The wispy white contrails that airliners streak across the sky may resemble harmless clouds, but scientists have long warned that they intensify climate change.
When a jet’s hot exhaust mixes with moisture and soot at high altitude, the lingering trail of ice particles that results can trap dangerous amounts of heat in the atmosphere.
Now, research scientists, air traffic controllers and the British government will test whether that warming effect can be reduced through minor flight path adjustments.
Over the next two winters, British air traffic controllers will order hundreds of pilots flying across an eastern portion of the Atlantic Ocean to adjust their trajectories, directing them to altitudes where it is less likely that they will generate a lingering contrail.
What experts call the contrail effect accounts for as much as 35 percent of the planetary warming caused by aviation, according to one study. Others have suggested the warmer effect is even greater. The result, according to Contrails.org, a nonprofit group that is involved in the trial, is that contrails contribute to 1 to 2 percent of all man-made global warming.
The trial, the largest of its kind, will occur in the Shanwick airspace, a busy flight corridor west of Ireland in the North Atlantic. Working with a combination of weather data and artificial intelligence analysis by Google, air traffic controllers will ask pilots to tweak their altitudes.
Around 10,000 flights are expected to fly through the designated air space during two four-month trial periods, with a small percentage asked to slightly deviate their paths. The trial will assess the climate impact of reduced contrails and the feasibility of scaling navigational adjustments more widely.
Even a tiny adjustment to an aircraft’s altitude can have a major influence on reducing the warming contrails it produces, said Edward Gryspeerdt, an assistant professor of atmospheric physics at Imperial College London. He is among a team of scientists at that institution and Cambridge University who will analyze satellite imagery to assess the trial’s effect.
Dr. Gryspeerdt said the contrails are generated when an aircraft flies through a humid layer of the atmosphere, an avoidable band of air that is often just a thousand feet thick.
“It’s where there’s enough water to form a cloud but a shortage of things for that water to freeze onto,” he said. “When the airplane comes along and provides this initial burst of ice crystals, those crystals then take the water from the atmosphere and grow into these persistent clouds.”
The clouds can linger for minutes, but sometimes they persist for hours, mixing with other contrails and trapping dangerous amounts of heat energy in the atmosphere.
“From our perspective, contrail avoidance is the most promising near-term climate opportunity,” said Marc Shapiro, the executive director of Contrails.org. “Through these small, targeted changes to flight paths when the conditions are right, we should be able to take a meaningful chunk out of aviation’s climate impact.”
If the program is proven to work and scaled globally, Mr. Shapiro estimates that navigational adjustments could theoretically reduce global contrails by 70 percent.
“We see this as being feasible within the next five years,” he said. “We want to encourage more trials like this globally, so that we can figure out how to do it as quickly as possible.”
The trial builds on previous research by Google, Breakthrough Energy and American Airlines. The aviation sector has framed reducing contrails as a low cost and relatively easy way to reduce the effect of flying on global temperatures.
Earlier this year, Google said that a previous effort with American Airlines to use contrail forecasts had reduced contrail formation by more than 60 percent.
But the strategy has drawbacks. Even minor adjustments to an airplane’s flight path can increase the amount of jet fuel that it burns. Flying at lower altitudes, where the atmosphere is less thin, also requires additional fuel.
Dr. Gryspeerdt said that previous studies have found the overall benefits of reducing the contrails to still outweigh the impact of burning extra fuel but stressed that the trial will examine that question.
“The weather, aviation, clouds, are complex problems and complex systems,” he said. “As we better understand how to route aircraft around these regions, I think we can at the very least reduce the warming from contrails, if not completely eliminate them.”
