To meet ambitious climate targets, the aviation sector needs to neutralize CO 2 emissions and reduce non-CO 2 climatic effects. Despite being responsible for approximately two-thirds of aviation's impacts on the climate, most of aviation non-CO 2 species are currently excluded from climate mitigation efforts. Here we identify three plausible definitions of climate-neutral aviation that include non-CO 2 forcing and assess their implications considering future demand uncertainty, technological innovation and CO 2 removal. We demonstrate that simply neutralizing aviation's CO 2 emissions, if nothing is done to reduce non-CO 2 forcing, causes up to 0.4 °C additional warming, thus compromising the 1.5 °C target. We further show that substantial rates of CO 2 removal are needed to achieve climate-neutral aviation in scenarios with little mitigation, yet cleaner-flying technologies can drastically reduce them. Our work provides policymakers with consistent definitions of climate-neutral aviation and highlights the beneficial side effects of moving to aircraft types and fuels with lower indirect climate effects.

The Representative Concentration Pathway 2.6 (RCP2.6), which is broadly compatible with the Paris Agreement’s temperature goal by 1.5–2°C, contains substantial reductions in agricultural non-CO2 emissions besides the deployment of Carbon Dioxide Removal (CDR). Failing to mitigate agricultural methane and nitrous oxide emissions could contribute to an overshoot of the RCP2.6 warming by about 0.4°C. We explore using additional CDR to offset alternative agricultural non-CO2 emission pathways in which emissions either remain constant or rise. We assess the effects on the climate of calculating CDR rates to offset agricultural emission under two different approaches: relying on the 100-year global warming potential conversion metric (GWP100) and maintaining effective radiative forcing levels at exactly those of RCP2.6. Using a reduced-complexity climate model, we find that the conversion metric leads to a systematic underestimation of needed CDR, reaching only around 50% of the temperature mitigation needed to remain on the RCP2.6 track. This is mostly because the metric underestimates, in the near term, forcing from short-lived climate pollutants such as methane. We test whether alternative conversion metrics, the GWP20 and GWP*, are more suitable for offsetting purposes, and found that they both lead to an overestimation of the CDR requirements. Under alternative agricultural emissions pathways, holding to RCP2.6 total radiative forcing requires up to twice the amount of CDR that is already included in the RCP2.6. We examine the costs of this additional CDR, and the effects of internalizing these in several agricultural commodities. Assuming an average CDR cost by $150/tCO2, we find increases in prices of up to 41% for beef, 14% for rice, and 40% for milk in the United States relative to current retail prices. These figures are significantly higher (for beef and rice) under a global scenario, potentially threatening food security and welfare. Although the policy delivers a mechanism to finance the early deployment of CDR, using CDR to offset remaining high emissions may well hit other non-financial constraints and can thus only support, and not substitute, emission reductions.

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<p class="HeaderandFooter"><span lang="EN-US">To meet ambitious climate targets, the aviation sector needs to neutralize CO<sub>2</sub> emissions and reduce non-CO<sub>2</sub> climatic effects. Despite being responsible for approximately two-thirds of aviation’s impacts on the climate, most aviation non-CO<sub>2</sub> species are currently excluded from climate mitigation efforts. Here we identify three plausible definitions of climate-neutral aviation that include non-CO<sub>2</sub> forcing and assess their implications considering future demand uncertainty, technological innovation, and CO<sub>2</sub> removal. We use empirical relationships to translate aviation emissions to climate forcing and a reduced-complexity climate model to assess the impacts of these climate neutrality frameworks, including the needed CO<sub>2</sub> removal, on global temperature in the context of the different demand and technology scenarios.</span> We demonstrate that simply neutralizing aviation’s CO<sub>2</sub> emissions, if nothing is done to reduce non-CO<sub>2</sub> forcing, causes up to 0.4 °C additional warming, thus compromising the 1.5 °C target. We further show that substantial rates of CO<sub>2</sub> removal are needed to achieve climate-neutral aviation in scenarios with little mitigation, yet cleaner-flying technologies can drastically reduce them. Our work provides policymakers with consistent definitions of climate-neutral aviation and highlights the beneficial side effects of moving to aircraft types and fuels with lower indirect climate effects.</p>

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