3.3.1 The impact of SAF assumptions on demand reduction
This section summarizes the results of including a constrained SAF supply assumption within the modelling (scenario E). Previously, under the dynamic modelling of fuel efficiency improvements (scenario D), which factored in the risk of future aircraft not meeting the ‘optimistic’ assumptions of the JZS (see Figure 8 – option 3) and applied the per capita carbon budget (644 MtCO₂), demand (PAX-km) growth in 2050 would need to be constrained to 27 per cent above 2019 levels (scenario D, Figure 6D). However, this assumed SAF scale-up followed option 1 for supply, namely 5.1 Mt/yr supply in 2050. With option 2 (supplying 10 per cent of jet fuel supply), where reliance risks are reduced due to supply limitations as a consequence of feedstock competition between sectors, and there are land tension risks with food production, demand (PAX-km) would need to decrease by 1 per cent to achieve a balanced carbon budget (scenario E, Figure 6E). Under this scenario, SAFs supply is 1.2 Mt/yr in 2050.
3.4 Negative emissions
Along with agriculture, aviation is one of the most difficult sectors to decarbonize. Such sectors are likely to represent the largest residual emissions as 2050 approaches. Under net zero, it is generally accepted that these sectors will require the greatest share of negative emissions to balance their residual emissions. As was highlighted in section 1.1, there may well be an overshoot of the 1.5°C temperature limit before negative emissions technologies (NETs) are deployed. However, by this time tipping points may have already been triggered, vastly accelerating climate change and generating catastrophic impacts. This indicates the primary risk of relying on supply-side CO₂ removal technologies – i.e. that by the time they are ready, it may be too late to have a sufficient impact.
An increasing tension exists between public support for negative emissions arising from GGRs in a general sense, and the level of reliance on these technologies within prominent, policy-informing, decarbonization models and associated pathways. To explore this, it is first important to differentiate between GGR options. Nature-based solutions (NBS) include storing more carbon by increasing the amount of wood in construction, restoring peatlands and wetlands, and afforestation. Meanwhile, engineered NETs mainly centre around BECCS and DAC. While the public are generally in favour of NBS, they are less supportive of the engineered approach, and policy-informing models (such as the IAMs and CB6 of the CCC) tend to foresee greater technical potential from these engineered approaches.
A common concern among assembly members regarding BECCS and DAC was that they are ‘treated as [a] magic solution’ that ‘takes the focus off the amount that we are emitting in the first place’.
For instance, 75 per cent of Climate Assembly members ‘strongly agreed’ or ‘agreed’ that they would like to see the aviation industry invest in GGRs, as a means of achieving net zero. And while ‘forests and better forest management’ was supported by 99 per cent of assembly members, BECCS and DAC did not feature in the top four favoured GGR methods. Indeed, only 42 per cent of assembly members ‘strongly agreed’ or ‘agreed’ that BECCS and DAC should be part of how the UK gets to net zero, respectively. Furthermore, 36 per cent of assembly members ‘strongly disagreed’ or ‘disagreed’ with the inclusion of BECCS, with 39 per cent for DAC. This is concerning, given that the balanced net zero pathway forecasts slightly more than 53 per cent of negative emissions coming from BECCS, and 5 per cent from DAC. Although BECCS has the least public support, prominent decarbonization pathways rely on this technology.
A common concern among assembly members regarding BECCS and DAC was that they are ‘treated as [a] magic solution’ that ‘takes the focus off the amount that we are emitting in the first place’. This concern has been echoed by a prominent analysis that highlights that ‘promises of GGR might instead deter or delay emissions reduction’ and that the possible extent of ‘mitigation deterrence’ could result in an additional temperature rise of up to 1.4°C, in pathways expected to limit increases to 1.5°C. In 2018, a report from the European Academies’ Science Advisory Council, which advises the EU and is comprised of the national science academies of the 27 member states, highlighted that relying on NETs, including BECCS, rather than pursuing greater emissions reductions, could catastrophically fail, resulting in ‘serious implications for future generations’.
In 2022, the sustainability of BECCS was questioned by the UK government. In a meeting with MPs, Kwasi Kwarteng, the then secretary of state for business, energy and industrial strategy, admitted that biomass was not developing at the pace of other renewables, and said, ‘I can well see a point where we just draw the line and say [biomass] isn’t working, this doesn’t help carbon emission reduction and so we should end it’, adding, ‘All I’m saying is that we haven’t quite reached that point yet’, and that imported biomass was ‘not something that the UK should be relying on at large scale’.
As with SAFs, a big risk of BECCS is potential land tensions with food production. In the near term, BECCS in the UK is likely to be deployed with the use of wood pellet feedstocks. The leading BECCS developer (Drax) uses 97 per cent woody biomass (3 per cent agricultural residues), and the global supply of pellets comprised of other feedstocks remains marginal. However, many decarbonization pathways envisage the use of a wide range of feedstocks, inclusive of bio-crops, with land tensions minimized as sustainable agricultural practices are assumed to increase the yield of crops per hectare of land use. However, future climate risks indicate crop yields could be under threat. Regardless, land tensions are likely to be significant. In 2022, the IPCC AR6 WGIII report indicated the cropland area to supply biomass for bioenergy and BECCS would equate to 199 (from a range of 56–482) million hectares (Mha) in 2100, equivalent to 13 per cent of global cropland.
To scale BECCS in the UK, based solely on the combustion of wood pellets to meet the CCC CB6 2050 target of 51 MtCO₂/yr would require the combustion of more than four times that currently burnt at Drax. It is also interesting to note that the BECCS removal target of 51 MtCO₂/yr would require 119 per cent of the 26 Mt of wood pellets consumed across the EU27 and the UK, which in turn represents 50 per cent of global consumption.
Given that the principal objective of BECCS is to remove CO₂ from the atmosphere and permanently store it in geological formations, feedstock choice should ensure the greatest carbon efficiency and hence net negativity. Carbon efficiency can be thought of as the proportion of carbon input to the whole BECCS system that is geologically stored. To produce any feedstock for BECCS the supply chain will involve processes that result in the emission of CO₂. For instance, from land-use change, farming practices, the drying of the biomass, palletization of the dried biomass, transport and any uncaptured emissions in the final process. The greater the carbon efficiency (proportion of CO₂ geologically stored) the less feedstock required to achieve a given removal target. If less feedstock is required, less land is needed, which in turn minimizes the risk of land tensions with food production. In the UK, wheat straw may be the feedstock with the optimal carbon efficiency: 74–72 per cent of CO₂ is geologically stored, and 26–28 per cent emitted to the atmosphere. As such, for a finite land area, wheat straw-based BECCS would remove more CO₂ from the atmosphere, compared to other feedstocks, when the risks of carbon debt of woody biomass are factored in.
