What fraction of the CO2 radiative forcing can be attributed to aviation?

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Estimating how much of the CO₂ radiative forcing can be attributed to the aviation sector may sound easy. Indeed CO₂ emissions from aviation are well known; the rise in CO₂ atmospheric concentration is well observed and the radiative impacts of CO₂ are well understood and quantified. However, there are also a number of complicating factors: the CO₂ radiative forcing depends logarithmically with the change in atmospheric concentration and the efficacy of natural sinks of CO₂ is changing over time. .All these effects need to be accounted for if a proper attribution is to be made.

A popular method, used by Lee et al. (2021) and others, is the residual attribution method, whereby the radiative forcing for a particular sector (the aviation sector in this case) is calculated as the difference between the total CO₂ radiative forcing and the CO₂ radiative forcing should that particular sector had not existed. However this method suffers from a major drawback that was overlooked by previous authors. Since the CO₂ radiative forcing is not linear in concentration, the total radiative forcing from all sectors considered together is not the same as the sum of the radiative forcings from each sector considered individually. Furthermore aviation is different from many other sectors in that it has occurred relatively late in the industrial period. It is thus essential to differentiate the impact of early and late emissions because they do not contribute equally to the current atmospheric concentration and radiative forcing. Aviation started only a few decades ago, its emissions can therefore contribute relatively more to the change in CO₂ concentrations, but relatively less to the CO₂ radiative forcing because of the logarithmic dependence.

Different methods exist to address those issues. In this study we used the proportional, differential, and time-sliced attribution methods. The last two methods require to compute the CO₂ concentration at time t due to emissions from aviation and all anthropogenic activities up to a time t’ before time t. We have used the OSCAR compact Earth System model and historical CO₂ emissions data to estimate the different values. This allows us to account for how the CO₂ concentration decreases as natural sinks sequestrate the emitted CO₂ over time.

We found that the more rigorous methods (the proportional, differential, time-sliced methods) lead to aviation CO₂ radiative forcing 20%, 13%, and 12% larger than the marginal method which underestimates the true CO₂ radiative forcing by aviation. However, this is compensated by the lower contribution to the increase in CO₂ atmospheric concentration that we estimated using our well calibrated model. We estimate that aviation contributed 2.18 ppm to the rise in CO₂ atmospheric concentration in 2018, which is less than the values of 2.9, 2.4 and 2.4 ppm found in a previous study relying on less sophisticated models. Our study thus provides a clear basis and methodology for future assessments of the aviation impact on the carbon cycle and CO₂ radiative forcing.