Carbon Offsetting & Air Travel
Part 2: Non-CO2 Emissions Calculations
June 2009, SEI Discussion Paper
Anja Kollmuss (SEI) and Allison Myers Crimmins
This is the second of two papers that examine the key factors that have to be taken into account when calculating air travel emissions for the purpose of climate footprint and offset calculations. The first paper examined methods of calculating CO2 emissions only, and provided a framework for how to allocate responsibility for these emissions among various aviation users (e.g. passengers, cargo customers). This paper has a broader scope in that it explains all the emission factors that affect climate change and discusses appropriate metrics that take into account all these factors. However, it does not directly address allocation of responsibility at the individual level.
In order to estimate the full effect of aviation on climate, it is necessary to account for CO2 as well as for all other, non-CO2 warming and cooling effects. This paper is written for a non-technical audience and explains how to account for non-CO2 impacts of air travel emissions. The non-CO2 warming effects of aviation are most commonly accounted for in emissions calculators through a Radiative Forcing Index (RFI) or a multiplier. These terms refer to a dimensionless factor which is multiplied by the calculated CO2 emissions in order to account for all warming effects. The multipliers used by different calculators lie between 1 (i.e. not accounting for non-CO2 warming effects) and 3 (i.e. the total warming effect is calculated at three times that of the CO2 emissions alone). These numbers are usually chosen in reference to the IPCC special report on aviation. Unfortunately, these multipliers are often scientifically flawed. This paper seeks to explain why.
Our discussion shows the limitations to developing a dimensionless multiplier for the integration of non-CO2 effects into emissions calculators. These limitations are to some extent caused by scientific uncertainty, such as limited knowledge about the effects of cirrus clouds. Yet we also show that developing a multiplier is influenced to a great extent by value-based decisions that underlie the chosen approach and parameters (i.e. climate impact parameters, time frame, damage function, and discount rate) and not just by the uncertainties that arise from a lack of scientific knowledge.
Though science-based reasoning discourages the use of a simple multiplier to account for non-CO2 effects, such a multiplier is desirable from a policy and climate protection point of view. We elaborate on a number of scientific and value-related issues and conclude that a multiplier of 2 or greater should be used for air travel emissions calculators to account for non-CO2 warming effects.