We’re starting a multipart series on the problem of plastics in the world’s oceans to find out the scope of the issue, whee and why plastics are reaching the ocean and what we can do to stop it.

This series is based upon articles and data presented by Hannah Ritchie a Senior Researcher and Head of Research at Our World In Data. These articles would not be possible with their hardworking and generous sharing policies.

Part 1: What’s Going On?

80% of the world’s ocean plastics enter the ocean via rivers and coastlines. The other 20% come from marine sources such as fishing nets, ropes, and fleets. To tackle plastic pollution we need to know where these plastics are coming from. Previous studies suggested that a very small number of rivers were responsible for the vast majority of ocean plastics: 60% to 90% of plastics came from only ten rivers. 

Higher-resolution mapping and consideration for factors such as climate, terrain, land use, and distance to the ocean suggests that many smaller rivers play a bigger role than we thought. It takes 1,600 of the biggest emitting rivers to account for 80% of plastic inputs to the ocean.

It is estimated that 81% of ocean plastics come from Asian rivers. The Philippines alone contribute around one-third of the global total. Since the number of contributing rivers is much higher than previously thought, we will need global efforts to improve waste management and plastic collection rather than targeting only a few of the largest rivers.

Where is This Plastic Coming From?

Previous studies suggested that most plastics come from only a few of the world’s rivers: one study estimated that the top ten rivers were responsible for 50% to 60%; another for more than 90%.³

The latest research, which was just published in Science Advances, updates our understanding of how these plastics are distributed.⁴ Lourens Meijer et al. (2021) developed higher-resolution modeling of global riverine plastics. They found that rivers emitted around 1 million tonnes of plastics into the oceans in 2015 (with an uncertainty ranging from 0.8 to 2.7 million tonnes). Around one-third of the 100,000 river outlets that they modeled contributed to this. The other two-thirds emitted almost no plastic to the ocean. It’s an important point because we might think that most, if not all, rivers are contributing to the problem. This is not the case.

But, importantly, the latest research suggests that smaller rivers play a much larger role than previously thought. In the chart we see the comparison of the latest research (in red) with the two earlier studies which mapped global riverine inputs. This chart shows how many of the top-emitting rivers (on the x-axis) make up a given percentage of plastic inputs (y-axis). Note that the number of rivers on the x-axis is given on a logarithmic scale.

We see that the latest research suggests that the top ten emitting rivers contribute a much smaller amount than previously thought: just 18% of plastics compared to 56% and 91% from previous studies. And to account for 80% of river plastics we need to include the top 1,656 rivers. This compares to previous studies which suggested the largest five or 162 rivers were responsible for 80%.

This makes a massive difference to how we tackle plastic pollution. If five rivers were responsible for most of the problem then we should focus the majority of our efforts there. A targeted approach. But if this comprises thousands of rivers we’re going to need to cast a much wider net of mitigation efforts.

Why are the latest results so different? The earlier studies relied on simpler models of waste generation within the world’s river basins. They took assessments of the amount of mismanaged waste generated in each basin; population density in the area; and correlative models of plastic concentrations in surface rivers. They then used these relationships to model the amount of pollution from basins where recorded data on river plastics was not available. This meant that the biggest emitters were believed to be the very large river basins home to large populations and poor waste management practices. The Yangtze, Xi, and Huangpu rivers in China; the Ganges in India; Cross in Nigeria; and the Amazon in Brazil were the big rivers that topped the list.⁵

The latest analysis builds on this earlier research with much higher-resolution data. It models the deterministic drivers of how plastic is transported using wind and precipitation patterns and river discharge data – plus factors that affect the probability of plastic first entering rivers, then entering the ocean such as the proximity of populations to the river; distance to the ocean; the slope of the terrain, and types of land use. This higher-resolution data was based on several years of research and calibrations of these models across 66 rivers in 14 countries.

This higher-resolution data shows that these factors that affect the probability of plastics not only reaching the river but then also reaching the ocean play a much more important role than the size of the river basin itself. This means many smaller rivers play a bigger role than we thought.