A new study published in the journal Science suggests that rivers may be transporting more microplastics into the world's oceans than previously estimated. Microplastics are tiny plastic fragments, typically ranging in size from 1 to 5,000 micrometers—about the size of a grain of sand or smaller. These particles come from sources like broken-down plastics, synthetic paints, and personal care products. Once in the ocean, microplastics can harm marine life and disrupt ecosystems when ingested by fish, shellfish, and other animals. Earlier estimates of microplastic flow from rivers to the ocean varied widely, ranging from 6,100 to over 2.31 million metric tons each year. This wide range was due to a lack of consistent field data, differences in how researchers collected samples, and models that failed to account for sudden changes, such as those caused by heavy rainfall. To address these gaps, a team led by Hehao Qin from Peking University developed a more precise model. This model tracks river conditions daily and uses a mathematical method to standardize measurements taken with different net sizes and sampling techniques. It also considers real-time factors like rainfall and river flow, which previous models overlooked. The model was trained using 1,386 real-world microplastic measurements collected from rivers around the world, combining this data with weather patterns, land-use information, and river flow rates to estimate microplastic levels globally. According to the study, rivers discharged approximately 263,000 metric tons of microplastics into the oceans in 2022. The majority—about 96%—came from countries in the Global South, with Southeast and East Asia contributing more than half of the total. The Yangtze River, one of the world's longest and most heavily used waterways, was identified as the single river releasing the highest volume of microplastic waste into the sea. The research also highlights how rainfall affects microplastic pollution. Heavy downpours can lead to sudden spikes in microplastic levels, a phenomenon the scientists refer to as "pulse enrichment." Intense storms can wash microplastics from land into rivers and also stir up plastic that has accumulated in soil and riverbeds. The team warns that without better upstream controls on plastic production and waste management, the increasing frequency of extreme rainfall events, driven by climate change, could worsen these spikes and lead to even greater microplastic pollution in the oceans.