Each year, air traffic controllers around the country handle 16.1 million flights that come and go through US airspace, leaving behind traces of thin, white clouds. New research from the German Aerospace Center gives us a reason to take a closer look at the trails that planes leave behind.
The white tendrils of vapor left behind when a plane crosses the sky are called condensation trails, or contrails, for short. They usually dissipate quickly, but in the right atmospheric conditions, they can spread out into a “trail,” forming high-altitude cirrus clouds. In a paper released Thursday in Atmospheric Chemistry and Physics, researchers predict that these contrails could significantly contribute to the Earth’s ongoing climate crisis.
As more planes take to the skies, contrail-induced warming could triple by 2050 compared to levels measured in 2006, write Lisa Bock, Ph.D., and Ulrike Burkhardt, Ph.D., researchers at the German Aerospace Center.
This tripling of contrail-induced warming will open up a whole new world of non-CO2 based climate change factors that humanity will have to grapple with, Bock says.
Contrails Could Impact Climate Change
Condensation trails are formed when humid air from jet exhaust meets the low temperatures and vapor pressures of certain cold, wet areas of the sky, forming ice crystals.
“The atmosphere must be cold and humid enough for contrail cirrus to form,” Bock tells Inverse.
The accumulation of contrail cirrus clouds causes two things to happen: They can reflect sunlight away from Earth and cause cooling, but they can also stop radiation that’s already emitting from the earth from escaping. Overall, the warming aspect of contrail cirrus wins out, which contributes to positive radiative forcing.
In similar fashion to the greenhouse effects of carbon dioxide, Bock explains that a greater volume of contrails could upset the balance between the heat that the Earth gives off into space, and the heat that remains trapped in our atmosphere.
“Regarding the overall climate impact of contrail cirrus and in particular their impact on surface temperatures, there are still some uncertainties,” she says. “Contrail cirrus themselves and their effects on the surface are ongoing topics of research. But it’s clear they warm the atmosphere.”
Right now, cirrus clouds’ contribution to climate change is far less than that caused by carbon dioxide from other industries and activities. In the paper, Bock and Burkhardt write that air traffic was only contributed 5 percent of the human-caused radiative forcing in 2005.
But taking into account the fact that air traffic is predicted to quadruple by 2050, and the fact that planes are beginning to fly higher (which favors cirrus formation in the tropics), Bock and Burkhardt’s work shows that contrail warming may start to play a bigger role in the ongoing climate crisis.
Contrails and “Chemtrails”
Bock’s work gives us good reason to think carefully about how air traffic will grow in the coming decades. But there’s already a community that has its eyes trained on contrails, so let’s get this out of the way now: These trails, no matter what some might have you believe, are not chemtrails.
Chemtrails are a spin-off of the term “contrails,” and the term refers to a geoengineering conspiracy theory that dates back to the mid-Sixties. The idea is that the contrails left behind by planes are actually chemicals and metals being spat into the sky as part of a large-scale secret atmospheric spraying program. Believers speculate that this happens for purposes ranging from mind-control to climate control. The chemtrails conspiracy has been debunked multiple times.
The chemtrails conspiracy theory has enough of a following that major research organizations and government agencies have debunked it. In 2015, the Environmental Protection Agency addressed the issue head-on in a statement. Harvard’s Keith Group, a solar geoengineering research group, maintains a permanent explanation of their chemtrail views on the group’s website because they get so many requests from chemtrails enthusiasts. They “have not seen any credible evidence that chemtrails exist.”
Preventing the Climate Effects of Contrails
The study released Thursday suggests that one way to reduce air pollution from contrails is to make contrails cleaner, which requires cleaner forms of jet fuel.
“We also show that a reduction of soot emissions could be a way to decrease the climate impact of contrail cirrus,” Bock says.
If fuel developers can decrease the soot emitted by 50 percent, Bock’s model shows it might be possible mitigate some of the effects of contrails on climate by 15 percent. Even a 90-percent reduction in soot, they note, won’t totally offset the increasing role that cirrus clouds will play as more planes take to the skies.
The climate impact of air traffic is to a large degree caused by changes in cirrus cloudiness resulting from the formation of contrails. Contrail cirrus radiative forcing is expected to increase significantly over time due to the large projected increases in air traffic. We use ECHAM5-CCMod, an atmospheric climate model with an online contrail cirrus parameterization including a microphysical two-moment scheme, to investigate the climate impact of contrail cirrus for the year 2050. We take into account the predicted increase in air traffic volume, changes in propulsion efficiency and emissions, in particular soot emissions, and the modification of the contrail cirrus climate impact due to anthropogenic climate change.
Global contrail cirrus radiative forcing increases by a factor of 3 from 2006 to 2050, reaching 160 or even 180 mW m−2, which is the result of the increase in air traffic volume and a slight shift in air traffic towards higher altitudes. Large increases in contrail cirrus radiative forcing are expected over all of the main air traffic areas, but relative increases are largest over main air traffic areas over eastern Asia. The projected upward shift in air traffic attenuates contrail cirrus radiative forcing increases in the midlatitudes but reinforces it in the tropical areas. Climate change has an insignificant impact on global contrail cirrus radiative forcing, while regional changes are significant. Of the emission reductions it is the soot number emission reductions by 50 % that lead to a significant decrease in contrail cirrus optical depth and coverage, leading to a decrease in radiative forcing by approximately 15 %. The strong increase in contrail cirrus radiative forcing due to the projected increase in air traffic volume cannot be compensated for by the decrease in initial ice crystal numbers due to reduced soot emissions and improvements in propulsion efficiency.