Sustainable aviation fuels (SAFs) come in different varieties. In this recent blog post, my colleague explained how SAF carbon intensity can vary widely depending on the feedstocks and technological pathways used. In this piece, I’ll dive into one aspect of this: an accounting convention adopted by the Intergovernmental Panel on Climate Change (IPCC) and most regulators, including the International Civil Aviation Organization, that treats biofuel combustion emissions as zero. As we’ll see, this treatment sometimes closely matches the carbon dioxide (CO₂) impact of burning biofuels and sometimes doesn’t capture potential increases in atmospheric CO₂.

The idea behind zero combustion CO₂ emissions is that biofuels contain carbon that was recently sequestered from the atmosphere by plant matter. When this carbon is released into the atmosphere during combustion, the quantity sequestered (the carbon “sink”) and the quantity released at combustion (the carbon “source”) are assumed to net out. The schematic in Figure 1 compares the transfer of carbon in the Earth’s carbon cycle for biofuels (left) and fossil fuels (right). While biogenic CO₂ sourced from plant matter results in no net change in atmospheric CO₂, fossil CO₂ sourced from plant matter that’s been stored underground for millions of years results in a net increase in atmospheric CO₂.

Source: Adapted from IEA Bioenergy. 

Viewed from another perspective, though, even a cycle like the left panel in Figure 1 can lead to a net increase in emissions in the atmosphere. It all comes down to timing. This ICCT paper highlighted the concept of “carbon debt,” or the amount of carbon stored in biomass that must be replaced by newly generated biomass when it is burned for energy or consumed as a transport fuel. There can often be a lag, sometimes a substantial one, between the time a tree is felled and burned for bioenergy and the time it takes for regrowth to sequester that same quantity of CO₂, particularly when burning roundwood and other mature trees that have stored carbon in their tree trunks for hundreds of years.

Indeed, biofuel combustion could lead to CO₂ impacts before the completion of a harvesting cycle, as research by Kendall et al. and others have demonstrated. While short-rotation crops such as eucalyptus and poplar that are replanted every 2 to 7 years may sequester that same quantity of carbon over short timescales, that’s not always the case for longer rotation crops like pine and other coniferous trees, and it can lead to atmospheric warming long before the carbon debt is paid off. Because of this, groups such as Fern have called on the European Commission to remove roundwood from the Renewable Energy Directive. Additionally, in 2021, the Roundtable on Sustainable Biomaterials, a third-party standard for sustainability certification, revised its guidance for woody biomass to exclude whole trees other than short-rotation crops due to their near-term CO₂ impacts.

Although assumptions also underlie the accounting treatment of CO₂ emissions from combusting e-fuels (also called synthetic fuels or power-to-liquids), which are a form of synthetic fuel derived from renewable electricity and waste CO₂, such assumptions closely represent their net CO₂ impacts. When e-fuels such as e-kerosene are combusted in aircraft, they release CO₂ emissions just as fossil fuels do. However, these emissions are typically assigned to the original point-source emitter rather than the e-fuels producer within standard life-cycle accounting guidance. For example, the EU Delegated Act on recycled carbon fuels (RCFs) and renewable fuels of non-biological origin (RFNBOs) assigns CO₂ emissions to the original point-source emitter rather than e-fuels producers that utilize the carbon downstream. While the European Union and United Kingdom are the only major markets that have published guidance on e-fuels life-cycle assessments, other methodologies are currently under development.

If we trace the carbon from its original source, such as a coal-fired power plant, through to its final combustion in an aircraft we can understand how these emissions are assigned, offset, and then re-assigned throughout the supply chain. Though coal plants produce a lot of CO₂, producers can capture it before it’s released into the atmosphere and store it underground to receive a carbon credit. If the captured CO₂ is then sold to another party, the carbon credit is transferred to the receiving party (in our case, the e-fuels producer) using chain of custody documentation. When this fuel is later burned in an aircraft engine, the CO₂ emissions from its combustion are offset by the carbon credit. We illustrate this accounting attribution in Figure 2.

Importantly, when two parties complete a financial transaction, only one party can benefit from the captured CO₂ credit; this is to provide consistency and avoid double claiming in mandatory emissions reporting. Because the input CO₂ came from the atmosphere or other waste sources, e-fuels are treated as zero carbon at the point of combustion and generally associated with very low upstream emissions, provided they are also derived from renewable electricity. Thus, in this case, the accounting closely matches the realistic impacts of producing and combusting the fuel.

However, if the input CO₂ used for e-fuel production is classified as non-waste, combusting it as SAF will not be treated as zero-emission in the European Union in the future. Indeed, the EU Delegated Act on RCFs and RFNBOs determined that captured emissions from non-renewable waste streams (e.g., flue gas from steel mills) can be considered as avoided in the near term, but that captured fossil CO₂ should no longer be considered as avoided beginning in 2036 for the power sector and 2041 for all other fossil industrial sources. This phaseout is to prevent a perverse incentive for industry to increase demand for fossil CO₂ at a time when decarbonization trajectories are expected to shrink fossil CO₂ availability. The guidance states that burning this carbon as SAF rather than reducing it at the source or sequestering it underground would lead to additional CO₂ in the atmosphere and thus it should no longer qualify as zero-carbon at the point of re-combustion.

As we see, burning biomass with a high carbon debt and the use of non-waste carbon inputs suggest that, in some cases, SAF combustion cannot be neatly defined as zero emission. The concept of carbon debt must be weighted relative to other sustainability risks when assessing the environmental impact of biofuels, as illustrated in a recent set of scientific recommendations prepared for the Dutch government. Policies that restrict the use of ecologically sensitive forestry biomass such as the U.S. Renewable Fuel Standard are a critical start. Further review of the long-established accounting convention that bio- and synthetic fuels are zero-emission at the point of combustion is also warranted.