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Pollution in the form of tiny aerosol particles—so small they’ve long been overlooked—may have a significant impact on local climate, fueling thunderstorms with heavier rainfall in pristine areas, according to a study released Thursday.

The study, published in the journal Science, found that in humid and unspoiled areas like the Amazon or the ocean, the introduction of pollution particles could interact with thunderstorm clouds and more than double the rainfall from a storm.

The study looked at the Amazonian city of Manaus, Brazil, an industrial hub of 2 million people with a major port on one side and more than 1,000 miles of rainforest on the other. As the city has grown, so has an industrial plume of soot and smoke, giving researchers an ideal test bed.

“It’s pristine rainforest,” said Jiwen Fan, an atmospheric scientist at the Pacific Northwest National Laboratory and the lead author of the study. “You put a big city there and the industrial pollution introduces lots of small particles, and that is changing the storms there.”

Fan and her co-authors looked at what happens when thunderstorm clouds—called deep convective clouds—are filled with the tiny particles. They found that the small particles get lifted higher into the clouds, and get transformed into cloud droplets. The large surface area at the top of the clouds can become oversaturated with condensation, which can more than double the amount of rain expected when the pollution is not present. “It invigorates the storms very dramatically,” Fan said—by a factor of 2.5, the research showed.

For years, researchers largely dismissed these smaller particles, believing they were so tiny they could not significantly impact cloud formation. They focused instead on larger aerosol particles, like dust and biomass particles, which have a clearer influence on climate. More recently, though, some scientists have suggested that the smaller particles weren’t so innocent after all.

Fan and her co-authors used data from the 2014/15 Green Ocean Amazon experiment to test the theory. In that project, the US Department of Energy collaborated with partners from around the world to study aerosols and cloud life cycles in the tropical rainforest. The project set up four sites that tracked air as it moved from a clean environment, through Manaus’ pollution, and then beyond.

Researchers took the data and applied it to models, finding a link between the pollutants and an increase in rainfall in the strongest storms. Larger storms and heavier rainfall have significant climate implications, Fan explained, because larger clouds can affect solar radiation and the precipitation leads to both immediate and long-term impacts on water cycles. “There would be more water in the river and the subsurface area, and more water evaporating into the air,” she said. “There’s this kind of feedback that can then change the climate over the region.”

The effects aren’t just local. The Amazon is like “the heating engine of the globe,” Fan said, driving the global water cycle and climate. “When anything changes over the tropics it can trigger changes globally.”

Johannes Quaas, a scientist studying aerosol and cloud interactions at the University of Leipzig, called the study “good, quality science,” but also stressed that the impact of the tiny pollutants was only explored in a specific setting. “It’s most pertinent to the deep tropics,” he said.

Quaas, who was not involved in the Manaus study, said that while the modeling evidence in the study is strong, the data deserves further exploration, as it could be interpreted in different ways.

Fan said she’s now interested in looking at other kinds of storms, like the ones over the central United States, to see how those systems can be affected by human activities and wildfires.