Decarbonization in civil aviation: transition strategies still don’t go far enough

As the effects of climate change and the associated risks are increasingly being felt, several organizations have developed roadmaps that outline potential steps towards carbon neutrality in the aviation sector. These strategies include multiple levers, such as developing alternative fuels and introducing more efficient aircraft, and are supported by the International Civil Aviation Organization (ICAO), the Air Transport Action Group (ATAG) and the European Commission, among others. They are typically evaluated based on a single criterion: greenhouse gas emissions.

Roadmaps that take a broader view

The research team, which included bioeconomy, environmental and aeronautics experts, tested the transition strategies put forward by various organizations against all the planetary boundaries¹. This was a first for this type of analysis, which sought to gain insights into issues such as the impacts of changes on the freshwater cycle or how much energy is needed to achieve a particular sustainability goal. To answer these questions, the scientists quantified the environmental impacts and associated resource needs for the roadmaps studied through 2050, while also testing various scenarios for changes in air traffic.

A scientific method based on “environmental budgets”

The scientists created an original method based on the concept of an environmental budget. It works in the same way as a carbon budget, which involves determining the maximum amount of CO₂ emissions that must not be exceeded to keep global warming within a certain limit. For this study, the scientists used the planetary boundaries framework to create environmental budgets for all human activities. For example, the environmental budget for freshwater was based on the maximum annual consumption volume that must not be exceeded in order to stay within the associated planetary boundary.

To assign a share of these global budgets to the aviation sector, scientists tested two allocation principles: an economic one (based on civil aviation’s share of global GDP) and an ethical one (based on aviation’s share of the total environmental impact of the reference countries, which meet several environmental and human development criteria).

They then used life cycle assessments (LCAs) to calculate the environmental impacts generated by the production, distribution and combustion of the fuels used in the various international roadmaps. With this method, the researchers could assess, for example, the amount of freshwater required for each roadmap and compare these values with the budgets allocated to aviation. With regard to climate change, the study tracked the share of the historical climate budget consumed by aviation – i.e., the cumulative greenhouse gas emissions to date.

The study results show that aviation remains within the allocated budget for some planetary boundaries, but none of the decarbonization plans studied respects all the boundaries simultaneously. The budgets related to climate change and biosphere integrity were consistently exceeded, regardless of the scenario. The scientists also noted that implementing the most ambitious strategies would entail significant resource costs related to electricity, biomass, water, agricultural land use and carbon sequestration capacity.

Reconsidering supply and demand policies

For the global civil aviation industry, the path to sustainability is still fraught with challenges. To address them, the study’s authors advocate for maintaining the measures already suggested by the roadmaps studied: developing alternative fuels, improving aircraft efficiency, optimizing operations (manoeuvres, tests, etc.) and implementing offset strategies. Additionally, to move more decisively towards a path of environmental sustainability (characterized by simultaneous respect of all planetary boundaries), the researchers demonstrated the need to maintain and complement these technological advances with a more comprehensive rethinking of usage patterns, especially by implementing policies to manage air traffic demand.

1.  There are nine planetary boundaries, which refer to the thresholds beyond which natural balances are destabilized and living conditions on Earth become unfavourable for humans. Seven of them have already been exceeded: climate change, biosphere integrity, biogeochemical flows (the nitrogen and phosphorus cycles), land system change (deforestation), freshwater change, novel entities (the release of new substances into the environment) and ocean acidification. Only two boundaries have not been exceeded: stratospheric ozone depletion and atmospheric aerosol loading.