Aviation Contrails: What We Know — and What We Don’t — about This Warming Phenomenon

RMI’s Contrail Impact Task Force is at the forefront of advancing solutions for the warming effect of these aviation-induced clouds. Our new comprehensive overview report shines a light on the science and potential solutions to aviation’s biggest non-CO₂ climate impact.

Understanding the full climate impact of aviation

The aviation industry, responsible for about 2 percent of global CO₂ emissions, has committed to reaching net zero by 2050, but eliminating the carbon emissions from jet fuel may not be enough to stop the climate impacts of air travel and transport. The full warming impact of aviation includes both CO₂ as well as non-CO₂ effects, with contrail-induced warming likely being the most significant driver of the non-CO₂ effects of aviation.

Condensation trails — or contrails — created by aircraft flying through cold and humid air can significantly warm the climate. Most contrails dissolve within a few minutes, but in certain conditions, they can persist in the atmosphere, spread out, and become artificial cirrus clouds that trap outgoing heat. This represents an additional climate impact alongside aviation’s direct emissions.

Exhibit 1: Contrail Formation and Warming Effects

Limiting uncertainty of contrails’ warming effect on the climate

Contrails have a significant climate impact, potentially ranging from half to over three times that of CO₂ emissions from aviation. However, quantifying the exact warming effect of contrails on the climate is difficult. There are numerous influencing factors such as dynamic atmospheric conditions (changes in humidity, temperature, wind); time of day, seasonal, and regional variations; cloud interactions (contrail cirrus clouds blending with natural cirrus clouds); and technological and observational limitations (it is difficult to assess the climate effect of a contrail over its entire life span using current ground- and space-based systems).

Uncertainty about the exact warming effect of contrails may always be present but we can increase our understanding of their climate impacts by investing in research that explores the myriad of influencing factors. Reducing the uncertainty about the warming effect of contrails on climate is critically important because the main solution: using additional jet fuel to fly around cold and humid areas to avoid contrail warming, emits more CO₂. Before making a contrail avoidance decision, the climate impact of the additional fuel burn and the potential contrail warming must be compared to ensure that the avoidance leads to a net climate benefit.

The challenges of predicting where persistent contrails will appear

In recent years, technologies and methods have been developed to predict and avoid some of the warming effects created by contrails. Contrail prediction models have begun to be integrated into flight planning tools, allowing airlines to identify and avoid warming contrail regions similar to the way they plan to avoid turbulence. Although contrail avoidance is a relatively simple solution to a very complex problem, it comes with its own set of challenges that need to be addressed before a full scale-up of this approach is viable.

Enhancing the reliability of contrail prediction models is critical to increasing confidence in their ability to address the climate impact of contrails. Contrail prediction models rely on a variety of input data, including flight trajectories, aircraft and engine parameters, fuel characteristics, and weather data. However, the availability and accuracy of some of these data inputs is a challenge. Efforts to improve data quality and validate prediction models through observation will help ensure the effectiveness of contrail prediction modeling.

For example, a primary challenge for model accuracy is the lack of sufficient humidity data at cruising altitude. Several efforts are currently underway to develop contrail-specific humidity sensors to retrofit aircraft, providing additional and more accurate data for contrail prediction modeling.

In addition to more accurate input data, observation techniques can also be used to validate and improve contrail prediction models. Validation can be done by ground observation using cameras pointed at the sky, in-flight observations by flight crews reporting persistent contrails formed by other aircraft, and satellite observations. To improve validation, more ground-based cameras are needed, use of in-flight observation and reporting procedures can be expanded, and the resolution and coverage of available satellite imagery must be increased.

Exhibit 2: Validation Techniques and Input Data for Contrail Prediction Modeling

Contrail avoidance flight trials and collaboration

Contrail management, including contrail avoidance by rerouting flights, presents the opportunity for stakeholders across the aviation sector to engage in radical collaboration to reduce non-CO₂ climate impacts alongside ongoing aviation decarbonization efforts. A growing number of contrail avoidance trials have been conducted in recent years through collaboration among airlines, scientific institutions, air traffic management, and tech companies. Some of the world’s biggest airlines are involved in real-world trials and research projects designed to improve the understanding of the climate impact of contrails and best operationalize potential contrail management solutions. Flight planning software providers have started integrating contrail avoidance processes into their software suites, enabling greater automation and reducing complexity.

To keep contrail avoidance moving toward operational viability, continued collaboration among stakeholders is essential in research, tech development, flight trials, and integration into airline systems and procedures.

A comprehensive report on contrail management

Addressing the climate impact of contrails presents challenges that can only be overcome through continued radical collaboration among stakeholders from across the aviation industry. Since its inception in November 2022, the RMI-convened Contrail Impact Task Force (CITF) has worked to share and expand upon the latest science on the climate impact of contrails, explore potentially actionable strategies to safely avoid warming contrails while minimizing additional CO₂ emissions, and identify the operational and financial challenges of implementing potential solutions. In a culmination of engagements between airlines, industry groups, scientific institutions, NGOs, tech companies, and government entities over the last 18 months, the CITF has assembled a comprehensive overview report on contrail management.

Serving as a thorough introduction covering the topics briefly discussed above and more, the report examines the state of the science around the mitigation of warming contrails, including an overview of the current approaches and efforts to better understand and address their significant climate impact. To advance these approaches, additional simulations and large-scale trials are needed to gain more knowledge and experience in the practical application of contrail avoidance.

With the aim of providing a comprehensive resource for aviation stakeholders – including researchers, commercial air carrier operators, international aviation industry organizations, civil society, and policymakers – it is our hope that these sector actors further support research efforts and work toward potential non-CO₂ industry standards and policies for the global aviation system. Through a collaborative approach, aviation has the opportunity to advance contrail solutions in the urgent need to address the full climate impact of aviation.