Several major airlines have committed to achieving net zero carbon emissions by mid-century to combat climate change. It’s an ambitious goal that will require a huge increase in sustainable jet fuels, but that alone won’t be enough, our latest research shows.
The idea of jet planes running solely on fuel made from used restaurant cooking oil or corn stalks might sound futuristic, but it’s not that far off.
Airlines are already experimenting with sustainable aviation fuels. This includes biofuels made from agricultural waste, trees, corn and used cooking oil. Other fuels are synthetic, made by combining carbon captured from the air, and green hydrogen made from renewable energy. Often they can go straight into existing aircraft fuel tanks that normally contain fossil jet fuel.
United Airlines, which uses a blend of used oil or residual fat and fossil fuels on some flights from Los Angeles and Amsterdam, announced in February 2023 that it has partnered with biofuel companies to power 50,000 flights a year between Chicago and Denver using sustainable ethanol-based aviation fuels by 2028.
In a new study, we examine different options for aviation to achieve net zero emissions and assess how air travel could continue without contributing to climate change.
In short: each path has important trade-offs and obstacles. Replacing fossil aviation fuel with sustainable aviation fuels will be crucial, but the industry will still need to invest in capturing and directly storing carbon in the air to offset emissions that cannot be cut.
Scenarios for the future
Before the pandemic, in 2019, aviation accounted for around 3.1% of total global CO₂ emissions from burning fossil fuels, and the number of miles traveled by passengers each year was increasing. If aviation emissions were a country, that would make it the sixth largest emitter, right behind Japan.
In addition to releasing carbon emissions, burning jet fuel produces soot and water vapor, known as contrails, which contribute to warming, and this is not avoided by switching to sustainable jet fuels.
Aviation is also one of the hardest sectors of the economy to decarbonize. Small electric and hydrogen-powered planes are being developed, but long-haul flights with many passengers are probably decades away.
We developed and analyzed nine scenarios covering a range of projected passenger and cargo demand, energy intensity and aviation carbon intensity to explore how the industry can achieve net zero emissions by 2050.
We found that up to 19.8 exajoules of sustainable aviation fuels could be needed for the entire industry to reach net zero CO₂ emissions. With further efficiency improvements, this could be reduced to just 3 exajoules. To put this in context, 3 exajoules is nearly equivalent to all biofuels produced in 2019 and far surpasses the 0.005 exajoules of biobased jet fuel produced in 2019. An exajoule is a measure of energy.
Flying less and improving the energy efficiency of planes, such as using more efficient “glide” landings that allow airlines to approach the airport with engines barely idling, can help reduce the amount of fuel needed. But even in our most optimistic scenarios – where demand grows 1% per year, compared to the historical average of 4% per year, and energy efficiency improves 4% per year instead of 1% – aviation would still need about of 3 exajoules of sustainable aviation fuel energy.
Why Offsets Are Still Necessary
A rapid expansion into sustainable aviation fuels with biofuels is easier said than done. This could require up to 300 million hectares of dedicated cropland to turn into fuel – about 19% of global farmland today.
Another challenge is the cost. The global average price of fossil jet fuel is around US$3 per gallon (US$0.80 per liter), while the cost to produce biofuels for jet fuel is typically twice as high. The cheapest, HEFA, which uses fats, oils and greases, costs from $2.95 to $8.67 per gallon ($0.78 to $2.29 per liter), but depends on availability of used oil.
Fischer-Tropsch biofuels, produced by a chemical reaction that converts carbon monoxide and hydrogen into liquid hydrocarbons, range from $3.79 to $8.71 per gallon ($1 to $2.30 per liter) . And synthetic fuels cost from $4.92 to $17.79 per gallon ($1.30 to $4.70 per liter).
Realistically, achieving net zero emissions will likely also depend on removing carbon dioxide.
In a future with air usage similar to today’s, up to 3.4 gigatons of carbon dioxide would have to be captured from the air and stored – pumped underground, for example – for aviation to reach net zero. This can cost trillions of dollars.
For these offsets to be effective, carbon removal would also have to follow robust eligibility criteria and be effectively permanent. This is not happening today in airline offset programs, where airlines are buying cheap non-permanent offsets such as those involving forest management and conservation projects.
Some caveats apply to our findings, which may further increase the need for tradeoffs.
Our assessment assumes that sustainable aviation fuels are net zero carbon emissions. However, the feedstocks for these fuels currently have lifecycle emissions, including fertilizers, agriculture and transport. The American Society for Testing Materials also currently has a maximum blend limit: Up to 50% sustainable fuels can be blended with conventional jet fuel in the US, although airlines are testing 100% blends in Europe.
How to overcome the final obstacles
To meet the climate targets the world has set, emissions across all sectors must decline – including aviation.
While reductions in demand will help reduce reliance on sustainable jet fuels, more and more people are likely to fly in the future as more people become wealthier. Efficiency improvements will help decrease the amount of energy needed to power aviation, but not eliminate it.
Increasing production of sustainable jet fuel can lower your costs. Quotas, such as those introduced in the European Union’s “Fit for 55” plan, subsidies and tax credits, such as those under the US Inflation Reduction Act signed in 2022, and a carbon tax or other carbon price, can help achieve this.
Furthermore, given the role that capturing carbon from the atmosphere will play in achieving net zero emissions, a more robust accounting system is needed internationally to ensure that offsets offset the non-CO₂ impacts of aviation. If these hurdles are overcome, the aviation sector could reach net zero emissions by 2050.
This article was originally published in The conversation by Candelaria Bergero and Steve Davis at the University of California. Read the original article here.