Plastic waste in the ocean - where does it all come from?

Reports of plastic pollution in the ocean first appeared in the scientific literature in the early 1970s, yet more than 40 years later, no rigorous estimates exist of the amount and origin of plastic debris entering the marine environment. In 1975, the estimated annual flux of litter of all materials to the ocean was 6.4 million tons [5.8 million metric tons (MT)], based only on discharges from ocean vessels, military operations, and ship casualties (1). The discharge of plastic from at-sea vessels has since been banned (2), but losses still occur. It is widely cited that 80% of marine debris originates from land; however, this figure is not well substantiated and does not inform the total mass of debris entering the marine environment from land-based sources.

Plastics have become increasingly dominant in the consumer marketplace since their commercial development in the 1930s and 1940s. Global plastic resin production reached 288 million MT in 2012 (3), a 620% increase since 1975. The largest market sector for plastic resins is packaging (3); that is, materials designed for immediate disposal. In 1960, plastics made up less than 1% of municipal solid waste by mass in the United States (4); by 2000, this proportion increased by an order of magnitude. By 2005, plastic made up at least 10% of solid waste by mass in 58% (61 out of 105) of countries with available data (5).

Plastics in the marine environment are of increasing concern because of their persistence and effects on the oceans, wildlife, and, potentially, humans (6). Plastic debris occurs on coastlines, in Arctic sea ice, at the sea surface, and on the sea floor (7, 8). Weathering of plastic debris causes fragmentation into particles that even small marine invertebrates may ingest (9). Its small size also renders this debris untraceable to its source and extremely difficult to remove from open ocean environments, suggesting that the most effective mitigation strategies must reduce inputs.

We estimated the annual input of plastic to the ocean from waste generated by coastal populations worldwide. We defined mismanaged waste as material that is either littered or inadequately disposed. Inadequately disposed waste is not formally managed and includes disposal in dumps or open, uncontrolled landfills, where it is not fully contained. Mismanaged waste could eventually enter the ocean via inland waterways, wastewater outflows, and transport by wind or tides. Estimates of the mass of plastic waste carried by particular waterways range from <<1 kg per day (Hilo, HI) to 4.2 MT (4200 kg) per day (Danube River) (10, 11). Because of their dependence on local watershed characteristics, these results cannot be easily extrapolated to a global scale.

Here we present a framework to calculate the amount of mismanaged plastic waste generated annually by populations living within 50 km of a coast worldwide that can potentially enter the ocean as marine debris. For each of 192 coastal countries with at least 100 permanent residents that border the Atlantic, Pacific, and Indian oceans and the Mediterranean and Black seas, the framework includes: (i) the mass of waste generated per capita annually; (ii) the percentage of waste that is plastic; and (iii) the percentage of plastic waste that is mismanaged and, therefore, has the potential to enter the ocean as marine debris (12) (data S1). By applying a range of conversion rates from mismanaged waste to marine debris, we estimated the mass of plastic waste entering the ocean from each country in 2010, used population growth data (13) to project the increase in mass to 2025, and predicted growth in the percentage of waste that is plastic. Lacking information on future global infrastructure development, the projection represents a business-as-usual scenario.

We estimate that 2.5 billion MT of municipal solid waste was generated in 2010 by 6.4 billion people living in 192 coastal countries (93% of the global population). This estimate is broadly consistent with an estimated 1.3 billion MT of waste generated by 3 billion people in urban centers globally (5). Approximately 11% (275 million MT) of the waste generated by the total population of these 192 countries is plastic. We expect plastic waste to roughly track plastic resin production (270 million MT in 2010) (3), with differences resulting from the time lag in disposal of durable goods (lifetime of years to decades), for example. Scaling by the population living within 50 km of the coast (those likely to generate most of the waste becoming marine debris), we estimate that 99.5 million MT of plastic waste was generated in coastal regions in 2010. Of this, 31.9 million MT were classified as mismanaged and an estimated 4.8 to 12.7 million MT entered the ocean in 2010, equivalent to 1.7 to 4.6% of the total plastic waste generated in those countries.

Our estimate of plastic waste entering the ocean is one to three orders of magnitude greater than the reported mass of floating plastic debris in high-concentration ocean gyres and also globally (14–17). Although these ocean estimates represent only plastics that are buoyant in seawater (mainly polyethylene and polypropylene), in 2010 those resins accounted for 53% of plastic production in North America and 66% of plastic in the U.S. waste stream (4, 18). Because no global estimates exist for other sources of plastic into the ocean (e.g., losses from fishing activities or at-sea vessels, or input from natural disasters), we do not know what fraction of total plastic input our land-based waste estimate represents.

