When daily life came to a sudden halt in March 2020 due to the Covid-19 pandemic, wondered a lot What effect does that have on the climate? One of the major changes was the sharp drop in air travel around the world as countries encouraged their residents to reduce non-essential travel. A recent study looked directly at the effect of reduced air travel on climate, by examining changes in ice crystals in thin clouds.
Thin clouds are thin and brittle and consist mainly of ice crystals. The size, shape, and density of these ice crystals determine how much energy from the Earth’s surface and atmosphere will be reflected or absorbed. This is referred to as the cloud radiological properties. When ice crystals trap heat, the temperature under the clouds increases. When ice crystals are smaller, they reflect that energy and do not contribute to the warming.
Why is this important for aircraft? former studies showed that soot particles in aircraft exhaust increase the density and size of cloud ice crystals, and thus increase the radiative properties of the cloud. Less soot means less reflection.
she was previously shown That the number of miles traveled by global flights, and thus the emission of soot from aircraft, was increasing by 5% annually between 2013-2019. It was expected that the distance traveled in the trip would 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 affect thinning clouds. First, they collected flight information between January and May in 2019 and 2020 from a commercial database, Flight Radar 24. They found a 0.3% decrease in global air miles in January and February 2020 compared to the same months in 2019. This decrease corresponds to fewer flights in and out of China during those months.
There was an even steeper 26% drop in global air miles in March 2020 as the pandemic slowed travel in Europe and North America. Then, they observed a 73% greater decrease in April and May 2020 compared to the same months in 2019. Assuming the shortest route was followed, they used the flight mileage data to calculate the amount of soot emitted by aircraft during these time periods.
The scientists then looked at data on the concentration of ice crystals in clouds collected by satellites during these same time periods. They used data collected by the observational Cloud-Aerosol Lidar and Infrared Pathfinder satellite, CALIPSO for short, a satellite jointly operated by NASA (USA) and the French National Center for Space Studies (France). Scientists divided the Earth into four accidentalor latitudes, and I looked at data for snow crystals 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 the same months in 2018 and 2019. However, the satellite she did Measurement of the increase in ice crystal concentration from April to May 2020 for the latitude range over China and the winds of Western Europe. Scientists performed a statistical analysis of these data and confirmed that the increase was significant at the >95% level, which means that the observed change is unlikely to be due to chance.
They did not see much 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 probably During mountains. The terrain downwind of Europe is mountainous while the area downwind of North America is the Atlantic.
The scientists used computer models to simulate the observed increase in the concentration of ice crystals over China and Europe’s winds during the period from April to May 2020, and found that it predicted an ice crystal concentration close to but slightly higher than what was collected by satellites. Scientists hypothesize that the satellite may have missed some freeze events because it passes through one location once every 16 days.
They then used this model to predict the effect on fluffy clouds if the number of flights continued to decline over the next five years. This showed an overall increase in the concentration of ice crystals in several regions of the Earth, most notably northern Eurasia and the northern Atlantic.
This increase in ice concentration means that clouds are more likely to reflect heat away from the Earth and will not contribute to warming. The scientists in this study acknowledge that their models have some uncertainties and that more work needs to be done to confirm these findings.
The reduction in flights during the COVID-19 pandemic appears to have increased the concentration of ice crystals in thin clouds. Modeling suggests that if this trend continues, these thinner clouds will continue to reflect heat radiation away from Earth rather than retain it. However, it is unlikely that air traffic will remain as low as it was in April and May 2020 when the pandemic first started.