Microplastics - tiny plastic particles less than 5 millimeters in size - have been found in some of Earth's most remote and pristine environments, from the Mariana Trench to the snow on Mount Everest, and even in clouds atop mountains in China and Japan.
They have also been detected in human brains, sea turtles' stomachs, and plant roots. Now, new research led by Penn State University scientists suggests that microplastics in the atmosphere may be influencing weather and climate.
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The study, published recently in Environmental Science and Technology: Air, reveals that microplastics can act as ice-nucleating particles - microscopic aerosols that facilitate ice crystal formation in clouds. This ability means microplastics they could alter precipitation patterns, weather forecasting, climate modeling, and even aviation safety by affecting how clouds form.
Over the last two decades, scientists have learned that microplastics are everywhere. Now they know they can trigger cloud formation, which means they could be interacting with the climate system in significant ways.
Proven in experiments
In a controlled laboratory setting, the researchers examined how four types of microplastics influenced ice formation: low-density polyethylene (LDPE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).
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They suspended these plastics in tiny water droplets and cooled the droplets, observing how the microplastics affected freezing. The team found that droplets containing microplastics froze at temperatures 5–10°C warmer than droplets without them. While typical atmospheric water droplets freeze at around -38°C, defects such as dust, bacteria, or microplastics provide structures that facilitate freezing at higher temperatures. For most plastics studied, 50% of droplets froze by -22°C.
This process, according to the study authors, could have implications for mixed-phase clouds, which contain both liquid and frozen water. These clouds, such as cumulus, stratus, and nimbus, are widespread in the atmosphere. Microplastics could influence how these clouds form and behave, affecting weather patterns.
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For example, in polluted environments with numerous aerosol particles, water is distributed among many smaller droplets, delaying rainfall but potentially leading to heavier precipitation once droplets grow large enough to fall.
Clouds play a dual role in regulating Earth's temperature: they cool the planet by reflecting sunlight but can also trap heat, depending on their composition and altitude. The ratio of liquid water to ice in a cloud is critical to determining whether it has a warming or cooling effect.
If microplastics influence mixed-phase cloud formation, they could also affect climate, but the exact nature of these effects is challenging to model.
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Graduate student Heidi Busse, the study's lead author, highlighted the broader implications: The fact that microplastics can nucleate ice has far-reaching consequences. This could affect everything from storm intensity to light scattering, which impacts climate.
Next, the researchers plan to explore how plastic additives, such as plasticizers, might influence the behavior of microplastics in the atmosphere. Why does it matter? Because the everyday plastics we use may be altering the physical and optical properties of Earth’s clouds and, by extension, the climate.
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