Assessment Methods

Land to River, Lake, and Ocean Leakage

Borrelle et al. (2020) update the annual amount of mismanaged plastic waste entering aquatic ecosystem (covering oceans, rivers, and lakes) from 2016 to 2030 in 173 countries. Applying a methodology similar to that of  Jambeck et al. (2015), the estimation integrates expected population growth, annual waste generation per capita, as well as proportion of plastic waste and mismanaged waste. Those variables were integrated using a distance-based probability function, considering the spatially explicit waste generation and downhill flow accumulation.

The leakage in 2016 becomes the baseline estimation (Table 2), while the leakage in 2030 is estimated for three scenarios: (1) business as usual, in which waste generation and plastic production follow current trajectories; (2) ambitious, which draws upon existing global commitments in reducing the leakage; and (3) target (<8 million metric tonnes), estimated in 2010 by Jambeck et al. (2015). Russia tops the list, while two East Asia countries (China and Japan) and five ASEAN countries (Indonesia, Thailand, the Philippines, Myanmar, and Viet Nam) are included in the top 20. Under the business-as-usual scenario, the global estimated leakage will reach up to 90 million metric tonnes/year by 2030.

River to Ocean Leakage

Meijer et al. (2021) recently estimated that amongst 31,904 rivers in 163 countries, more than 1,500 rivers account for 80% of global plastic waste leakage. The global leakage of 0.8 million–2.7 million metric tonnes/year estimated by Meijer et al. (2021) is far below the amount estimated by Jambeck et al. (2015) in 2010. The reason behind the lower estimate is not the reduction of single-use plastics or the improvement of waste management systems but the estimation methodologies. In addition to common variables, such as population, waste generation per capita, and proportion of mismanaged waste, Meijer et al. (2021) utilised a probabilistic model that considered additional variables, including land use, terrain slope, wind, and precipitation. The model was then calibrated and validated against recent field observations from 2017 to 2020. Despite the difference, the results find that ASEAN countries remain as main contributors.

Table 3 lists five ASEAN countries as the top 10 contributors. Amongst these countries, the largest contributor is the Philippines, with seven rivers in the top 10 plastic-emitting rivers (Table 4), followed by Malaysia (3rd), Indonesia (5th), Myanmar (6th), Viet Nam (8th), and Thailand (10th).

The results are consistent with Lebreton et al. (2017) and Schmidt et al. (2017), who found that 1.15 million–2.41 million metric tonnes and 0.41 million–4 million metric tonnes, respectively, of plastic flows from rivers to oceans annually. The top 20 polluting rivers were mostly in Asia (Table 5) and accounted for more than two-thirds (67%) of the global leakage (Lebreton et al., 2017). Amongst the top 20, seven rivers—Brantas (6th), Pasig (8th), Irrawaddy (9th), Solo (10th), Mekong (11th), Serayu (14th), and Progo River (19th)—are in ASEAN countries.

Using underlying mismanaged plastic waste data similar to that used by Lebreton et al. (2017), Schmidt et al. (2017) concluded that 10 rivers  (Table 6) account for 88%–95% of global leakage to the ocean. Eight out of the 10 rivers are in Asia, including the Mekong (10th), which flows through five ASEAN countries. The estimated leakage is higher because Schmidt et al. (2017) compiled a larger data set and treated microplastic and macroplastic separately. Reducing plastic leakage by 50% in the 10 top-ranked rivers would reduce total river-based leakage by 45%.

Recent estimation by Meijer et al. (2021) shows a significant increase of leakage from several rivers in the Philippines, compared with the estimation by Lebreton et al. (2017). For instance, the amount of leakage from the Pasig River increases more than 60%, from only 0.039 million metric tonnes/year in 2017 (Table 5). Rivers in ASEAN countries, especially in the Philippines, have toppled rivers in China from the top spot as emitters of plastic to oceans since Meijer et al. (2021) considered spatial variability of the amount of mismanaged plastic waste within a river basin and utilised climate and terrain characteristics to differentiate the probability of leakage. With these assumptions, relatively small river basins, including those in ASEAN countries (e.g. the rivers in the Philippines), contribute proportionally more leakage than larger river basins, where the amount of mismanaged plastic waste is similar but located further upstream. Meijer et al. (2021) answer the limitation on Lebreton et al. (2017) and Schmidt et al. (2017), who overestimated the leakage from large rivers and underestimated the leakage from smaller rivers due to the exclusion of those important assumptions. The trend is a backstep and a warning that the region must reduce marine plastic debris.

Harmonised Methodology to Support Effective Countermeasures

Although several estimations have been undertaken to capture plastic leakage into the marine environment, more data should be estimated periodically. The results of estimations differ from one another depending on the scope and methodologies applied. To avoid any underestimation, harmonising the estimation methodologies is important. With harmonised methodology, data can be compared and validated against each other. Using a larger set of data, as done by Schmidt et al. (2017) and Borrelle et al. (2020), will further increase estimations’ accuracy. However, some countries might not have country-specific data, so that the data estimated using a proxy value with some assumptions and a certain level of uncertainties might lead to underestimation. For instance, Schmidt et al. (2017) extended by 41 countries the estimation of Jambeck et al. (2015) of mismanaged plastic waste generation rate from 192 coastal countries. The waste generation rate and plastic composition for these 41 countries were taken from Hoornweg and Bhada-Tata (2012) based on past regional estimation, while the mismanaged plastic waste was calculated based on average values for each World Bank economic classification (high income, upper middle income, lower middle income, or low income). In most developing countries, including India (Nandy et al., 2015), where plastic waste is mostly recovered by the informal sector, a significant amount of plastic waste is excluded in such estimation.

To address this issue, government shall support such research by monitoring leakage in rivers and providing valid waste management data regularly. The lack of actual waste management data, especially in ASEAN countries, might lead to lower or higher leakage estimations, depending on the proxy data. The lower estimation by Meijer et al. (2021) does not necessarily mean the reduction of leakage. By utilising the appropriate harmonised methodology and supported by valid data from the respective governments, various policies can be formulated and/or evaluated to create effective countermeasures against marine plastic leakage.