Research update

Less aviation during the global lockdown had a positive impact on the climate

High levels of aviation drive global warming, not only through greenhouse gas emissions, but also through additional clouds.

This is the conclusion reached by scientists at Leipzig University, Imperial College London and the Institut Pierre-Simon Laplace (IPSL) in Paris. They studied the extent to which cirrus clouds caused by aircraft occurred during the global lockdown between March and May 2020, and compared the values with those during the same period in previous years.

The study, led by Johannes Quaas, CONSTRAIN scientist and Professor of Theoretical Meteorology at Leipzig University, and also involving Dr Olivier Boucher of IPSL is now published in Environmental Research Letters.

Condensation trails, known as “Cirrus homogenitus ”, often form behind aircraft. These can spread to form cirrus clouds. Photo: Colourbox

Cirrus clouds, known for their high, wispy strands, contribute to warming the climate. When cirrus clouds occur naturally, large ice crystals form at an altitude of about 36 kilometres, in turn reflecting sunlight back into space – albeit to a small extent. However, they also prevent radiated heat from escaping the atmosphere, and thus have a net heating effect.

When the weather conditions are right, condensation trails form behind aircraft. These may persist and spread to form larger cirrus clouds. In this case, their effect on the climate is much greater than that of narrow contrails alone.

The study team analysed satellite images of clouds in the northern hemisphere, between 27° and 68° North, from March to May 2020. They then compared these with images from the same period in previous years.

“The study clearly demonstrates that aircraft contrails lead to additional cirrus clouds and have an impact on global warming.”

Professor Quaas explained:“Crucially, our studies reveal a clear causal relationship. Since clouds vary considerably depending on the weather, we would not have been able to detect the effects of air traffic in this way under normal circumstances. The period of lockdown due to the COVID-19 pandemic offered a unique opportunity to compare clouds in air traffic corridors at very different traffic levels. Analysis of the data collected showed that nine per cent fewer cirrus clouds formed during the global lockdown, and that the clouds were also two per cent less dense.”

“The study clearly demonstrates that aircraft contrails lead to additional cirrus clouds and have an impact on global warming.”

The data collected also confirmed previous estimates based only on climate models, and may also improve the ability to simulate these effects in climate models.

Despite the team’s findings, there has still not been enough research into the impact of aviation on global warming.  But Professor Quaas’s research group is part of a European collaboration currently investigating the precise mechanisms in detail.

“The tough global lockdown has been helpful in terms of our research. In order to mitigate or even avoid the warming effect on the climate, flight routes could be adapted in the future to avoid cirrus cloud formation, for example by separating flight corridors.”

Adapted from the original article by Ulf Walther, Leipzig University.

The full publication, now available in Environmental Research Letters is:
Climate impact of aircraft-induced cirrus assessed from satellite observations before and during COVID-19,