss of Arctic sea ice, see e.g. Shepherd and Wingham, 2007) and triggering positive feedbacks (e.g. methane emissions in Siberia, see e.g. UNEP, 2007) that may lead to unprecedented and possibly irreversible changes. At the same time, anthropogenic emissions are growing faster than was predicted by even the highest IPCC emissions scenario (Raupach et al., 2007). It is therefore no exaggeration to say that we are facing a climate emergency. Addressing this emergency will require changes on a scale we have never undertaken as a human society.

Given the urgency of the climate change challenge, it is an ethical imperative to proceed following the precautionary principle and include all warming effects to the best of our knowledge.


The wealthy are disproportionately responsible for air travel, yet the impacts of climate change will be felt disproportionally by the poor.

We believe that an ethical argument can be made that the effects of aviation should be accounted for to their fullest extent, so that mitigation policies and offset options are based on fully internalized climate costs of aviation.