Our framework was designed to compute, from the best-available data, an order-of-magnitude estimate of the amount of mismanaged plastic waste potentially entering the ocean worldwide. It is also a useful tool to evaluate the factors determining the largest sources of mismanaged plastic waste. The amount of mismanaged plastic waste generated by the coastal population of a single country ranges from 1.1 MT to 8.8 million MT per year, with the top 20 countries’ mismanaged plastic waste encompassing 83% of the total in 2010 (Fig. 1 and Table 1). Total annual waste generation is mostly a function of population size, with the top waste-producing countries having some of the largest coastal populations. However, the percentage of mismanaged waste is also important when assessing the largest contributors of waste that is available to enter the environment. Sixteen of the top 20 producers are middle-income countries, where fast economic growth is probably occurring but waste management infrastructure is lacking (the average mismanaged waste fraction is 68%). Only two of the top 20 countries have mismanaged fractions <15%; here, even a relatively low mismanaged rate results in a large mass of mismanaged plastic waste because of large coastal populations and, especially in the United States, high per capita waste generation.

Assuming no waste management infrastructure improvements, the cumulative quantity of plastic waste available to enter the marine environment from land is predicted to increase by an order of magnitude by 2025 (Fig. 2 and table S1). The predicted geographic distribution of mismanaged plastic waste in 2025 does not change substantially, although the disparity between developing and industrialized countries grows (table S2). For example, mismanaged plastic waste in the United States increases by 22%, whereas in the top five countries it more than doubles. The increase in these middle-income countries results from population growth, waste generation rates for 2025 that are consistent with economic growth (5), and a projected increase in plastic in the waste stream.

The analytical framework can also be used to evaluate potential mitigation strategies. For example, if the fraction of mismanaged waste were reduced by 50% (i.e., a 50% increase in adequate disposal of waste) in the 20 top-ranked countries, the mass of mismanaged plastic waste would decrease 41% by 2025. This falls to 34% if the reduction is only applied to the top 10 countries and to 26% if applied to the top 5. To achieve a 75% reduction in the mass of mismanaged plastic waste, waste management would have to be improved by 85% in the 35 top-ranked countries. This strategy would require substantial infrastructure investment primarily in low- and middle-income countries.

Alternatively, reduced waste generation and plastic use would also decrease the amount of mismanaged plastic waste. If per capita waste generation were reduced to the 2010 average (1.7 kg/day) in the 91 coastal countries that exceed it, and the percent plastic in the waste stream were capped at 11% (the 192-country average in 2010), a 26% decrease would be achieved by 2025. This strategy would target higher-income countries and might require smaller global investments. With a combined strategy, in which total waste management is achieved (0% mismanaged waste) in the 10 top-ranked countries and plastic waste generation is capped as described above, a 77% reduction could be realized, reducing the annual input of plastic waste to the ocean to 2.4 to 6.4 million MT by 2025 (table S3).

Sources of uncertainty in our estimates result from the relatively few measurements of waste generation, characterization, collection, and disposal, especially outside of urban centers. Even where data were available, methodologies were not always consistent, and some activities were not accounted for, such as illegal dumping (even in high-income countries) and ad hoc recycling or other informal waste collection (especially in low-income countries). In addition, we did not address international import and export of waste, which would affect national estimates but not global totals. Although national estimates are somewhat sensitive to the model predicting the percentage of mismanaged waste, the global estimate and ranking of top countries are not. The long-term projections are also sensitive to the model predicting growth of plastic in the waste stream; historical growth may not be a good indicator of future trends (12). The inclusion of the economic cost of implementation, as well as socio-cultural, environmental, and other factors that affect infrastructure development or behavioral change, would improve the evaluation of mitigation strategies (19).

We will not reach a global “peak waste” before 2100 (20). Our waste will continue to grow with increased population and increased per capita consumption associated with economic growth, especially in urban areas and developing African countries (see supplementary materials). Historically, waste management by burying or burning waste was sufficient for inert or biodegradable waste, but the rapid growth of synthetic plastics in the waste stream requires a paradigm shift. Long-term solutions will likely include waste reduction and “downstream” waste management strategies such as expanded recovery systems and extended producer responsibility (21, 22). Improving waste management infrastructure in developing countries is paramount and will require substantial resources and time. While such infrastructure is being developed, industrialized countries can take immediate action by reducing waste and curbing the growth of single-use plastics.

Supplementary Materials

www.sciencemag.org/content/347/6223/768/suppl/DC1

Materials and Methods

Supplementary Text

Fig. S1

Tables S1 to S6

References (23–30)

Data S1

References and Notes

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12. Materials and methods are available as supplementary materials on Science Online.
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Acknowledgements

We thank M. A. Barlaz and N. Starr for critical discussions and L. Amaral-Zettler, M. G. Chapman, G. Leonard, and R. Williams for helpful reading of the manuscript. We also thank the anonymous reviewers for their constructive comments. This work was conducted within the Marine Debris Working Group at the National Center for Ecological Analysis and Synthesis, University of California, Santa Barbara, with support from the Ocean Conservancy. Our data and model can be found at http://jambeck.engr.