2.4. Aviation specific carbon budgets
The modelling here was constructed to investigate the scale of demand reduction that may be needed to enable the UK aviation sector to stay within a fair share of global carbon budgets, taking into consideration realistic supply-side decarbonization options.
Defining carbon budgets for individual sectors is fraught with trade-offs and misleading simplifications. Global carbon budgets from the beginning of 2020 until global net zero CO₂ emissions are reached, as defined by the IPCC (2021), are reasonably robust for a given probability of staying below 1.5°C or 2°C of global warming. However, defining a carbon budget for a country, and indeed a sector within a country, is more complex and requires assumptions and simplifications. These simplifications stem from the relative speed and scale of decarbonization that may, or may not, take place within each sector and country. Questions arise as to which countries should reduce their emissions the most, and which sectors are the most difficult technically and economically to decarbonize. For instance, while proven low-cost electricity sector decarbonization options exist (solar, wind etc.), given the high historical emissions of the US and low access to electricity in India, should India be allowed a greater per capita carbon budget? And, as electricity decarbonization is cheaper and more viable based on today’s technology than the agricultural sector, should the electricity sector be assigned a smaller carbon budget than agriculture?
The answers to these questions are complex and are traditionally dealt with by cost-optimization, whole-economy models, which investigate the allocation of carbon budgets either within a singular nation state’s economy, or on a global basis. These models make assumptions and forecasts as to the relative rate of decarbonization between sectors, based on minimizing costs. Here, and within the JZS, the aviation sector is modelled in isolation, meaning to define a carbon budget requires an assumed input that is defined outside of the model.
This paper considers two different carbon budget scenarios for the UK aviation sector. In the first scenario, a proportion of the global carbon budget, defined by the IPCC (2021) as 400 GtCO₂ between 2020 and when global net zero CO₂ emissions are reached, is allocated to the UK’s aviation sector. This carbon budget gives the world a 67 per cent chance of limiting temperature increases to 1.5°C, the goal of the Paris Agreement. One adjustment and two assumptions are made to translate this 400 GtCO₂ global carbon budget down to the UK aviation sector. Firstly, adjusting for global emissions since 2020, and then assuming that the UK is allocated 1 per cent of this budget, on the basis of current emissions, and that the aviation sector is allocated 24.5 per cent of the UK’s carbon budget. The latter is based on the time-averaged yearly emissions from the aviation sector as a proportion of economy-wide emissions to 2050 within the all-sector modelling of the Committee on Climate Change (CCC) CB6. This proportional allocation results in the UK aviation sector being allocated 767 MtCO₂ between 2022 and 2050.
The second carbon budget scenario is based on a per capita allocation of the 400 GtCO₂ IPCC carbon budget, and allocation to the aviation sector based on the 24.5 per cent time-averaged yearly emissions from the aviation sector as a proportion of economy-wide emissions within the all-sector modelling of CB6. This results in a carbon budget of 644 MtCO₂ for the UK aviation sector. It should be noted that even this lower carbon budget is arguably an overallocation of the carbon budget to the UK aviation sector, as Article 4.1 of the Paris Agreement specifies that developed countries will need to have a proportionally higher reduction in emissions. This is due to developing countries’ emissions peaking later in time than developed countries, and on ‘the basis of equity, and in the context of sustainable development and efforts to eradicate poverty’.
By defining a carbon budget for the UK’s aviation sector, the paper is not proposing to put in place sector emission caps, rather the model utilizes a sectoral carbon budget as an analytical tool to investigate the challenges of decarbonizing the aviation sector within the time frame of the UK net zero target: 2050.
2.4.1 The implications of carbon budgets for demand reduction
Under the larger UK aviation sector carbon budget of 767 MtCO₂ (scenario A, Figure 6A), demand (PAX-km) by 2050 could increase by 70 per cent, with a small carbon budget surplus of 58 MtCO₂ remaining, equivalent to 1.5 years of 2019 emissions. Under the more equitable 644 MtCO₂ carbon budget, allocated on a per capita basis (scenario B, Figure 6B), demand (PAX-km) could increase by 48 per cent. Hence, under the more equitable carbon budget based on a per capita basis, demand would need to be constrained by an additional 22 percentage points. These emission abatement pathways can be seen in Figure 6A and Figure 6B. It should be noted that both these scenarios broadly follow the input assumptions of the JZS high-ambition scenario, where the displacement of jet fuel by low-carbon alternatives and availability of negative emissions is optimistically high, and hence these scenarios embody a high degree of reliance risk (see Chapter 3).
The results of re-modelling the JZS high-ambition scenario do not mean that demand (PAX-km) can increase by 48–70 per cent, between 2019 and 2050. The abatement assumptions prescribed by DfT in relation to fuel efficiency improvements, SAFs and negative emissions are all optimistic, and as such embody risks associated with under delivery. Chapter 3 unpacks these supply decarbonization assumptions, lowering the associated reliance risks, and exploring the required demand management to stay within the aviation sector specific carbon budget of 644 MtCO₂.