Facts about contrails and the climate
Contrails/condensation trails are clouds created by jet engines:
Contrails – short for condensation trails (and sometimes called vapor/vapour trails) – are the long line-shaped clouds that aircraft sometimes leave in the sky.
Condensation trails are NOT the same as chemtrails. Chemtrails are a conspiracy theory, while condensation trails are science.
When a jet aircraft burns aviation fuel, it emits CO2, water vapor, soot, and other particles.
If the air is cold and humid enough, the water vapor condenses around the particles, creating ice crystals that make up the long white clouds.
Most contrails disappear again after a few minutes.
But if it is cold and humid enough, the ice crystals in the contrails can spread over time and create high-altitude ice clouds (cirrus clouds) that can linger in the atmosphere for hours.
So, contrails are artificial ice clouds created by the water and soot emitted from jet engines.
Contrails are closely connected to climate change and global warming:
The artificially created contrail clouds – or cirrus clouds – influence radiative forcing and global warming.
Radiative forcing is what scientists talk about when the incoming energy from the sun is larger than the outgoing energy that Earth sends back into space.
In the daytime, the cirrus clouds produced by condensation trails will reflect some of the sun’s incoming energy back into space. This is good.
But contrail cirrus clouds also block Earth’s heat from escaping into space, thereby keeping the planet warmer than it would otherwise have been (this is also why a cloudy winter day can sometimes be warmer than a sunny one).
Contrails are particularly bad for the climate at night when there is no heat from the sun to reflect back into space, and the only climate effect the contrail clouds have is acting like a blanket, trapping heat on Earth.
The climate impact of condensation trails is about the same as the CO2 emissions from aviation:
The climate impact of contrails varies greatly from flight to flight. It depends on local weather conditions, time of day, time of year, geographical area, the albedo effect (whether contrail clouds are created over the dark ocean, which absorbs much of the sun’s heat in the daytime, or over mountains full of snow that reflect most of the heat back into space), and more.
However, contrails have a historic warming effect that is 0.5 to 3 times greater than all the CO2 emissions from aviation. Scientists are working to bring down the big margin of uncertainty. Still, it is common to use the estimation that the climate impact of contrails is about the same as the CO2 emissions from aviation.
In 2019, aviation was responsible for a little over one gigaton of CO2 – or about 2% of human-induced global warming. So, when we include the effect of contrails and other non-CO2 effects, aviation is responsible for roughly 4% of global warming.
There are big differences in how CO2 and condensation trails warm the planet: While CO2 has a very low, immediate impact on global warming, the greenhouse gas will linger in the atmosphere for centuries, and its impact will accumulate over time. With condensation trails, it is the opposite: The immediate climate impact of contrails is extremely high, but when the contrails clouds are gone after a few hours, so is their climate impact.
On average, the contrails that are in the atmosphere now will warm the planet more over the next six hours than all the accumulated CO2 from aircraft since the birth of aviation will in the next six hours.
There is already an easy, cheap, and available solution for mitigating contrails:
Most persistent condensation trails can be relatively easily avoided by adjusting the altitude of a small number of aircraft and simply directing them a couple of thousand feet over or under the areas predicted to create contrails.
Scientists and engineers are improving their ability to predict which areas produce persistent condensation trails. They are also improving systems to verify whether specific contrail clouds were produced or avoided and what the resulting climate impact would be.
This research has already been implemented in software used for flight planning and other climate tech.
While every flight path should be analyzed, only about 5% (1 in 20) of aircraft must have their flight path adjusted to avoid about 80% of the climate impact from contrails.
Because these few flights are being redirected, they often spend a little more fuel (and CO2). However, research indicates that the fuel penalty to avoid 80% of the climate impact from contrails is 0.1% - 0.3% more fuel consumed.
Google Research used numbers from the IPCC to calculate that rerouting aircraft around contrail zones resulted in a reduction in contrail warming that is 20 times greater than the additional CO2 from fuel burned.
So mitigating contrails is easy to implement, requires no big new investments, is very cheap to run, and is great for the climate.
Why are airlines not mitigating contrails and bringing down their climate impact?
Airlines – and the world – have been focused on CO2 emissions. Everything is measured in CO2 or CO2 equivalent (CO2e). Contrails are part of the so-called “non-CO2 effects” of flying (the other big contributor is emissions from nitrogen oxide, NOx). The warming effect of contrails can be translated into CO2e.
Until recently, there was no useful solution for mitigating contrails, but now there is, and the systems are constantly improving.
Aviation is notoriously hard to decarbonize, so the industry has perhaps not been eager to admit that its climate problem is twice as big as previously stated (CO2 emissions + non-CO2 (contrails)).
Other methods for mitigating contrails include using lower aromatics fuels or sustainable aviation fuels (SAF), which release fewer soot particles for the water vapor to condense around, thereby reducing contrail formation.
Go here for further reading - and here for a list of relevant scientific literature.
