Life Cycle Assessment in Aviation: Focus on Freshwater Use

The civil aviation sector has pledged to become net-zero carbon by 2050. Manufacturers and airlines have undertaken intense efforts to decarbonize flights and approximately 39,000 new, fuel-efficient aircraft will be delivered to replace the existing fleet by 2040. Nevertheless, the sustainable strategies of aviation should avoid mitigating climate change impacts at the expense of ecosystems, human health, and resource conservation. Moreover, preventing rebound effects is critical because civil aviation is expected to grow at a rate of 4% per year in the next decades. Life cycle assessments (LCA), combined with absolute environmental sustainability assessments (AESA), provide a reliable framework to ensure that new technologies, sustainability strategies, and policies consider these aspects. The Life Cycle Impact Assessment (LCIA) phase of an LCA study systematically quantifies the potential environmental impacts of large-scale systems such as the aviation value chain. In particular, freshwater biodiversity and resources require special attention because the populations of freshwater organisms are particularly affected by the ongoing biodiversity crisis, and a quarter of humanity already lives in water-stressed areas. Climate change and the growing human population will increase water use and the resulting pressure on freshwater ecosystems and resources. Therefore, my thesis aims at developing LCIA methods relevant to aviation with a special focus on freshwater use. It tackles the following research questions: (i) What are the main environmental impacts of aviation? (ii) Why is freshwater use critical for aviation today and will continue to be in the future? (iii) How to quantify the impacts of water use on freshwater ecosystems in LCIA? (iv) How can the LCA framework support decisions in balancing ecosystems and humanity’s water needs?

A systematic literature review of LCA studies applied to civil aviation confirmed that the main environmental issues of civil aviation are climate change, noise disturbance, and air quality at and in the vicinity of airports. LCA studies have frequently investigated the climate benefits of using alternative aviation fuels, e.g. biofuels and electro-fuels, in long-haul flights, as well as all-electric and hybrid-electric propulsion for shorter flights. Beside the climate aspects, the large-scale adoption of alternative aviation fuels is anticipated to increase the demand for agricultural land, irrigation, fertilizers, and pesticides, which may compromise food security, impair terrestrial and freshwater ecosystems, and increase water scarcity globally. In comparison with kerosene, the climate advantages of alternative propulsion systems, alternative aviation fuels, and aircraft fleet renewal depend upon the availability of renewable electricity, metals, and low-carbon hydrogen, which also requires freshwater.

Only a small fraction of the studies analyzed in the literature review focus on aircraft manufacturing. To fill this gap, a mapping approach was developed as part of this thesis to estimate key environmental indicators of the aircraft manufacturing industry. Financial and environmental data for 2017 from 88 large and medium-size companies were used to estimate the global manufacturing value chain's energy requirement, carbon footprint, water withdrawals, and waste generation. The highest water withdrawals occurred in the production of aluminum and titanium alloys used in manufacturing. Moreover, the results indicated high rates of material loss during the manufacturing stage, estimated at 20% for aluminum alloys and 90% for titanium alloys at sector scale. In another study, we quantified the potential environmental benefits of recycling titanium alloy scraps generated in the manufacture of a turbine component using primary data from the laboratory and LCA. The results support the conclusion that material choice and efficient metal use are a relevant axis for reducing the potential impacts of water use in aviation.

Therefore, the transition to low-carbon fuels and the fleet renewal rely heavily on freshwater use and should consider the potential impacts on freshwater scarcity, pollution, land use, and ecosystems to avoid burden-shifting. To do so, regionalized LCIA methods are needed. Accordingly, LCIA methods development is the focus of the second part of the thesis.

An operational LCIA method quantifying the impact of water consumption on freshwater ecosystems was developed in three steps. First, regionalized depletion factors were proposed to quantify the intertwined effect of surface and groundwater consumption on freshwater availability. They were computed builds on a state-of-the-art global hydrological model comprehensively modelling water transport. Depletion factors were calculated for 8,664 river basins and represent the freshwater availability change in stream flow, groundwater storage, evapotranspiration, and soil moisture induced by water consumption from 1960 to 2000. We argue that these can be used as fate factors in LCIA methods modelling the impacts of water consumption.

Second, a regionalized effect model was developed to quantify the consequences of streamflow reductions on the richness of freshwater species. This was derived from a novel species-discharge relationship (SDR), including simulated streamflow data, 11,450 ranges of fish species, elevation, and climate information. The best SDR was selected from five SDR candidates through a cross-validation procedure. This updated effect model increases the geographical coverage and includes more species data than previous similar models.

Third, the combination of the regional fate model, effect model, and global extinction probability resulted in globally differentiated characterization factors for LCA in 2,320 river basins covering 88% of the global landmass, which is a higher coverage than previous methods. For the first time characterization factors were divided into four sets differentiating the impacts of marginal and average water consumption and damage to regional and global freshwater ecosystems. This family of characterization factors offers greater flexibility to LCA practitioners. An illustrative case study comparing rice production in five locations demonstrated the importance of this flexibility and showed how to utilize the factors.

Finally, a holistic representation of the impacts of water use in aviation and other sectors requires going beyond water consumption alone. Research efforts have been undertaken to integrate water use impacts and water productivity into water footprint assessments. We expanded the existing proposal for a water footprint framework bringing consumptive and degradative water use impacts under the same roof. Using data from the European Commission, scientific literature, and a global hydrological model, we tested our method to Finally, a holistic representation of the impacts of water use in aviation and other sectors requires going beyond water consumption alone. Research efforts have been undertaken to integrate water use impacts and water productivity into water footprint assessments. We expanded the existing proposal for a water footprint framework bringing consumptive and degradative water use impacts under the same roof. Using data from the European Com-mission, scientific literature, and a global hydrological model, we tested our method to assess whether water use in the European Union (27 countries) was sustainable in 2010. The proposed water footprint assessment offers an advantage to existing methods because the impact assessment on ecosystems and freshwater resources is more accurate. Moreover, we brought an ecological and a social welfare perspective to the water footprint by proposing absolute limits to impacts reflecting these aspects. The aviation industry can use the water footprint assessment framework to map the holistic impacts of its supply chains on local water resources and ecosystems.

This PhD thesis contributes to implementing sustainable water use in all sectors by providing operational decision-support tools based on LCA. We developed operational LCIA models quantifying the impact of water consumption on freshwater ecosystems. We integrated this research into the water footprint assessment and developed new approaches to combine pollution and scarcity impacts. In doing so, we contribute to increasing the relevance of water foot-printing and water use LCIAs for decision-making. The productivity of land and aquatic ecosystems is critical to the successful transitioning to a fossil-fuel free society. This requires clean, abundant freshwater and healthy freshwater ecosystems.