Heavy Rain Boosts River Microplastic Flow Globally

Rivers may be delivering far more microplastic pollution to the oceans than previously realized, with developing countries accounting for nearly all of the global flow, according to a new study. The worldwide analysis also finds that heavy rainfall can unleash surges of microplastics into rivers – a climate-sensitive pollution threat that could intensify as extreme weather becomes more common. Microplastics have become a pervasive and long-lasting form of pollution, with recognized threats to ecosystems, water quality, and human health. Each year, tens of millions of metric tons of microplastics enter the environment, and much of that material ultimately reaches the oceans. Rivers are a major pathway for transporting these particles from inland regions to coastal waters. However, estimates of global riverine microplastic pollution vary widely because of inconsistent sampling methods and limited understanding of how rainfall and runoff affect their transport. Here, Hehao Qin and colleagues developed a new framework to produce more reliable, high-resolution estimates of microplastic concentrations and export from rivers. According to the authors, their approach harmonizes differences in particle size and sampling methods, while using machine learning to account for both human activity and natural processes. It also tracks how rainfall can either increase or dilute microplastic concentrations. Using this approach, Qin et al. estimate that rivers worldwide delivered roughly 263,000 tons of microplastics to the ocean in 2022, with 96% originating from the Global South – Southeast and East Asia alone contribute to more than half of this total. Plastic use, poorly managed waste, and human development were the strongest drivers of geographic differences in microplastic concentrations, while weather and seasonal conditions influenced short-term fluctuations. The findings show that extreme rainfall events can substantially increase the movement of microplastics into rivers, particularly in regions with high plastic use and poor waste management. Qin et al. warn that climate change could worsen this problem as extreme rainfall becomes more frequent and intense, potentially creating short-lived but highly concentrated pollution pulses into rivers and marine environments.