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

They studied the extent to which cirrus clouds caused by aircraft occurred during the global hard lockdown between March and May 2020, and compared the values with those during the same period in previous years. The study was led by Johannes Quaas, Professor of Theoretical Meteorology at Leipzig University, and has now been published in the renowned journal “Environmental Research Letters“.

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. This is the dominant effect in cirrus clouds.

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 researchers led by Professor Quaas analysed satellite images of clouds in the northern hemisphere, between 27° and 68° North, in the period from March to May 2020. They then compared these with images from the same period in previous years. “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,” said Professor Quaas. “The study clearly demonstrates that aircraft contrails lead to additional cirrus clouds and have an impact on global warming.” According to Professor Quaas, the data collected confirmed previous estimates based only on climate models: “Our study may improve the ability to simulate these effects in climate models.”

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