Flying cloud factories: planes produce more than carbon dioxide alone

In 2018, planes around the world, both passenger and freight, emitted 1 billion tons of CO₂, accounting for 2.5% of global anthropogenic CO₂ emissions[1]. The aviation industry is a known contributor to climate change, which is why these CO₂ emissions are regulated by the EU Emissions Trading Scheme (ETS). Unfortunately, the CO₂ emissions do not provide the whole picture regarding aviation’s climate impact. Indeed, there are also emissions of nitrogen oxides (NO_x), heat, water vapor, and particulate matter. These emissions lead to so-called non-CO₂ impacts of aviation, of which two types are particularly important regarding climate impact.

The first impact comes from the NO_x emissions, which affect two greenhouse gasses present in the atmosphere. On one hand, NO_X molecules break down atmospheric methane, thereby having a cooling effect. On the other hand, they promote the formation of ozone[2], another important greenhouse gas, leading to a warming effect larger than the cooling effect from breaking down methane. As a result, the net effect of NO_x emissions contributes to the effect of aviation on climate change.

The second non-CO₂ impact of aviation comes from the emissions of heat, water vapor, and particulate matter, which create thin clouds in the sky, known as contrails. These contrails are generally formed in certain areas of the atmosphere that are cold and humid. They trap radiation escaping from the Earth’s surface, thereby acting as a blanket[3].

The consequence of these non-CO₂ impacts of aviation is that the climate impact is much higher than would be inferred from the CO₂ emissions alone. Indeed, if the non-CO₂ impacts are included, aviation’s share in climate change increases to 5%[4]. A logical next step, then, would be to include these non-CO₂ impacts in calculation methodologies for greenhouse gas emissions to foster accurate reporting and monitoring. Such inclusion could be achieved by using Effective Radiative Forcing instead of Radiative Forcing, but there is still uncertainty about the specific metric to use[5].

On the positive side, small adjustments can considerably lower aviation’s non-CO₂ climate impact. For example, between 2% and 10% of flights are responsible for 80% of the contrails[6]. By combining satellite imagery, weather data, software models, and AI prediction tools, areas of the atmosphere where contrails are likely to form can be avoided. Doing this could decrease the formation of contrails by more than 50%[7]! Of course, rerouting comes at a cost, as it will increase fuel use. However, studies have shown that considerable formation of contrails can be avoided at very limited costs[8]. Smart tools are useful to balance the reduced warming effect of contrails and the increased CO₂ emissions from fuel use. Finally, in addition to rerouting, the use of sustainable aviation fuels (SAF), especially on flights prone to contrail formation, can significantly reduce the climate impact of the aviation industry[9].

Econopolis Strategy has written a position paper on the Flemish regional airports. If you want to discuss this topic further, feel free to contact me on this topic via email: luca.campion@econopolis.be