We can understand the differing greenhouse gas (GHG) emission benefits of using sustainable aviation fuels (SAFs) by comparing their life-cycle emissions with the emissions of fossil jet fuel. While many sources indicate that SAFs reduce GHG emissions by up to 80%, today this applies only to SAFs produced using waste fat and oil feedstocks that are in limited supply. In contrast, other SAFs that use crops as feedstock may not reduce life-cycle GHG emissions at all. Let’s explore why this is and why transparency will be increasingly important as SAF production scales up.

Emissions accounting

Life-cycle emissions are the full set of GHG emissions associated with producing, processing, and delivering a fuel to the point of consumption and its final combustion. These are measured in terms of carbon dioxide equivalent. In the case of fossil jet fuel, the life cycle includes the emissions from extracting crude oil from the ground, refining it into jet fuel, and transporting the crude oil and finished fuel. When these emissions are divided by a fuel’s energy content, the resulting value is its carbon intensity.

For biofuels, the life-cycle emissions likewise include emissions from feedstock production (in this case, growing the biomass that will eventually become the fuel), conversion, and transport (Figure 1). However, there are three important distinctions when calculating the carbon intensity of biofuels. First, when wastes or byproducts are the feedstock, the emissions from producing the primary feedstock are not counted. For example, in the case of SAF derived from used cooking oil, emissions from producing the virgin (unused) vegetable oil are outside the life-cycle assessment system boundary. Second, CO₂ released during biofuel combustion is treated as zero because this carbon was taken up from the atmosphere during photosynthesis (when the plant was growing) rather than extracted from underground. My colleague dives deeper into the complexities around this “netting out” assumption in another blog post, but as it doesn’t change the big picture I’m highlighting, I won’t discuss it further here. Finally, for crop-based fuels, indirect emissions from increased demand for agricultural land are also included.

Source: Adapted from the International Civil Aviation Organization (ICAO)’s Carbon Offsetting and Reduction Scheme for International Aviation.

Figure 2 compares the GHG savings of current SAF pathways, according to a life-cycle analysis conducted by ICAO. The vast majority of today’s SAF is produced using hydroprocessed esters and fatty acids (HEFA) technology that converts fats and oils into fuel, and only a small fraction is produced using the alcohol-to-jet (ATJ) technology that processes ethanol into jet fuel. Other pathways are under development, such as e-fuels produced from renewable electricity and second-generation SAF produced from cellulosic biomass. Collectively, we refer to these as “advanced” SAF pathways.

Figure 2. Emission reductions from current SAF pathways

Source: Default life-cycle emissions values from ICAO’s Carbon Offsetting and Reduction Scheme for International Aviation. E-kerosene emissions estimated by ICCT assuming 100% use of renewable electricity.

As we can see, the frequently cited 80% emissions reduction applies to waste-derived SAFs such as those made from used cooking oil, tallow, and distillers corn oil (a byproduct of ethanol production). That these are currently the majority of the SAF produced likely explains why this figure is so commonly referenced.

The limits to waste

The reality is there are limits to the amounts of used cooking oil, tallow, and distillers corn oil—generated by frying, meat processing, and ethanol production, respectively—that are going to be available to make SAF. Aviation climate policy won’t prompt people to fry additional food or process additional cattle. Instead, for SAF production to grow in the coming years, the industry will need to use feedstocks other than waste fats and oils.

Converting vegetable oil feedstocks to SAF via the HEFA process is currently the lowest-cost alternative. The cost of converting corn grain and sugarcane ethanol via the ATJ process is also competitive relative to advanced fuel pathways. For this reason, growth in SAF production could increase demand for purpose-grown crops. Unfortunately, as shown in Figure 2, the life-cycle GHG emissions from fuels like corn grain ATJ and palm oil HEFA may be the same as or even higher than the fossil baseline. Crop-based biofuels tend to have higher life-cycle emissions because of their feedstock cultivation emissions.

For crop-based fuels, there is also the risk of emissions from land-use change. When land is cleared and repurposed for agriculture, carbon stored in the soil and vegetation can be released. These effects are often indirect. For example, if U.S. soybean oil that’s currently exported is re-directed to domestic SAF production, a likely consequence is the expansion of soybean or oil palm cultivation elsewhere. To quantify this impact, economic models are used to estimate how increased demand for biofuel feedstocks affects global land use patterns. The resulting indirect land-use change (ILUC) emissions are a major source of uncertainty in life-cycle assessments. Different models produce different results, but some suggest that when ILUC emissions are taken into account, the GHG impact of crop-based fuels are worse than conventional jet fuel. In recognition of this risk, these fuels are excluded from SAF mandates in the European Union and United Kingdom. Conversely, if ILUC emissions are not considered, there is a risk that SAF policies morph into agricultural subsidies untethered from real-world emission impacts.

Toward genuine decarbonization

Fuel producers can reduce the life-cycle emissions of SAF by swapping out fossil-based electricity or heating sources with renewable ones, utilizing carbon capture and storage, and implementing on-farm management practices such as more efficient fertilizer use. Other methods to reduce the life-cycle carbon intensity of SAF are less certain. For example, estimating the GHG reduction benefits of soil carbon accumulation due to the use of cover crops or no-tillage farming has been the subject of substantial debate in the environmental community due to wide ranges in uncertainty and the possibility for carbon reversals (i.e., when the carbon removed is released back into the atmosphere) if on-farm management practices are suspended. Crediting other on-farm management practices such as organic soil amendments within a SAF life-cycle assessment may even reward business-as-usual practices that provide no additional climate benefit.

Despite these uncertainties, industry groups have urged governments to accelerate the adoption of soil carbon crediting and updated ILUC assessments that estimate a much lower GHG impact than current ICAO consensus in fuels policies. Ensuring that SAFs reduce emissions in the future when volumes are expected to be much larger thus requires transparency from SAF producers about which feedstocks and cultivation practices are being used and rigorous life-cycle accounting methods.

As we have seen, SAFs are hardly a monolith. There is wide variation in the GHG emissions and resource availability of different SAF pathways. To extrapolate from the small volumes of waste-based SAF currently in the market to project future emissions reductions is an oversimplification. Instead, to achieve emission reductions in the future, the SAF industry and policymakers should prioritize scaling up fuel pathways with measurable and traceable (more certain) GHG emission reductions.