The world faces a critical waste crisis fueled by escalating waste generation and unsustainable waste management practices. This poses substantial threats to the environment, climate, and human health. Global municipal solid waste (MSW) generation is projected to increase significantly by 2050, ranging from 20% to 68% depending on socioeconomic pathways. The increasing complexity of waste composition, coupled with stagnant waste treatment levels, exacerbates these negative consequences. Currently, a substantial portion (64%) of global MSW is mismanaged, leading to open burning, dumping, and scattering of waste, which significantly impacts both terrestrial and aquatic ecosystems. Land-based waste is the primary source of marine litter, with plastic waste accounting for 80%. While various initiatives address plastic pollution, including amendments to the Basel Convention and UN resolutions, effective waste management systems remain crucial to prevent waste leakage into the environment. Existing research often focuses on plastic waste, overlooking the broader issue of inadequate waste management systems. This study addresses this gap by providing a comprehensive global assessment of future waste leakage scenarios, considering various socioeconomic pathways and the potential of circular waste management systems to mitigate this issue.

Current discussions surrounding marine litter primarily center on plastic waste (macro- and microplastics) due to its toxicity and harmful effects on human health and aquatic life. Global efforts to combat plastic pollution include the 2019 Basel Convention amendments and the 2022 UN Environment Assembly resolution. Many countries have implemented legislation to regulate plastic bags and single-use plastics. However, global and regional studies on marine litter predominantly focus on plastic, neglecting the underlying problem of inefficient waste management. Estimates of plastic waste entering the oceans vary significantly across studies, reflecting differences in methodologies and scope. Some studies track macroplastics throughout their lifecycle, while others assess micro- and macroplastics across a product's entire lifecycle. Existing research also highlights the heterogeneity of the litter problem, necessitating sub-national approaches for effective solutions. Studies examining the impacts of mismanaged plastic waste on various regions, such as the Carpathian region, Jakarta, and Bandung, underscore the local and regional variations in plastic pollution. Existing research lacks comprehensive global analyses that combine socioeconomic pathways, waste generation/management storylines, and spatial analysis to project future waste leakage and assess the mitigation potential of circular waste management systems.

This research combines a recently developed method for assessing global MSW generation and composition with spatial analysis to identify MSW leakage hotspots in aquatic environments. The study uses the IIASA-GAINS model, a framework with global coverage (180 countries/regions) and five-year temporal resolution. The model differentiates between urban and rural areas, considering disparities in lifestyles, income, and resource consumption. Five future socioeconomic pathways (SSPs) are analyzed, each with a "Baseline" and a "Maximum Technically Feasible Reductions" (MFR) scenario. The Baseline scenario reflects waste-related legislation up to 2018. The analysis goes beyond plastic waste, quantifying MSW leakage across eight waste streams. MSW generation and composition are estimated using elasticities representing four different income averages, assuming that MSW composition depends on average national income levels. The amount of scattered MSW (uncollected waste) is calculated by subtracting open-burned waste from uncollected MSW, with adjustments for food waste potentially used as animal feed or composted. Potential MSW reaching rivers, lakes, and coastal areas is determined based on the amount of uncollected scattered MSW generated by populations within 1 km of aquatic systems. Four buffer zones (250m intervals) are used, each with a leakage factor reflecting MSW fate based on distance. While the same fate factors are applied to all streams, the study acknowledges that different streams may behave differently based on their characteristics and environmental conditions. The baseline scenarios assume the implementation of existing MSW management legislation up to 2018. Mitigation scenarios are developed based on SSP narratives, representing circular MSW management systems according to the EU's waste management hierarchy, including waste reduction policies (food and plastic), maximum feasible recycling rates, incineration with energy recovery (after recycling capacity is reached), anaerobic digestion for food and garden waste, landfill diversion, and dumpsites upgrading. A Monte Carlo simulation and sensitivity analysis were conducted to assess uncertainty, focusing on uncollected waste as the most impactful parameter.

In 2020, global MSW generation was estimated at approximately 2560 Mt, projected to increase to 3320-3790 Mt by 2040 depending on the socioeconomic pathway. Food waste comprised 43% of MSW in 2020. Scattered MSW accounted for about 14% (350 Mt) of total global MSW generation in 2020, with China, South Asia, Africa, and India contributing over 87%. Food waste constituted the largest fraction of scattered waste (52%) in 2020. Urban areas accounted for 70% of scattered MSW in 2020. Scattered MSW is expected to rise to 427-475 Mt by 2040 if current management practices persist. The study estimates that 74 Mt of MSW potentially reached rivers in 2020 (21% of global scattered MSW, 3% of global MSW generation), with China, South Asia, Africa, LCAM, and India accounting for 80%. Urban areas accounted for 70% of this leakage. Potential food waste leakage into rivers was 40 Mt, and plastic waste leakage was 7.4 Mt. The study estimated that 1.35 Mt of MSW entered lakes in 2020, with Africa and China responsible for 55%. 5.79 Mt of scattered MSW near coastlines potentially entered seas in 2020. Overall, 80.8 Mt of MSW leaked into aquatic environments in 2020, projected to increase by up to 36% by 2040 under baseline scenarios. Mitigation scenarios show that implementing circular waste management systems, focusing on waste reduction and increased recycling, significantly reduces scattered MSW. Even under the most optimistic scenario (SSP1_MFR), achieving the UN's 2030 waste-related SDGs is unlikely, highlighting the need for stronger MSW reduction strategies. The study also identifies significant regional variations in MSW leakage, with South Asia, China, Africa, and India accounting for a large proportion of potential leakage.

This study's findings underscore the critical role of circular waste management systems in mitigating MSW leakage into aquatic environments. The strong correlation between waste collection rates and MSW leakage highlights the importance of improving waste collection infrastructure and practices, particularly in regions with high levels of mismanaged waste. The results show that focusing solely on one waste stream (e.g., plastics) or one strategy (e.g., recycling) is insufficient; a holistic approach is needed. The significant regional disparities emphasize the necessity of tailored strategies that address the specific challenges faced by different regions. The study also demonstrates that even under the most favorable scenarios, achieving the UN's 2030 waste-related SDGs requires a significant acceleration of waste reduction and management efforts. This necessitates global cooperation, enhanced policies, increased public awareness, and improved waste management infrastructure.

This study demonstrates the crucial role of adopting circular MSW management systems in significantly reducing waste leakage into aquatic environments. While even the most optimistic scenarios fall short of achieving waste-related SDGs by 2030, this research provides critical evidence for the urgent need for enhanced waste management strategies. Future research should focus on further refining the models to incorporate additional environmental, meteorological, and geographical variables and developing a global standardized MSW reporting framework to reduce uncertainty and improve the effectiveness of mitigation strategies.

This study has several limitations. The accuracy of MSW generation, composition, and management estimates relies on existing data sources which may contain uncertainties. Assumptions are made due to data gaps in certain areas, potentially affecting the accuracy of scattered MSW estimations. The study's model applies the same fate factors to all waste streams, though actual behavior may vary. The use of a fixed share of urban and rural population living near water bodies across all years and scenarios due to data limitations may result in under- or overestimation of MSW leakage. The model also doesn't account for MSW leakage during collection and transport or in dumpsites. Despite these limitations, the study robustly demonstrates the pivotal role of circular waste management in reducing MSW leakage.