SA E NV FE I RTO YN M E N TA L The hidden enemy? CO2 gets the bad press, but its contrails that make up most of the aviation industrys contribution to climate change By Raimund Zopp, co-founder, Flightkeys F it mainly warms the northern hemisphere, where most flights take place. Reducing this warming effect will immediately benefit those areas where weather extremes have been devastating over the past years. As pilots, well be playing a key role in this effort to reduce contrail cirrus, but well need proper support to apply the strategy efficiently and without too much extra cost. At Flightkeys, were dedicated to providing this decision support in our flight planning and EFB tools. irst the bad news: most of aviations climate impact isnt caused by CO2. In addition, the already accumulated amount of CO2 wont stop impacting our climate, even if we reduce the output of new CO2 to zero. CO2 is very sticky and stays in the atmosphere for centuries unless we manage to actively remove it. Measures to reduce aviations CO2 emissions are The facts absolutely necessary, but will take decades to be fully Lets look at facts and physics. Ice clouds, including effective. However, we need to reduce our climate impact natural and contrail-induced cirrus clouds, are urgently, and focusing only on CO2 might be a mistake. Now, the good news. Almost 50% of aviations climate better at trapping outgoing longwave (warming) impact contrail-induced cirrus clouds acts on a radiation emitted by the Earth than reflecting very short time horizon and contrails warming stops incoming shortwave (light) radiation from the immediately we get rid of them. One flights sun. This means, in general, that cirrus 2% Water contrail can result in a warming effect clouds trap more heat in the Earths vapour equivalent to CO2 from hundreds of atmosphere than they reflect. flights but as only a very small These clouds continue to absorb 20% NOx percentage of flights actually create outgoing radiation at night, even 23% CO2 these sort of contrails, avoidance when there is no solar radiation measures can be very effective at to reflect. extremely low extra cost. This has The contrail impact on already been proven in realistic incoming and outgoing large-scale simulations and even radiation is defined as in a small number of trial flights. radiative forcing. Contrail 55% Contrail cirrus But theres more. Contrail radiative forcing depends on cirrus, because of its rather short the time of day, season, cloud lifetime, acts locally and because cover, and albedo (reflectivity) of of weather and traffic flow patterns, the surface of the Earth. Collectively, Above: Aviation climate impact 20-year time horizon (Lee et al, 2021) 52 THE LOG Spring 25 pp52-55 BALPA Contrails : Environmental study group.indd 52 17/03/2025 14:28