Decarbonizing aviation through low-carbon fuels will be beyond difficult
The International Civil Aviation Organization (ICAO) appears to believe that the international aviation industry can meet its steep carbon targets under the Paris Agreement almost entirely through the use of low-carbon fuels. This seems very unlikely, if you consider everything that would need to happen to realize that vision.
Most biofuels produced today are made from corn, palm oil, and other food crops associated with high land-use change emissions. Replacing conventional jet fuel with food-based biofuels won’t meaningfully help reduce greenhouse gas (GHG) emissions, and in the case of palm oil will actually worsen climate impacts. The readiest supply of alternative jet fuels, from waste oils and fats such as tallow, is limited, and the diversion of this resource from animal feed and other uses can indirectly drive up global GHG emissions. Very low-carbon fuels need to be produced from wastes or energy crops grown on low-carbon land. There is the potential for producing drop-in power-to-liquids for aviation, which can deliver significant GHG reductions, but the pathway will likely be prohibitively expensive for the next decade or two.
Cellulosic biofuels from sustainable biomass sources such as responsibly harvested agricultural and forestry residues and municipal and industrial waste offer the best potential for scaling up truly low-carbon biofuels. The development of this industry has been extremely slow to date, and it won’t be easy to accelerate. Cellulosic biofuel facilities take several years to construct and ramp up, and many projects have failed before reaching commercial capacity. Technological improvements are needed to bring down costs, but also to improve conversion yields. An immediate turnaround would be necessary if the industry is to scale up even by 2050.
While the availability of agricultural residues, forest residues, and municipal waste is greater than that of waste fats, it’s still limited by crop and timber production. Substantially increasing the total supply of sustainable biomass would require a large-scale expansion of dedicated cellulosic energy crops such as switchgrass, Miscanthus, and short-rotation poplar. However, even if all the world’s grassland was converted to energy cropping, only around 15% of world energy requirements in 2050 could be replaced with biomass. All of this would be needed to displace total jet fuel demand in 2050.
Far more bioenergy produced today is used in heat and power production than biofuels. Much of this is used for traditional cookstoves that may be replaced with more efficient (and perhaps solar) models, particularly in developing countries. But even in the U.S. and EU, more biomass is used by industrial electricity and heat generation plants than for transport, and this is likely to continue. To free up this resource for aviation fuel, heat and power generation globally would need to be supplied entirely by solar, wind, and other zero-carbon sources. If the heat and power sectors are still partially dependent on fossil fuels in 2050, diverting biomass from these uses would indirectly increase the consumption of coal, gas, and oil to replace them.
In addition to the supply issues outlined above, widespread use of biofuels in aviation is hindered by economic factors. Producing “drop-in” fuels suitable for aviation requires further processing and increases production costs compared to the ethanol and biodiesel commonly used in the road sector. There is little if any additional value to producers in adapting those fuels for use in aviation, since kerosene commands a lower price than road fuels. There’s also the question of willingness to pay for cleaner fuels, which will be lower for airlines if you buy the argument that they are more fuel price sensitive than drivers. As long as there is liquid fuel demand for cars and trucks, it is likely that low carbon fuel will preferentially be consumed in the road sector.
Lastly, fuel conversion processes limit the amount of jet fuel that can be produced; typically, a mixture of jet fuel, gasoline, diesel, and light ends comes out the end of an advanced biorefinery. Refiners can increase the jet fuel fraction but can’t completely eliminate the other products, leaving substantial amounts of gasoline and diesel to be used in the road or marine sectors.
The figure below summarizes the major changes that would have to occur to replace all petroleum demand in aviation with low carbon fuel by 2050. All these massive developments would have to happen to replace all jet fuel demand with low carbon fuel in 2050.The first four steps (commercialize cellulosic biofuel through phasing out biomass in heat and power) are dependent on electrification of the road sector to make low-carbon fuel available for aviation. The electrification of all cars and trucks in the medium term would require an acceleration compared to current trends — something few analysts are willing to predict.
While it is possible for aircraft to run entirely on low-carbon fuel by 2050, that possibility will not be realized solely through aviation sector policies aimed at decarbonization. Many of the changes needed require action by automakers, electricity generators, fuel producers, farmers, and policymakers focusing on all energy sectors and land use. A holistic approach to global climate mitigation across sectors is needed, and we’re not moving fast enough now to make it happen.