February 08, 2023

“. . . and the skies are not cloudy all day”: Tackling Contrails

Posted on February 8, 2023 by Susan M. Cooke


On October 7, 2022, member states of the International Civil Aviation Organization adopted a long term aspirational goal (LTAG) of net-zero CO2 emissions from aviation by 2050, marking a turn away from an emission offset approach. The agreement directs the ICAO Council to develop guidance and monitoring measures for achieving LTAG, but doesn’t set any specific targets or intermediate goals for member states and invites them to regulate aviation emissions instead of relying on the ICAO to develop minimum global standards.

One issue left unaddressed is the non-CO2 impact of aviation. Studies indicate that persistent contrails constitute the most important factor driving the non-CO2 climate impact. Such contrails are created when aircraft fly at cruising altitude high in the troposphere through ice-supersaturated areas where ice crystals form around soot in the aircraft exhaust. They then merge with cirrus clouds in the area and primarily reflect heat back to earth, as can be seen here. This effect constitutes more than half of aviation’s total climate warming impact

In a November 2020 report, the European Union’s Aviation Safety Agency (EASA) identified several options for abating aviation’s non-CO2 climate impact, including aircraft avoidance of ice-supersaturated areas. According to a study published in 2022 of aviation’s contrail climate effects over the North Atlantic airspace between the U.S. and Europe, around 12% of the flights cause 80% of their annual “energy forcing” effect. This is good news, since optimizing the trajectories of a relatively limited number of flights could have an appreciable and positive effect on the formation of so-called persistent contrail-cirrus clouds.

Recent advances in the computer mapping of contrails indicate that timely and accurate flight plans can be generated to avoid tropospheric regions of supersaturated ice, and several collaborative efforts are now underway to address the warming impact of contrails. They include the Etihad Greenliner Programme developed in partnership with Boeing and GE, a Delta/MIT project employing computer models and flight tests, and the Contrail Impact Task Force with participants from the aircraft industry, academia, the tech sector, and the nonprofit community. Efforts are also underway to quantify the value of contrail avoidance as tradable carbon credits for use in emissions trading programs like that being implemented for the aviation sector by the European Union.       

Nevertheless, it is important to remember that flight path optimization is a complex process, as such adjustments may involve the expenditure of more fuel and result in greater emissions of CO2 and other pollutants. Furthermore, a contrail’s climate impact is dependent on a number of factors, including the season and time of day, the humidity, the fuel constituents, and the aircraft’s size and fuel consumption. Another consideration is what value can be agreed upon and assigned to the amount of warming caused by a contrail, and what regulatory body will govern that decision. Given the number of governmental entities that could play a role in regulating contrails, particularly for international flights, resolution of such issues may take considerable time and effort.

In light of these complexities, the growth in air traffic, and the importance attached to addressing climate change, a data driven and flight specific approach may well be necessary in order to optimize flight paths and ensure the safety of all aircraft in flight, especially given the time constraints involved. So, make way for a collaborative (and complicated) machine learning regulatory approach aimed at ensuring, in the words of your favorite “Home on the Range” cowboy crooner, that “the skies are not cloudy all day”.