Industrial Pollution Can Increase Snowfall and Reduce Cloud Cover

“This is the first direct evidence of industrial pollution causing the water in clouds to freeze and fall to earth as snow,” said Watson-Parris, an atmospheric physicist. “Our findings show something that could be an important missing piece from climate models and a clear place to look for future studies.”

Particles so small they can be suspended in the air, known as aerosols, have long been known to help clouds form as well as make them more reflective. Aerosols can come from natural sources such as dust and human ones like the smoke stacks of coal-fired power plants. These airborne particles can help clouds form by encouraging water vapor to condense into droplets – the building blocks of clouds. The presence of aerosols can also increase the number of water droplets inside clouds, which makes them more reflective and bounces more sunlight back into space, preventing it from heating the planet. 

More cloud cover and more reflective clouds mean that human-created aerosol pollution has created a somewhat paradoxical cooling effect by increasing the amount of the sun’s energy that is sent back into space. There is still uncertainty about the precise size of this cooling effect, but in 2021 the Intergovernmental Panel on Climate Change reported that these aerosols may have masked 0.4 °C (0.72 °F) of warming from human activities. 

In 2022, lead study author climate scientist Velle Toll from Estonia’s University of Tartu and his colleagues were studying satellite imagery looking for evidence of increased cloud cover and reflectiveness near industrial sites when they encountered something puzzling: Gaps in cloud cover near industrial facilities located in colder climates.

The team wondered whether industrial air pollution in Earth’s colder climes might be responsible for these gaps in cloud cover. Specifically, the researchers set out to explore whether industrial aerosols were encouraging what are known as supercooled water droplets inside clouds to freeze at warmer temperatures than usual. Water is supercooled when it remains liquid below 0 °C (32 °F). Some eight percent of the world’s clouds contain supercooled droplets that are so pure and free of the aerosols that allow ice crystals to form that the droplets won’t freeze until the temperature drops to -36°C (-32.8°F).

To investigate, the researchers studied cloud patterns near 67 industrial sites in the colder parts of North America, Europe and Asia by analyzing 21 years of satellite imagery along with ground-based weather radar data. The study authors compared affected clouds to nearby unaffected clouds and used computer models to track how far pollution dispersed. 

The study found that industrial pollution, particularly from metal processing and cement facilities, caused supercooled clouds to freeze at temperatures between -10°C (14°F) and -24°C (-11.2°F), when they can remain liquid all the way down to -36°C (-32.8°F). 

The study also found that industrial pollution triggered local snowfall of up to 15 mm (0.6 inches) per day. By causing water from clouds to fall as snow, the industrial aerosols shrank the clouds themselves, resulting in an 8.3 percent decrease in cloud cover. The loss of cloud cover in turn resulted in 13.7 percent less sunlight being reflected back to space compared to unaffected clouds nearby.

Watson-Parris said that the only plausible explanation for their observations was that the aerosol particles released by industries including metal processing, cement production, coal-fired power plants and oil refineries were triggering the formation of ice in supercooled clouds by acting as scaffolding for the formation of ice crystals. 

“If it turns out this phenomenon is really common then many climate models have been missing a way that aerosols have been contributing to global warming,” said Watson-Parris. “It would also mean that cleaning up industrial processes like cement production or moving from burning dirty fossil fuels to clean energy could help combat climate change by helping to cool the planet by increasing cloud cover in addition to reducing greenhouse gas emissions.”

The researchers said further research is needed to understand whether these effects are common enough to make an impact at the global scale. They call for additional studies to better understand how different types of industrial particles can encourage clouds to freeze and to investigate the potential broader implications for climate change.

In addition to Toll and Watson-Parris, Jorma Rahu of the University of Tartu, Hannes Keernik of the University of Tartu, Heido Trofimov of the University of Tartu, Tanel Voormansik of the University of Tartu, Peter Manshausen of the University of Oxford, Emma Hung of Environment and Climate Change Canada, Daniel Michelson of Environment and Climate Change Canada, Matthew W. Christensen of Pacific Northwest National Laboratory, Piia Post of the University of Tartu, Heikki Junninen of the University of Tartu, Benjamin J. Murray of the University of Leeds, Ulrike Lohmann of the Swiss Federal Institute of Technology, Philip Stier of the University of Oxford, Norman Donaldson of Environment and Climate Change Canada, Trude Storelvmo of the University of Oslo, Markku Kulmala of the University of Helsinki and Nicolas Bellouin of the University of Reading also co-authored the study.

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