Could forests make flying sustainable?

Forests residues can make flying more sustainable. The use of renewable jet fuels promotes sizeable climate mitigation benefits, but sometimes at cost of unexpected implications to other Sustainable Development Goals. Yet, most of these adverse side effects are attenuated when improved conversion technologies and cleaner inputs are used for renewable jet fuel production.

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Nov 19, 2018

Read the full paper, "Contribution of jet fuel from forest residues to multiple Sustainable Development Goals" in Nature Sustainability at  

My girlfriend lives on the other side of the Atlantic. She works for an airline company and, as a perk of the job, is able to travel quite often. Every time she comes to visit, she expends about 600 liters of jet fuel. This figure represents her share of the total jet fuel consumption in a large, 500-odd seat plane. And this is just her share. Many other people fly, and often. A holiday in Greece, the Maldives, or Australia, or even just a return flight to attend a two-hour meeting in a faraway city, being back home for dinner. Global aviation currently contributes to about 2% of human-made CO2 emissions, but has historically been one of the most rapidly growing economic sectors, and is expected to take-off further.  

Compared to the road transport sector, options to reduce climate impacts in the aviation sector are intrinsically more limited due to stricter fuel quality standards and expectations about electric airplane capacity. As my girlfriend and I want to continue our currently long-distance relationship, I have been thinking a lot about the possible solutions to decarbonize the aviation sector.

When I came onboard with my colleague Francesco Cherubini at the Centre for Environment-friendly Energy Research (Bio4Fuels) at the Norwegian University of Science and Technology, we decided to focus to aviation fuels. In particular, we wanted to quantify the climate impacts of renewable jet fuels from forest residues compared to conventional jet fuels, using a Norwegian context. If you have ever been to Norway, you will know why forest resources was an obvious choice for us. We only included currently available forest residues, to minimize the additional pressure on terrestrial ecosystems and land competition, while stimulating a circular economy perspective. 

My girlfriend arriving in Norway in winter. A lot of forest a few kilometers away from the city.

When jet fuel is burned in the airplane turbines, various emissions are generated high in the sky, not at ground level like with cars, trucks, and ships. In the case of flying, there are also significant contributions to climate from so-called near-term climate forcers (NTCFs) such as aerosols, ozone precursors, and the formation of condensation trails (those line-shaped clouds formed from airplane exhaust emissions). Our analysis evidently needed to go beyond climate impacts because our society is facing many other interconnected environmental challenges related to our consumption patterns. After several rounds of coffee, Francesco and I decided to propose an ambitious framework to unravel the contributions of renewable jet fuels to the recent list of Sustainable Development Goals (SDG). We chose to ground our analysis on the well-documented life cycle assessment models and adapt them to SDG analysis, as it appeared a promising bridge to this research gap. Ironically, we drew the outline of our paper on the back of a boarding pass whilst delayed at Oslo airport, waiting to return home from a two-hour meeting.

We looked at complementary aspects (global warming potential and global temperature change potential) with different temporal (e.g. 20 years versus 100 years) and geographical (e.g. Europe and global) dimensions of how jet fuel interferes with the climate. Our results show that the longer the time frame for climate analysis, the larger the mitigation benefit of using renewable jet fuels. This is because the contributions of the NTCFs are much higher than we initially expected, especially the significant shorter-term impacts from condensation trails. Thus, a longer-term climate perspective - more aligned with the global temperature stabilization objectives stated in the Paris agreement – favors renewable jet fuels.

Going beyond climate mitigation benefits, we also mapped all the associated direct and indirect emissions to air, soil, and water bodies from aviation fuels, from forest residues extraction all the way to the airplane turbine emissions. Renewable jet fuels caused unexpectedly large impacts in several other Sustainable Development Goals. These adverse side effects are mainly related to zero hunger (SDG 2), good health and well-being (SDG 3), clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11), responsible consumption and production (SDG12) and life below water (SDG 14). However, with improved renewable jet fuel production technologies and cleaner inputs in the value chain (renewable electricity, for example), most of these adverse environmental impacts can be substantially alleviated.

Thinking to the future, I am confident I will be able to continue seeing my girlfriend with lower climate impacts than today and, with the potential of biofuel technology advances, without major impacts on other relevant SDG. Forests may offer us more than timber and ecosystems services, but also a more sustainable way of flying.

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Otavio Cavalett

Researcher, Norwegian University of Science and Technology (NTNU)

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