For decades, scientists have tracked the relentless spread of microplastics into nearly every corner of our planet. These tiny particles — some too small to see with the naked eye — have turned up in the depths of the Mariana Trench, atop the snow-capped peaks of Everest, and even in human blood.

The health effects of microplastic pollution are still poorly understood, although some suggest they cause inflammation in the body. Now, a new study suggests that microplastics may be affecting something far grander: the atmosphere, with potential consequences for our climate.

Microplastics: Unexpected Ice Makers in the Sky

Researchers from Penn State University uncovered a surprising role that microplastics might be playing high above our heads. Their experiments showed that these particles can act as ice nucleating agents, tiny bits of debris around which ice crystals can form in clouds.

“Throughout the past two decades of research into microplastics, scientists have been finding that they’re everywhere, so this is another piece of that puzzle,” said Miriam Freedman, a chemistry professor at Penn State and senior author of the study. “It’s now clear that we need to have a better understanding of how they’re interacting with our climate system, because we’ve been able to show that the process of cloud formation can be triggered by microplastics.”

To explore how these microplastics might influence cloud dynamics, the team focused on four types of common plastics: low-density polyethylene (LDPE), polypropylene (PP), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). In their lab, they suspended microplastics in tiny droplets of water and cooled them. The results were striking: droplets containing microplastics froze at temperatures 5 to 10 degrees Celsius warmer than those without. Normally, water droplets high up in the atmosphere only freeze at around -38 degrees Celsius unless they contain impurities.

“In the case of our microplastics, 50% of the droplets were frozen by minus 22 degrees Celsius for most of the plastics studied,” explained Heidi Busse, the study’s lead author. “It turns out that if you introduce something insoluble, you introduce a defect into that droplet, and it can nucleate ice at warmer temperatures.”

A New Variable in Climate Models?

The implications of these findings could be far-reaching. Clouds play a critical role in regulating Earth’s climate, both by reflecting sunlight away from the planet and by trapping heat close to its surface. The balance between liquid water and ice in clouds helps determine whether they cool or warm the Earth. Microplastics, it seems, could be shifting this balance.

“When air patterns are such that a droplet gets lifted into the atmosphere and cools, that’s when microplastics could be affecting weather patterns and forming ice in clouds,” Freedman said. If microplastics are influencing cloud formation, it could alter everything from rainfall to storm intensity.

In polluted areas, where microplastic concentrations are higher, clouds might contain more (but smaller) droplets. This can delay rainfall, leading to heavier downpours when the clouds finally release their moisture. However, it’s not clear at all how much of an influence microplastics have on weather formation in practice. Future studies should count microplastics in clouds and do experiments to learn more.

The study also touched on how microplastics might complicate climate predictions. “We know that the fact that microplastics can nucleate ice has far-reaching effects,” said Busse. “We’re just not quite sure yet what those are.”

The researchers are now turning their attention to the long-term effects of microplastics on atmospheric chemistry. They suspect that environmental aging — where plastics break down due to light, ozone, and other chemical exposures — may alter their ice-forming abilities. Intriguingly, aging decreased the ice nucleating ability of some plastics like LDPE and PET but enhanced it in PVC. This suggests that as microplastics age, they might impact the atmosphere in unpredictable ways.

What Comes Next?

The team plans to investigate how additives in plastics, such as plasticizers, might further alter their interactions with the atmosphere. Freedman and her colleagues emphasize that while this study opens a new chapter in understanding microplastics, it also underscores just how little we know about their impact on our environment.

“We know the full lifecycle of these plastic items we use every day could be changing the physical and optical properties of the Earth’s clouds and, therefore, changing the climate in some way,” Busse said. “But we still have a lot to learn about exactly what they are doing.”

The findings appeared in the journal Environmental Science and Technology: Air.