A conversation with atmospheric scientist Prof. Andreas Petzold
It has long been known that the CO2 emissions from aircraft have an impact on the global climate. However, the impact of air traffic goes beyond harmful CO2 emissions. In recent years, the focus has shifted to the condensation trails of commercial aircraft and the ice clouds - contrail cirrus - that they can produce under certain conditions.
International commissions and several research projects are currently looking into how these long-lasting contrails, which are particularly harmful to the climate, can be avoided or at least reduced. Through the European research infrastructure IAGOS (In-service Aircraft for a Global Observing System), Forschungszentrum Jülich is playing a major role in these studies, which have recently been published in a report by the International Air Transport Association (IATA).
Prof. Andreas Petzold is head of the "Global Observation" group at the Jülich Institute of Climate and Energy Systems - Troposphere (ICE-3), coordinator of IAGOS Germany, and a member of the IAGOS executive board. IAGOS has been running for almost 30 years with the long-term support of the German Federal Ministry of Education and Research (BMBF), monitoring changes in the chemical composition of the atmosphere. As part of its research work, conventional commercial aircraft are equipped with compact measuring instruments. During regular flights, they measure the concentration of important trace gases such as ozone, water vapour, greenhouse gases, and nitrogen oxides. Ten aircraft from eight airlines are currently fitted with IAGOS equipment.
What exactly did the scientists look at in the research projects and what did they find?
You'll be familiar with condensation trails - linear ice clouds that form behind commercial aircraft and usually dissolve quickly. On some days, however, the condensation trails remain in the sky and after a while the sky is covered by a thin, milky-looking layer of clouds. These are the anthropogenic ice clouds that have a major impact on the climate and need to be avoided.
We have investigated the conditions under which these special clouds form and the mechanisms by which they affect the climate. They form at an altitude of 10-12 kilometres, i.e. where commercial aircraft fly and the air is so cold and humid that the water vapour is supersaturated compared to the ice phase. We are all familiar with this effect in the case of liquid water, when small droplets of mist form in the autumn as a result of the cooling of humid and warm exhaled air. A similar process occurs in the cooling exhaust gases from aircraft engines, except that it is so cold up at that altitude that ice crystals form, which then grow in the cold and humid air and can exist for several hours.
The thin, milky-looking ice clouds only weaken the incoming sunlight to a small extent and the cooling effect of a reduction in solar radiation on the ground is rather small. However, these thin ice clouds are very effective at blocking thermal radiation from the Earth, which would otherwise radiate into space, in a similar way to CO2 as a greenhouse gas - otherwise known as the greenhouse effect. This means that the warming effect of these clouds is greater than the blocking of solar radiation in the other direction. In other words, there is an overall warming effect for the climate. These processes are largely understood and can now be well represented in atmospheric models.
Can anything be done about it? Could the harmful condensation trails be avoided?
Yes, in theory at least. The formation of such long-lasting condensation trails and the resulting contrail cirrus clouds, which are very harmful for the climate, could be avoided if commercial aircraft were routed around these cold, humid air masses or flew above or below them. That sounds simple, but it is difficult to implement.
Why? What exactly are the difficulties?
The main difficulty is that it is not yet possible to predict these cold and humid air masses with sufficient accuracy. Every flight plan is based on a weather forecast made several hours in advance. The route is then calculated to ensure that the aircraft reaches its destination as quickly and as directly as possible. The planning process thus aims to optimize flight time and fuel consumption for the prevailing weather conditions. In order to avoid areas where condensation trails are likely to form, a further criterion must be added to this optimization algorithm.
In particular, a decision must be made as to whether the fastest route should be flown, thus saving CO2 emissions, or whether a detour should be taken to avoid condensation trails, which would, however, result in higher CO2 emissions. You therefore have to consider which option has the greater impact on the climate - that of the condensation trail, which lasts for several hours and can have a direct impact on the temperature on the ground during this time, or that of the additional CO2 emissions, which remain in the atmosphere for at least 100 years and can have an impact on the climate over this long period of time? This is an unresolved question and the development of suitable metrics and criteria for this decision-making process is currently the focus of research.
So it is a very complex problem. Who is involved in finding a solution?
The implementation of such contrail avoidance strategies requires the involvement of a large number of stakeholders from different areas, such as atmospheric research, weather services, airlines, air traffic control and airspace management, aircraft manufacturers, and aviation authorities - including the European Union Aviation Safety Agency (EASA), the World Meteorological Organization (WMO), and the International Air Transport Association (IATA).
Ultimately, weather forecasting must be further developed on the basis of scientific findings in order to provide the airlines with the necessary information for flight planning. Precise knowledge of the location of a contrail area can then be taken into account when deciding whether to fly around the area or save on CO2 emissions. Air traffic control must also be in a position to divert aircraft en route at short notice and safely. And the authorities must ensure that the operational systems are safe and well coordinated. This overall package could reduce the negative impact of aviation on the climate, but it remains complicated.
How did Forschungszentrum Jülich contribute to the study?
Forschungszentrum Jülich plays a central role in improving weather forecasts through IAGOS. The IAGOS aircraft, with their instruments for measuring water vapour - which are managed from Jülich - are currently the only such aircraft that can determine the humidity of the atmosphere accurately enough in routine operation. In the European research project CICONIA, which is coordinated by Airbus and has been running since July 2023, the corresponding measurement data from Jülich are used to assess the quality of weather forecasts. To this end, each flight is compared with the predicted humidity along the flight path. The results are then used by the German and French weather services to improve their forecast models.
What happens next? What are the next steps for the researchers?
This autumn, a number of workshops and conferences on this topic will take place to coordinate the various international activities. The International Civil Aviation Organization (ICAO), a United Nations agency, and the World Meteorological Organization will play an important role here. They want to set up a network comprised of many commercial aircraft that continuously report the data from their humidity measuring devices to the weather services. The weather services, in turn, can use these data to improve the starting conditions of their models. Ultimately, this should lead to improved weather forecasts for flight planning.
With their scientific expertise concerning water vapour in the atmosphere and decades of experience in operating humidity sensors on commercial aircraft, the Jülich researchers can help to put international activities in the right scientific context and ensure that the key technical aspects of setting up such a measurement infrastructure are adequately taken into account.
The various activities planned this autumn should result in a roadmap for an international strategy to reduce the impact of aviation on the climate. The avoidance of condensation trails is one of the levers for doing so, along with the use of fuels from renewable raw materials or synthetic fuels. Intensive research is also being carried out in this area.
What do the results mean for society and the future of air transport?
All the stakeholders involved are aware that aviation will remain part of a global transport system. Aviation will also be the industry that is dependent on fossil fuels for the longest period of time for reasons of safety and the technology available. The aim here is to find a good balance that will enable aviation to continue while also reducing its impact on the climate.
