High-flying jets often create long white lines across the sky. Called contrails, those narrow clouds can vanish within minutes or last for days. Like other clouds, long-lasting contrails can trap heat in the atmosphere. Scientists have found in the last year or two that these contrails can boost the warming of Earth’s atmosphere. But it may be fairly simple to lower that contribution: Just make a few planes fly higher throughout much of their route.
Engineers in England recently studied flights and contrails in a high-traffic part of the sky above Japan. Along the way, they turned up a surprising pattern. About 80 percent of the warming from contrails came from just 2 percent of the flights.
The scientists published their findings March 5 in Environmental Science and Technology. It’s led them to now propose a solution for those 2 percent of trips: Fly higher.
“The fact that it’s such a small [number] of flights means we think it’s a feasible solution,” says engineer Marc Stettler. He led the study and works at Imperial College London.
Sending just 1.7 percent of flights to a higher altitude should lower the atmospheric-warming impact of contrails by 59 percent, his team calculates. If those planes fly higher and use more efficient ways to burn fuel, the scientists predict that the warming impact could drop by more than 90 percent.
Not just any flights should be changed. “We only looked at diverting a flight if the number of the planes in the sky was low,” he says. “[With fewer flights] late in the evening, for example, we have more space to move planes around.”
How contrails foster warming
Jets contribute to climate change in at least two ways. Both come from burning fuel.
First, they spew carbon dioxide, a greenhouse gas. Greenhouse gases trap heat in the atmosphere. That can lead to higher temperatures.
Second, they emit tiny particles of black carbon, known as soot. These particles act as seeds for cloud-forming. “Water molecules in the air are looking for a surface on which they can condense,” Stettler explains. The most difficult challenge in his study, he says, was estimating the size of those soot particles — and then linking those to the making of new clouds.
Contrails usually form at 8 to 13 kilometers (5 to 8 miles) above the ground. That’s the “cruising altitudes” of long-distance jets. When the air up there is saturated with moisture, molecules of water vapor can condense onto the soot particles. This forms ice crystals. And if there are enough of them, they appear as contrails.
However, the science of cloud formation is complex. Researchers can’t always predict when these clouds will form or how long they’ll stick around.
“It’s a hard-nosed scientific problem,” says Bernd Kärcher. He did not work on the new study. He’s a physicist who studies cloud formation at the DLR Institute of Atmospheric Physics in Oberpfaffenhofen, Germany. His research has linked soot particles to contrail formation.
Scientists have been looking for links between contrails and climate since the 1970s. These high clouds reflect sunlight back into space, which has a cooling effect. But they also act like a thermal blanket. That holds heat in the atmosphere. The effect of the trapped energy far exceeds that of the reflected light.
Kärcher says reducing the contrails’ role in global warming is an important issue. The number of airline flights increases every year. “Aircraft emissions might rise dramatically within the next decades,” he says. As a result, he argues, every tiny drop in the heat that contrails add to global warming matters.
Not all agree the solution will be simple
Climate scientists have largely ignored contrails, Kärcher says. In one 2011 study, he and his colleagues found contrails have a bigger impact on climate change than the carbon dioxide from jets. “There can be no doubt,” he says, “that the global atmospheric warming induced by contrail cirrus [clouds] is bound to increase if nothing is done to prevent their formation.”
He doubts that simply re-routing planes will solve the problem. Right now, the coronavirus has sidelined many flights. But in normal times, more than 100,000 flights take off and land every day around the world. Controlling all that air traffic is challenging, he notes. Small changes in flights can cause unexpected problems. What’s more, modern jets take paths designed to keep fuel use as low as possible. That’s why he worries that changing those paths might backfire.
“Controlling contrail formation in this way will almost certainly lead to increases in aircraft CO2 emissions,” Kärcher says. He thinks that developing fuels that release less soot — as well as more efficient ways to burn that fuel — seem like better tactics.
Stettler disagrees. He thinks it’s possible to make small changes without burning more fuel. Diverting fewer than 2 percent of planes, for example, leads to only a very small increase in carbon dioxide, he says. By making changes that didn’t increase carbon dioxide at all, his team calculated that contrail warming of the air would fall by one-fifth.
The study looked only at flights over Japan. Now, the German team is looking at other locations to test whether the same boost in some plane’s altitude might show similar benefits. “We want to know if we’ll see the same story,” Stettler says — “to know if changing a very small number of flights would curtail the amount of contrail warming.”