Cut the contrail? Tweak a few aircraft flight levels, researchers say

David Wagman | February 13, 2020

Researchers suggest that jet aircraft flight altitude changes of as little as 2,000 ft could lessen the climate effects of contrails, the white streaks that aircraft leave in the sky.

Flight level adjustments, combined with cleaner aircraft engines, could cut contrail-caused impacts on the environment by up to 90%.

Lead author Dr. Marc Stettler of Imperial College London's Department of Civil and Environmental Engineering said that changing the altitude of a small number of flights could "significantly reduce the climate effects of aviation contrails."

The research was published in Environmental Science & Technology.

Contrail formation

The researchers said that when hot exhaust gases from aircraft meet the cold, low-pressure air of the atmosphere, they produce white streaks called "condensation trails," or contrails.

Contrail fumes include black carbon particles, which provide surfaces on which moisture condenses to form ice particles. Most contrails last only a few minutes, but some spread and mix with other contrails and cirrus clouds that can linger for hours.

Previous research suggests that contrails and the clouds they help form have as much of a warming impact on the climate as aviation's cumulative carbon dioxide (CO 2 ) emissions, because of an effect known as "radiative forcing." This is where the balance is disrupted between radiation coming to Earth from the sun and heat emitted from the surface of the Earth going out to space, forcing a change in the climate.

One difference between CO 2 and contrails, however, is that while CO 2 may have an impact for hundreds of years, the impact of contrails is short-lived and could quickly be reduced.

As the basis for their work, the researchers used computer simulations to predict how changing aircraft altitudes might reduce the number of contrails and how long they linger. They said that contrails only form and persist in thin layers of the atmosphere that have very high levels of humidity. Because these layers are relatively thin, small changes to flight altitudes would mean that aircraft could avoid these regions, leading to fewer contrails forming.

Using data from Japan's airspace, the researchers found that 2% of flights were responsible for 80% of radiation forcing within the airspace.

The team simulated these planes to fly either 2,000 ft higher or lower than their actual flight paths and found that the contrail climate forcing could be cut by 59% by altering the altitudes of 1.7% of flights.

The diversion in flight paths caused less than a 0.10% increase in fuel consumption. The researchers said the reduced contrail formation more than offset the CO 2 released by the extra fuel.

The researchers said aircraft engines themselves also play a part in how harmful contrails may be. Black carbon particles are produced by incomplete fuel combustion, so new, more efficient engine combustion technology could help to reduce them by around 70%. This, combined with small altitude changes, could help reduce overall contrail harm by around 90%.

The researchers next plan to refine their simulations to more accurately predict the characteristics and impact of contrails, and to evaluate the wider effects and practicalities of contrail mitigation strategies such as altering flight paths.