There is more than just carbon – 5 facts about the non-CO2 emissions of flying
When we think about the climate impacts of flying, we often think of the warming effect of carbon dioxide (CO2) emissions. However, carbon dioxide alone does not fully describe the impact of flying on the climate; we should also pay attention to other emissions. We'll go through five facts that everyone should know about contrails and other so-called non-CO2 emissions.
What should everyone know about non-CO2 emissions, Tuomo Karppinen, Head of Environment at Finnair?
1. What climate-warming emissions are produced by flying?
Flying releases carbon dioxide into the atmosphere, which warms the climate. The effects and principles of carbon dioxide are well known, and as an airline, we have clear ways to reduce our carbon dioxide emissions.
However, there is a less known aspect to aviation's climate impacts: the so-called non-CO2 emissions. These emissions can have a significant warming effect on the climate, and understanding them is crucial as we, as an industry, strive to promote more environmentally friendly aviation.
Non-CO2 emissions include nitrogen and sulfur oxides (NOx, SOx), soot, and water vapour. Of these, water vapour is particularly significant in its effects and is related to aircraft contrails. Contrails, the white streaks you see behind aircraft, are ice clouds. They form when the water vapour in aircraft exhaust condenses in cold conditions. Depending on the weather conditions in the upper atmosphere, these contrails can persist and spread, forming fibrous high clouds called cirrus clouds.
2. What do we know about the climate impacts of contrails?
Contrails form only when the atmospheric conditions are suitable for their formation. These weather conditions are like pancakes floating in the sky – they are wide and broad, but relatively shallow. When an aircraft flies through such a “pancake”, the water vapour from the engines freezes and forms a cloud composed of ice crystals of various shapes. The exact shape and persistence of ice crystals in the trails can vary, which affects their reflective properties and thus their climate impact.
The time of day affects the climate impacts of contrails. During the day, contrails can cool the atmosphere by reflecting sunlight back into space. At night, they work in reverse by reflecting the Earth's thermal radiation back to the ground, which promotes warming. At night, the cooling effect is absent because there is no sunlight to reflect. Therefore, nighttime contrails primarily promote warming by trapping heat that would otherwise escape into space. This makes nighttime contrails particularly concerning for the climate.
The warming effect of contrails varies according to the time of day and atmospheric conditions, and their impact time is short-term. Usually, the effect lasts for a few hours. Carbon dioxide, on the other hand, remains in the atmosphere for hundreds of years. This difference in impact duration makes it challenging to compare these two different greenhouse gases, but it is clear that both carbon dioxide and contrails significantly contribute to aviation's climate footprint.
3. What methods can airlines use to try to reduce the climate impacts of contrails?
To reduce the climate impacts of contrails, we must first be able to verify their effect. Additionally, we need to be able to predict when contrails are likely to form. Verification requires satellite observations and advanced modelling techniques, while prediction requires temporal integration of weather conditions, their forecasting, and flight routes. Current models still have uncertainties, which makes it difficult to provide accurate estimates of the impacts and their causes.
It is generally thought that the polluter is responsible for the environmental impact they cause. Therefore, it's important to be able to verify how much each airline has caused contrails through its operations, for example. For this, we must first be able to confirm that both the results of contrail analyses and measured observations are reliable. This means that forecast models are compared with actual observations, and the models are refined based on these comparisons. This work involves the use and analysis of satellite data, information collected by aircraft, fuel data, and ground-based observations.
Despite the uncertainties, there are several promising possibilities for mitigating the effects of contrails. Reducing soot particles in aircraft emissions can help reduce contrail formation. This can be achieved by developing engines and using cleaner synthetic fuels.
Airlines can modify their flight routes to avoid areas, those “pancakes”, where contrails are likely to form. With advanced weather forecasts and machine learning, it's possible to detect and avoid contrail-forming areas in advance and even in real-time. This approach requires the development of current tools available to airlines and extensive cooperation between different data providers.
4. How are non-CO2 emissions reported?
Companies' carbon dioxide emissions are reported as so-called carbon dioxide equivalents, which include other greenhouse gases in addition to just carbon dioxide. For example, Finnair's science-based climate target covers carbon dioxide emissions and other Kyoto Protocol greenhouse gas emissions, which include methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride.
So far, the so-called non-CO2 emissions, such as contrails, have not been included because there is still too much scientific uncertainty associated with them. To change this, airlines in the EU have had to collect and report related data since the beginning of this year. The aim is to better calculate and predict the climate impacts of non-CO2 emissions in the future based on the reported information.
In Europe, there has also been an initiative under development for some time concerning a unified European airspace, which aims to reform and unify European air traffic management to improve safety, efficiency, and environmental protection levels. This Single European Sky program could also play a significant role in reducing contrails by optimising air traffic management and flight routes across Europe. By reducing the fragmentation of European airspace and implementing more efficient flight routes, the program could help minimise conditions that lead to contrail formation.
5. What is Finnair doing to promote the investigation of this issue?
At Finnair, we have begun studying contrail formation. Initially, we want to understand the warming effect our flights' contrails have caused over the past couple of years. After that, we can analyse whether we can possibly influence their formation with the tools currently in use or if we need automated planning and flight management software for support. This understanding also gives us the opportunity to help current software providers develop their services.
There are clearly impacts, and as an industry, we need to better understand how they work and bring tools to the market that can, for example, reduce the climate impact of contrails. By combining technological advancements, political measures, and operational practices, we can move towards more sustainable aviation. It's a journey that requires joint actions from airlines, decision-makers, researchers, and passengers.