The warming effect of aeroplane contrails on Earth’s climate could increase threefold by 2050, according to new research done by Lisa Bock and Ulrike Burkhardt at the German Aerospace Centre in Oberpfaffenhofen. Their study highlights the importance of looking beyond carbon dioxide emissions when considering the environmental impacts of the aviation industry.
Contrails are the familiar line-shaped clouds of ice crystals that appear behind aircraft and are usually created by the freezing of water vapour in engine exhaust. Ice formation is aided by soot particles – also emitted by jet engines – which provide nucleation sites. Contrails can also be created from ambient humidity when the wings of an aircraft cause a rapid drop in the local air pressure and temperature.
Contrails have the potential to linger and become artificial cirrus clouds. These clouds reflect infrared radiation coming up from the Earth’s surface far more than they reflect incoming solar radiation back into space. As a result, such clouds tend to trap heat in the Earth’s atmosphere and have an overall warming effect on climate.
Important, but overlooked
Indeed, contrail cirrus clouds are the single largest source of the aviation industry’s contribution to climate change, far outpacing the impact of aircraft carbon dioxide emissions. Despite this, however, their role is often overlooked and is not included in the United Nation’s upcoming Carbon Offsetting Scheme for International Aviation (Corsia).
“It is important to recognize the significant impact of non-carbon dioxide emissions, such as contrail cirrus, on climate and to take those effects into consideration when setting up emission trading systems or schemes like the Corsia agreement,” says Bock.
To this end, Bock and Burkhardt used a special atmospheric climate model to calculate the climate impart of contrail cirrus formation to explore the clouds’ projected climate impact by the mid-21st century.
The researchers incorporated the predicted increase in global air traffic – with four times more traffic expected in 2050 than in 2006 – which is the baseline year of the study. Also considered were improvements in fuel and engine efficiency and the slight upwards shift in cruising altitudes expected to result from ongoing changes in aircraft design . Such higher flying, the researchers report, is likely to increase the rate of contrail formation over the tropics.
Faster rise
From their modelling, the researchers estimate that the radiative forcing from contrail cirrus will be three times larger in 2050 than 2006. Furthermore, the climate impact of contrails will rise faster than that coming from the aviation industry’s carbon dioxide emissions, thanks to anticipated increases in fuel efficiency.
“Contrail cirrus’ main impact is that of warming the higher atmosphere at air traffic levels and changing natural cloudiness,” says Burkhardt. However, she added, “how large their impact is on surface temperature and possibly on precipitation due to the cloud modifications is unclear”.
Bock adds, “There are still some uncertainties regarding the overall climate impact of contrail cirrus, and in particular their impact on surface temperatures,” noting that the effects of the clouds are an active area of research. Nevertheless, she concludes, “it’s clear they warm the atmosphere”.
The team found that the climate impacts of contrail cirrus will be highest in busy air traffic areas above North America and Europe. They also predict a significant increase in the busier sectors of Asian airspace.
An often-proposed method for mitigating the overall impact of contrail cirrus is to reroute flights around those regions of the atmosphere that are particularly susceptible to the effects of the clouds. The researchers caution, however, that this approach risks increasing the emission of long-lived carbon dioxide, potentially making such a cure worse than the original disease.
Cleaner emissions
Instead, a better course of action may be to reduce soot emissions from aircraft engines to make ice nucleation less likely. “This would enable international aviation to effectively support measures to achieve the Paris climate goals,” Burkhardt says.
“Larger reductions than the projected 50% decrease in soot number emissions are needed”, Burkhardt says, however, adding that even a 90% reduction might not be enough to keep the climate impact of the clouds at the same level as was seen in 2006.
Climate impact of aircraft contrails could treble by mid-century
University of Leeds climate researcher Piers Forster highlights the importance of Bock and Burkhardt’s demonstration that the warming effect scales with increasing flights. “There was the possibility of the warming effect saturating, as contrails don’t have much effect if the sky is already full of them,” he says, adding however that “this study shows that any saturation effect is small”.
“Overall, their results indicate that more radical solutions will be needed,” Forster concludes. “Flying lower and slower could reduce both carbon dioxide and contrailing,” he adds, “but in the end we might need to take the train more or simply stay home”.
The study is described in Atmospheric Chemistry and Physics.
