Discussion

Considering trend traffic growth and allocation of energy resources, even with optimistic technological breakthrough, aviation share of emissions will grow as the wider economy decarbonizes in a 2°C temperature increase scenario. This is in line with scenarios from the IPCC’s 6th Assessment Report [98] (Figure 3.19), in which transport emissions take the longest to reach net zero even in scenarios below 1.5°C with little overshoot.

Regarding the implications for the energy system, fulfilling aviation’s needs will require significant volumes of low-carbon electricity and biomass. Limited production amounts will require dedicated policy on which sectors to prioritize the access to these resources. These findings are also in line with recent studies focused on the aviation sector [2, 17, 42]. Dedicated production facilities integrated near airports might be a way to reducing associated supply chain losses and costs [18].

As the efficiencies of energy conversion processes can vary strongly depending on the primary energy source, their comparison must be made on a per unit of service basis. While some studies on several transport modes [41] consider the base service unit as 1 MJ of thrust (or to wheels), this work instead, considers it to be a traffic measure (passenger-kilometer). This distinction is fundamental for aviation due to the different weights of the propulsion architectures to power these vehicles.

Air transportation is highly unequal sector, both across (see the regional demand calibration) and within [99] countries. Given that most of the traffic growth in the next decades will happen in emerging economies, which have not yet reached demand saturation. Many of these, however, are still steering public policy to increase access to air travel, e.g., [100]. The public acceptance and effectiveness of demand-side measures can be questioned, and using pricing mechanisms to address this problem also raises questions of social justice [101].

Also, climate change is but one within the Planetary Boundaries [102, 103]. Some recent assessments that expand the scope for other Planetary Boundaries [86] stress on the need for acknowledging other limits to avoid shifting the problem, such as biodiversity loss and eutrophication of freshwater ecosystems, especially when considering a strong uptake of biofuels, which are present in most of the industry decarbonization roadmaps [2].

Finally, given a set of technology forecasts, optimization can be useful for deciding which technologies to prioritize and when, especially when they compete for similar resources. Yet, in many IAM applications that use optimization, the formulation of the optimization problem is often left unchanged or is little discussed, even if the choice of policy goal (objective) and non-negotiables (constraints) are a fundamental part of the process.