Alternative aviation fuels offer limited mitigation potential

Alternatives beyond fuel switching are necessary to align the aviation sector with climate neutrality targets, according to a new study recently published in Environmental Science & Technology.

Aviation emission expected to grow

Aviation contributes to about 2,4 % of global co₂ emissions, but the sector’s contribution to global warming is considerably higher due to short-lived climate forcers such as contrail cirrus, black carbon (BC), organic carbon (OC), nitrogen oxides (NOx), and sulfur oxides (SOx). These short-lived forcers are estimated to be responsible for about two-thirds of the aviation sector’s global climatic impact. Aviation emissions are also expected to continue to grow. Hence, mitigation measures beyond efficiency improvements are key to achieve emissions reductions in the sector in line with ambitious climate targets.

“Aviation is an integral part of modern society but is currently heavily dependent on fossil fuels, thus contributing to global warming. To move towards a more sustainable aviation sector, we need mitigation measures that address both co₂ and non-co₂ effects”, said research director at CICERO, Marianne Lund, one of the authors behind the study.  

Replacing today’s fossil jet fuels with alternative fuels is one key mitigation strategy that is considered to lower aviation’s climate impact. Such fuels could be kerosene synthesized from different carbon feedstocks other than fossil and carbon-free fuels such as hydrogen or ammonia. In a new study by researchers at the Norwegian University of Science and Technology (NTNU) and CICERO Center for International Climate Research in Norway, the researchers assess the mitigation potential of alternative fuel for shorter flights by integrating a geospatial fuel and emission model with life cycle analysis. The researchers used data from 210 000 domestic flights in Norway in 2019, roughly the annual domestic aviation activity in Norway. The distance of the flights ranged from 40 to 2 200 km. 

Considerable renewable energy required

“The alternative aviation fuels considered show relevant mitigation potentials. However, they are not a silver bullet, as non-co₂ emissions causing climatic impacts remain and their production would require substantial amounts of renewable energy,“ said Jan Klenner from NTNU, the lead author of the study.

The renewable energy needed for producing the alternative fuels for the 210 000 flights (approximately the annual domestic aviation activity in Norway) would be 10 TWh, or approximately a tenth of Norway’s current annual renewable electricity production, according to the study. If fuel needed for international flights departing from Norway would be included, that would exceed 30 TWh annually.

Mitigation potential vary with flight distance and fleet composition

In an optimistic case, when liquid hydrogen (LH2) and power-to-liquid fuels are produced with renewable electricity, they may reduce emissions, and the mitigation potentials range from 44% on shorter flights to 56% on longer flights as compared to today’s fossil jet fuel. This is a substantially lower mitigation potential than some other life cycle analyses, as the results presented here include short-lived climate forcers.

“It was very interesting to see how the mitigation potential of the fuels vary with flight distance and how contrails could potentially be particularly important to keep in mind when considering hydrogen as an alternative fuel,“ said Helene Muri from NTNU, one of the authors of the study.

Neither liquid hydrogen nor power-to-liquid fuels that are considered here have reached market readiness and would require substantial further investments.

“Our study consider how climate impacts and mitigation potential of alternative fuels vary with flight distance, fleet composition and other parameters and can thus be of value to researchers and decision-makers engaged in climate change mitigation in the aviation and transport sectors,“ Klenner said.

“This work demonstrates the value and importance of interdisciplinary modeling and analysis that combines engineering and climate sciences. We need more of this type of collaboration across fields to address our complex transition challenges”, says Anders Hammer Strømman, Professor and Vice-Dean of Sustainability, Faculty of Engineering NTNU.  

Read the full study in Environmental Science & Technology.