The non-CO2 Effects of Aviation: Mapping, valuation, and policy Instruments for Swedish domestic and international air travel

Research Project, 2026 – 2029

The climate impact of aviation extends beyond its carbon dioxide emissions. The most significant warming effect from aviation is caused by persistent contrails, which form due to aircraft emissions of particles and water vapor under specific conditions related to temperature and humidity. The uncertainty is large, and the formation and warming effect of contrails is dependent on local weather conditions. It has been estimated that approximately 2% of global flights account for 80% of the warming effect from contrails. These non-CO₂ effects are therefore crucial both (i) in economic analyses of aviation’s climate impact and (ii) in the design of cost-effective measures to reduce this impact. The Swedish Transport Administration’s valuation of carbon dioxide emissions is based on the abatement cost required to achieve Sweden’s climate targets. To account for the so-called high-altitude effects, a standardized factor is currently used: 1.3 for domestic flights and 1.9 for international flights. These factors are based on global averages and simplified assumptions rather than specific Swedish conditions. The project will: 1. Calculate high-altitude factors for domestic and international flights, providing both annual averages and seasonal values. 2. Map high-altitude factors for alternative flight altitudes in Swedish airspace, presenting both annual averages and seasonal distributions. 3. Analyse how high-altitude factors depend on fuel type by comparing different blend ratios of fossil jet fuels and sustainable aviation fuels (SAF). 4. Estimate the social cost of contrail cirrus. 5. Analyse policy instruments to handle the problem of contrails based on Swedish conditions. The project focuses on contrails, as they have by far the largest climate impact among aviation’s non-CO₂ effects. The Swedish Transport Administration’s estimate of the high-altitude factor is currently based on Global Warming Potential with a time horizon of 100 years. We will include such an estimate but will also explore alternative time horizons and metrics, including the socioeconomic damage cost of CO₂ and contrails. These values are crucial for developing and evaluating potential policy instruments to reduce the high-altitude effect without risking an increase in aviation’s CO₂ emissions.