Researchers have found that pollutants are strengthening storms above the Pacific Ocean, which feeds into weather systems in other parts of the world.
The effect was most pronounced during the winter.
The study is published in the Proceedings of the National Academy of Sciences (PNAS).
Lead author Yuan Wang, from the Jet Propulsion Laboratory at the California Institute of Technology, said: “The effects are quite dramatic. The pollution results in thicker and taller clouds and heavier precipitation.”