When everyday life came to an abrupt halt in March 2020 due to the Covid-19 pandemic, many have wondered what would be the impact on the climate. A major change has been the drastic reduction in air travel around the world, with countries encouraging their residents to limit non-essential travel. A recent study looked directly at the impact of this reduction in air travel on the climate, by studying the evolution of ice crystals in cirrus clouds.
Cirrus clouds are thin and wispy and composed mostly of ice crystals. The size, shape, and density of these ice crystals determine how much energy from Earth’s surface and atmosphere will be reflected or absorbed. It’s called the cloud radiative properties. When ice crystals trap heat, the temperature under the clouds rises. When ice crystals are smaller, they reflect this energy and do not contribute to rising temperatures.
Why is this important for airplanes? Previous studies showed that soot particles in aircraft exhaust increase the density and size of cirrus ice crystals, and therefore increase the radiative properties of the cloud. Less soot means less reflection.
It was previously shown that global flight mileage and therefore aircraft soot emissions increased by 5% per year between 2013 and 2019. Flight mileage was expected to continue to increase at this rate, before the pandemic.
Scientists in the United States, Hong Kong and China wanted to test their hypothesis that fewer flights would impact cirrus clouds. First, they collected information on flights between January and May in 2019 and 2020 from a commercial database, FlightRadar24. They found a 0.3% decrease in global flight mileage in January and February 2020 compared to those same months in 2019. This decrease corresponded to fewer flights to and from China during those months. .
There was a more drastic 26% decrease in global flight mileage in March 2020 as the pandemic slowed travel in Europe and North America. Then they noticed an even bigger decrease of 73% in April and May 2020 compared to those same months in 2019. Assuming the shortest routes were taken, they used this flight mileage data to calculate the amount of soot emitted by aircraft during these periods. .
The scientists then looked at cloud ice crystal concentration data collected by satellite during those same time periods. They used data collected by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations, CALIPSO for short, a satellite jointly operated by NASA (USA) and CNES (France). Scientists have divided the earth into four horizontalor latitude, bands and looked at ice crystal data in these four bands from January to May in 2018, 2019 and 2020.
There was no significant change in ice concentration in January-February 2020 compared to those same months in 2018 and 2019. However, the satellite did measure an increase in ice crystal concentration for April-May 2020 for the latitude band above China and downwind of Western Europe. The scientists performed a statistical analysis of this data and confirmed that the increase was significant at a >95% level, meaning that the observed change was unlikely to be due to chance.
They saw no significant difference in the concentration of ice crystals above and downwind of North America. This is likely because the type of ice formation that occurs from aircraft soot is most likely more mountains. The leeward terrain of Europe is mountainous while the leeward area of North America is the Atlantic Ocean.
Scientists used computer models to simulate the observed increase in ice crystal concentration over China and downwind of Europe in April-May 2020, and found that it predicted a concentration of ice crystals close to but slightly higher than that collected by satellite. Scientists speculate that the satellite could have missed some freezing events because it only passes one location once every 16 days.
They then used this model to predict what the impact on cirrus clouds would be if the reduced number of flights were to continue for the next five years. This showed an overall increase in the concentration of ice crystals in many regions of the Earth, including northern Eurasia and the northern Atlantic Ocean.
This increase in ice concentration means clouds would be more likely to reflect heat from Earth and not contribute to rising temperatures. The scientists in this study acknowledged that their modeling had some uncertainties and that more work needed to be done to confirm these results.
The reduction in flights during the COVID-19 pandemic appears to have led to an increase in the concentration of ice crystals in cirrus clouds. Modeling suggests that if this trend continues, these cirrus clouds will continue to reflect thermal radiation away from Earth rather than trap it. However, air traffic is unlikely to remain as low as it was in April-May 2020 when the pandemic began.
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