While airports are currently jammed by a revived post-pandemic Fernweh and most of the Northern hemisphere is vexed by heatwaves, international aviation still flies outside of the domain of the Paris Agreement. Although aviation’s carbon footprint may seem small enough to justify the exclusion of these emissions, the climate impacts of aviation appear much larger once we consider the non-CO2 effects. In fact, non-CO2 emissions and their indirect effects e.g., the formation of contrail cirrus, account for around two-thirds of the total impact of aviation on the climate. 

In a new study published in Nature Climate Change, we show that neglecting the non-CO₂ impacts of aviation and striving, at best, for net-zero CO₂ emissions could compromise the alignment of the aviation sector with the Paris climate goal. Lacking to address the non-CO₂ effects of aviation, as in current mitigation efforts of the international aviation organization, means locking about 0.1-0.4°C of warming due to aviation sector alone. Instead, we assess pathways for a Paris-aligned, climate-neutral aviation sector. 

Yet, the definition of climate neutrality is not as unequivocable for aviation’s non-CO₂ effects as it is for a well-mixed, long-lived greenhouse gas such as CO₂. In fact, the only way to halt the warming caused by CO₂ emissions is to stop emissions altogether i.e., reach net-zero CO₂ emissions. This is not the case, however, for the non-CO₂ effects caused by aviation, which are to a large extent short-lived: ceasing emissions would revert the short-lived climate impacts, resulting in a cooling relative to the period preceding the cessation. Thus, defining a baseline for climate neutrality is in first place necessary.

Our modelling shows that – across a number of scenarios of aviation’s future demand and technology – only those climate neutrality baselines that prescribed a reduction or elimination of the non-CO₂ effects are compatible with the Paris climate targets. Simply stabilizing their climate impacts is not sufficient, in most scenarios, to fairly contribute to the 1.5°C.

Yet, efforts to achieve those climate neutrality targets that are aligned with the Paris Agreement are enormous. Getting rid of CO₂ emissions is an imperative across all scenarios and can be achieved either via a switch to zero-carbon fuels and zero-emissions flying modes, such as electric airplanes, or using carbon removal to counterbalance emissions. Yet, to meet climate neutrality the non-CO₂ effects also need to sufficiently decrease. We found that sufficiently reduce non-CO₂ effects via a shift to alternative fuels alone is impossible without measures to reduce demand for aviation. If aviation demand continues growing, and battery technology fails to improve to the level required to enable electric transatlantic flights, substantial amounts of carbon removal will always be needed to ensure the climate neutrality of the sector. If the sector is left unmitigated – with kerosene fuels used and increased demand – the amount of carbon removal needed to counterbalance would be enormous – in the oder of one or two Germany’s surfaces covered in forest – and possibly unfeasible, seen the large amount of carbon removal needed by other sectors, such as agriculture, to meet net-zero pledges.