A comparative analysis of global models for riverine plastic input to the ocean

The plastic pollution crisis demands establishing a global science-policy framework to achieve a circular economy for plastics. Such a framework should be based on scientific evidence to evaluate the success of mitigating plastic pollution in terrestrial, freshwater, and marine environments. This work focuses on the role of rivers as main pathways connecting land-based plastic to the marine environment. Existing large-scale estimates of river plastic input to the ocean used different and contrasting choices in their modelling approaches, e.g., including highly variable number of rivers in the global outputs, differing item-to-mass conversion factors, and extrapolations from microplastic to macroplastic loads. We observed that estimates can diverge up to five orders of magnitude when global models are applied to individual rivers, denoting large uncertainties in the data and approaches used to extrapolate results for the most polluting rivers at World and European scales. These uncertainties would not allow for a quantitative assessment of the effectiveness of plastic mitigation measures, as the expected reduction of plastics in the environment would vary within a much lower range than the modelled estimates. The way forward to provide meaningful assessments involves collecting comparable data using harmonized sampling methods, increasing fundamental understanding of plastic transport and retention dynamics, and a better understanding of spatio-temporal variability of plastic transport in rivers.