In an unprecedented study overturning long-held assumptions about contrail formation, an international team of scientists has discovered that the majority of persistent aircraft contrails develop not in the pristine skies as previously believed, but within pre-existing cirrus clouds. This research, spearheaded by institutions including Forschungszentrum Jülich, the University of Cologne, the University of Wuppertal, and Johannes Gutenberg University Mainz (JGU), leverages extensive atmospheric observations to reveal complex interactions between artificial and natural ice clouds. The study challenges conventional wisdom by showing that more than 80% of long-lived contrails emerge within natural cirrus cloud formations, raising profound questions about their elusive climatic impacts and opening new avenues for aviation climate strategy.
Contrails, the streaks of cloud trailing aircraft engines at cruising altitudes near 10 kilometers, arise from the condensation of water vapor in engine exhaust mixing with the cold atmospheric air. Conventional understanding assumed these ice clouds formed predominantly in blue, clear sky conditions where ambient humidity and temperature favor their persistence. However, through meticulous analysis of temperature and humidity data collected by commercial aircraft under the IAGOS (In-service Aircraft for a Global Observing System) program, researchers found the opposite: contrail longevity is overwhelmingly linked to pre-existing cirrus clouds, environments with subtly different thermodynamic properties than clear air. This insight fundamentally reshapes the understanding of contrail microphysics within the earth’s upper troposphere.
Cirrus clouds, composed of minuscule ice crystals and appearing as wispy veils in the sky at altitudes between 8 to 12 kilometers, are climatically significant due to their complex radiative properties. Their dual role involves trapping longwave infrared radiation emitted from the Earth’s surface and atmosphere, which tends to warm the planet, while concurrently reflecting incoming solar radiation, which can have a cooling effect. The net climate impact of cirrus clouds is therefore a delicate balance influenced by factors such as optical thickness, crystal size, and background atmospheric conditions. Contrails morphing into cirrus clouds—or forming within them—thus alter this radiative balance in ways that have remained poorly quantified until now.
One crucial finding from the research is the differential climatic effect of contrails depending on their atmospheric setting. When developing in clear or near-clear skies, contrails often act to warm the climate. Their thin, ice-crystal-laden structures allow sunlight to penetrate but trap outgoing longwave radiation, functioning similarly to a greenhouse blanket. Conversely, contrails embedded within already thick cirrus clouds can lead to a slightly cooling effect by increasing the cloud’s reflectivity and bouncing more sunlight back to space. These nuanced interactions underscore that the simple presence or absence of contrails is insufficient to fully understand their climate forcing; instead, atmospheric context and cloud microphysics must be accounted for.
The technical challenge of extracting these insights lies in the complexity of measuring atmospheric conditions at flight altitudes over vast geographical scales. The IAGOS program provided a unique observational platform equipped with sophisticated instruments continuously monitoring parameters such as temperature, water vapor content, and pressure during commercial flights over the North Atlantic corridor from 2014 to 2021. This unprecedented dataset allowed scientists to map the interplay between contrail persistence and local environmental variables systematically, leading to robust statistical conclusions rather than isolated case studies. This kind of data-driven insight is critical for improving climate models reliant on accurate representations of cloud-radiation interactions.
Adding to the observational work, the study incorporated advanced model calculations to estimate the radiative forcing—how much contrails contribute to climate warming or cooling. Professor Peter Spichtinger of JGU led this modeling effort, revealing that contrails within dense cirrus have limited climate forcing individually. The complexity arises when multiple layers of overlapping contrails and natural clouds interact, a scenario often encountered in busy flight routes. These multilayer effects present challenging variables that current models struggle to encapsulate fully, indicating a research frontier that calls for enhanced computational methods and data assimilation techniques.
Understanding the climate implications of contrails is imperative because their net radiative forcing is considered larger than the impact of direct CO₂ emissions from aviation. This finding thrusts contrail science to the forefront of climate-sensitive aviation policy and technology development. The study’s insight, that contrail formation is frequently tied to existing cirrus clouds, suggests that mitigating their climatic footprint may require route planning and flight altitude adjustments that consider not just cloud-free conditions but also the microphysical state of the upper troposphere. This approach represents a paradigm shift away from simpler heuristics toward integrated atmospheric dynamics in flight management.
Moreover, the findings hold potential significance for international climate regulation frameworks and aviation industry practices. Organizations such as the World Meteorological Organization (WMO), the International Civil Aviation Organization (ICAO), and the European Aviation Safety Agency (EASA), along with aviation stakeholders, are now examining how to incorporate these insights into sustainable aviation strategies. Flight planning that actively considers cirrus cloud distributions and their radiative characteristics could reduce contrail-related climate effects without substantially compromising safety or logistics. The continuation of IAGOS and similar observational networks will be key to validating such strategies and ensuring adaptive responses to atmospheric variability.
The study also underscores an underappreciated feedback loop between anthropogenic and natural components of the climate system. By embedding within natural cirrus clouds, contrails may modify cloud lifetimes, ice crystal habits, and cloud optical properties, potentially altering tropospheric humidity and temperature profiles. The subsequent effects on Earth’s energy budget, precipitation patterns, and even atmospheric chemistry remain areas ripe for future research. Detailed microphysical characterization studies combined with remote sensing and in situ measurements will be essential to unravel these complex responses.
From a technical perspective, the challenge extends beyond atmospheric measurement to computational representation, where cloud microphysics parameterizations often simplify natural variability. The study’s authors advocate for enhanced synergy between observations, laboratory experiments on ice crystal formation, and high-resolution climate modeling to build predictive tools capable of simulating contrail-cloud interactions with fidelity. Such advancements would empower climate policy makers and aviation planners with evidence-based metrics to evaluate trade-offs among emission reductions, flight efficiency, and contrail mitigation.
The broader implications of this research transcend aviation alone. Cirrus clouds influence global climate dynamics and hydrological cycles, playing a role in feedback mechanisms sensitive to greenhouse gas forcing. By demonstrating that human activities can modify cirrus cloud characteristics at a global scale via contrails, the study spotlights a subtle yet potent anthropogenic climate agent. This realization aligns with broader scientific agendas emphasizing the importance of understanding aerosol-cloud-climate interactions, further integrating aviation’s role within the planetary climate mosaic.
In conclusion, this landmark study redefines prevailing conceptions about the formation and climate impact of persistent contrails. The discovery that most long-lived contrails form embedded within cirrus clouds, rather than in open skies, challenges scientific and operational norms. This necessitates a re-evaluation of flight route planning, climate impact assessments, and modeling approaches to accurately capture these cloud-climate interactions. As the aviation sector seeks pathways to sustainable growth amidst mounting climate concerns, integrating this complex atmospheric science represents both a challenge and an opportunity for innovation, collaboration, and ultimately, meaningful climate action.
Subject of Research:
Not applicable
Article Title:
Most long-lived contrails form within cirrus clouds with uncertain climate impact
News Publication Date:
3-Nov-2025
Web References:
http://dx.doi.org/10.1038/s41467-025-65532-2
References:
The study is published in Nature Communications, DOI: 10.1038/s41467-025-65532-2
Image Credits:
Photo/© Andreas Petzold
Keywords:
Contrails, Cirrus Clouds, Climate Impact, Radiative Forcing, Aviation Emissions, Atmospheric Science, Cloud Microphysics, Flight Route Planning, IAGOS, Climate Mitigation
