Understanding Contrails
They look like simple clouds, but contrails play a complex and significant role in our planet's climate. This report explores what they are, how they impact global warming, and what we can do about it.
What Are Contrails?
"Contrail" is short for "condensation trail." This section breaks down how they form and why some last for hours while others vanish in minutes. Understanding this difference is key to understanding their climate impact.
How They Form
Contrails are human-induced clouds. They form when the hot, humid exhaust from a jet engine mixes with the surrounding cold, low-pressure air at high altitudes. This causes the water vapor to condense and freeze into tiny ice crystals, creating the visible lines we see in the sky.
Persistent vs. Non-Persistent
The atmosphere's humidity determines a contrail's lifespan. This is the most critical factor in their environmental effect.
Persistent Contrails
These form in air that is "ice-supersaturated," meaning it has excess moisture. The contrail's ice crystals don't sublimate (evaporate); instead, they draw in more moisture from the air, growing and spreading out into large, thin clouds that can last for hours. These are the contrails responsible for the vast majority of the climate impact.
Non-Persistent Contrails
These form in dry air. The ice crystals quickly sublimate (turn from solid to gas) and disappear, often within minutes. They look like short white lines trailing the plane that fade away. These contrails have a negligible effect on climate.
The Climate Impact
While CO2 from aviation has a long-term warming effect, persistent contrails have a significant, short-term one. They act like a blanket, trapping heat (infrared radiation) that would otherwise escape into space. This section visualizes their estimated contribution to aviation's overall climate impact.
Aviation's Climate Forcing Components (Illustrative)
Click on the bars to see more info. "Radiative Forcing" is a measure of the change in energy balance in the atmosphere.
Key Takeaways
- Contrails are a major factor: As this illustrative chart shows, the warming effect from contrails (Contrail Cirrus) is believed to be a dominant component of aviation's climate impact, potentially larger than the cumulative effect of all CO2 ever emitted by planes.
- It's a "net warming" effect: While contrails also reflect some sunlight back to space (a cooling effect), their heat-trapping effect is much stronger.
- A short-term problem: Unlike CO2, which lasts for centuries, contrail effects are short-lived. If we stop creating them, the warming effect disappears in hours. This also means it's a solvable problem.
What Can Be Done?
Because contrail formation depends on specific atmospheric conditions, it's a solvable problem. Small changes could lead to big results. Explore the primary solutions being researched today by clicking on each topic below.
This is the most promising near-term solution. Since persistent contrails only form in small, specific regions of "ice-supersaturated" air, planes could make minor altitude or route adjustments (often just a few thousand feet) to avoid these zones. Studies suggest that rerouting a small percentage of flights could eliminate a large majority of the contrail climate impact, often with only a minimal increase in fuel burn.
The formation of ice crystals is seeded by soot (particulate matter) in the engine exhaust. Sustainable Aviation Fuels (SAFs) burn much cleaner and produce significantly less soot. Widespread adoption of SAFs could reduce the number of ice crystals that form, making contrails thinner and less impactful. Likewise, more efficient modern engines also produce less soot.
To make flight path adjustments viable, we need highly accurate weather models that can predict the exact location, altitude, and duration of ice-supersaturated regions. Continued research, supported by satellite observations and in-flight measurements, is crucial to building reliable prediction tools that airlines can integrate into their flight planning systems.
About ContrailsWatch.org
ContrailsWatch.org is an informational project dedicated to raising public awareness about the significant and solvable climate impact of aviation contrails. We believe that by clearly communicating the science, we can help build support for the research, policy, and technological solutions needed to create a cleaner, more sustainable future for aviation. This site is for educational purposes, built from publicly available research and data.