A mass budget and box model of global plastics cycling, fragmentation and dispersal in the land-ocean-atmosphere system

Since 1950 humans have introduced over 10 000 million metric tons (Mt) of plastic polymers into the Earth’s surface environment. Accounting for the dispersal and fate of mismanaged plastic waste and fragmented microplastics (MP) in the environment has been challenging. Recent studies have fueled debate on the magnitude of plastic transport from land to oceans, on the sinking and beaching of marine plastics and on the emission of marine microplastics to the atmosphere. In this study we define a global plastics cycle and budget, and develop a box model of plastics cycling, including the fragmentation and transport of MP within coupled terrestrial, oceanic and atmospheric reservoirs. We force the model with historical plastics production and waste data, and simulate global plastic and MP dispersal for different OECD policy scenarios towards 2060. Based on published observations, we suggest floating surface ocean plastics (2.1 Mt) are the tip of the marine plastics iceberg, as considerable amounts of plastics reside in the deep ocean (82 Mt), in shelf sediments (175 Mt), and on beaches (1.8 Mt). The model constrains current land to sea plastic transport to 14 Mt per year, implying 4 to 9 times larger leakage than OECD estimates. Model simulation of two ambitious policy scenarios show a peak in land to sea transport of total plastics of 23 Mt per year around 2045 and a decrease thereafter. Environmental concentrations of small MP remain high after 2060 due to continuous fragmentation of legacy mismanaged plastic waste on land and indicates the need for remediation of legacy plastic waste in policy instruments.