Credit: Pixabay.

Global heat trapped by high-altitude airplane contrails could more than triple by 2050, according to a study published yesterday in Atmospheric Chemistry and Physics. The rapid rise in contrail cirrus clouds is due in large part to an expected rise in air traffic in the coming decades, the researchers said.

“Contrail cirrus’ main impact is that of warming the higher atmosphere at air traffic levels and changing natural cloudiness,” coauthor Ulrike Burkhardt, an atmospheric scientist at the German Aerospace Center (DLR) in Oberpfaffenhofen, said in a statement.

It’s clear that the contrail cirrus clouds will warm the atmosphere, according to lead author Lisa Bock, an atmospheric scientist at DLR. However, “there are still some uncertainties regarding the overall climate impact of contrail cirrus and in particular their impact on surface temperatures,” she said.

Simulating Cirri

When heat from the Sun strikes Earth’s surface, Earth bounces some of it back toward space. But some of the radiation that Earth tries to send away is forced back to the surface by clouds. This process is a normal part of Earth’s energy balance and part of what makes the planet warm enough to live on.

Airplanes, however, are messing up that balance. Soot in airplane exhaust seeds icy clouds, contrail cirri, that crisscross the sky at altitudes greater than 6 kilometers. In 2005, air traffic contributed about 5% to the global amount of anthropogenic radiative forcing, not from its carbon dioxide (CO2) emissions but from the contrails left behind.

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Using climate models, Bock and Burkhardt calculated the global amount of radiative forcing from contrail cirrus clouds under current climate conditions and a predicted 2050 climate. They included the expected level of air traffic in 2050 and calculated values for current and improved emissions standards.

The team found that global radiative forcing from contrail cirrus clouds will increase threefold by 2050. This increase was mostly due to the rise in air traffic and not the different climate conditions in the future. Airspace with the densest air traffic, including southeast Asia, western Europe, and the eastern United States, will be the most heavily affected.

These maps show simulated global radiative forcing levels from contrail cirrus clouds from (a) current climate and current airplane traffic, (b) current climate and expected 2050 airplane traffic, (c) predicted 2050 climate and expected 2050 airplane traffic, and (d) predicted 2050 climate, expected 2050 airplane traffic, and improved fuel efficiency and emission standards. Credit: Bock and Burkhardt, 2019.

These maps show simulated global radiative forcing levels from contrail cirrus clouds from (a) current climate and current airplane traffic, (b) current climate and expected 2050 airplane traffic, (c) predicted 2050 climate and expected 2050 airplane traffic, and (d) predicted 2050 climate, expected 2050 airplane traffic, and improved fuel efficiency and emission standards. Credit: Bock and Burkhardt, 2019.

Corralling Contrails

Contrails have warmed the atmosphere more than all of the CO2 released by airplanes since the birth of flight. This contribution cannot be ignored by climate mitigation policies, Bock said.

“It is important to recognize the significant impact of non-CO2 emissions, such as contrail cirrus, on climate and to take those effects into consideration when setting up emission trading systems or schemes,” Bock said.

In the face of an inevitable rise in airplane traffic, the best way to minimize the impact of airplane contrails on the climate is to cut airplane soot emissions by at least 50%, the researchers found. Switching to more efficient fuels would also help to a lesser degree. These measures would reduce a small amount (15%) of the heat trapped by contrail cirrus clouds, they concluded.

Adopting stricter fuel standards “would enable international aviation to effectively support measures to achieve the Paris climate goals,” Burkhardt said.

—Kimberly M. S. Cartier (@AstroKimCartier). This article originally appeared on Eos.