Diverting flights into a higher cruising altitude could reduce contrail climate effects by up to 60 percent.
Like hole punch clouds, contrails are a reaction between particulate and freezing water vapor.
At a higher cruising altitude, warmer temperatures and less humidity means far fewer contrails.
A new study suggests that a few small tweaks could greatly reduce how much airplanes affect the climate via contrails. By raising the altitude of certain flights by just 2,000 feet, scientists say we could reduce climate-affective airplane exhaust by almost 60 percent. These flights would be shifted just outside the altitude range where contrails are able to form.
Most long-haul passenger planes cruise at 35,000 to 36,000 feet. This is about 11 kilometers, putting these planes at the very lowermost reaches of the stratosphere.
There are major benefits to flying this high, namely much thinner air that makes for better fuel efficiency. This band of altitude is (mostly) free of clouds, which is why it feels like the plane is flying above a floor of fluffy cloud mass when you're in the sky. Air traffic controllers plan routes and tandem flights using odd and even altitudes as cues to keep planes separated.
But the stratosphere isn’t like Earth’s surface, where higher altitudes mean colder temperatures. The lowest part of the stratosphere is the coldest, because the sun’s UV rays heat the upper portions much more quickly. Regular planes can reach as high as about 42,000 feet safely, but after a certain point, the thin air is too thin and too warm. Climbing that high burns even more fuel for diminishing returns.
It’s in this lower stratospheric sweet spot that you can sometimes find naturally occurring clouds and so-called “contrail cirrus” clouds, NASA explains. Move just a little farther up and there’s no longer any naturally occurring or incidental weather. This is where British and German scientists suggest a select group of flights choose to cruise.
“The climate forcing of contrails and induced-cirrus cloudiness is thought to be comparable to the cumulative impacts of aviation CO2 emissions,” the researchers explain in their new paper. How does a plane generate a contrail that in turn becomes a climate trigger? Beginning at about 25,000 feet up, planes at their cruising altitudes disturb the surrounding air and make contrails.
“Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and, some is added by the exhaust of the aircraft,” NASA explains. The air must be both humid enough to have water vapor present and cold enough for that vapor to freeze. In a way, it’s a related counterpart to cloud seeding, where water vapor again coalesces around and reacts to particulate passing through.
The air conditions, both temperature and humidity, determine which flights are most at risk for generating contrails. In this study, the researchers looked at flight data over Japan. The country is technically an archipelago, which, combined with its distance from any “mainland,” means flying is frequent and normalized within a relatively confined coverage area for researchers to study.
“Only 2.2 [percent] of flights contribute to 80 [percent] of the contrail [energy forcing (EF)] in this region. A small-scale strategy of selectively diverting 1.7 [percent] of the fleet could reduce the contrail EF by up to 59.3 [percent], with only a 0.014 [percent] increase in total fuel consumption and CO2 emissions,” the researchers conclude.
In other words, raising the flights in that contrail sweet spot just a few feet out of it could reduce contrail generation by almost 60 percent and cost very, very little in extra fuel. The corresponding tiny bump in carbon dioxide would be majorly offset by a clearer sky without climate-altering contrail coverage.
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