Plastic in the ocean is bad news. Marine species become entangled in larger pieces, and the smaller fragments enter the food web. Recent studies showed that the average seabird has a belly full of plastic – equivalent to a human trying to digest 6 kg of plastic at any one time. "We don't know the effect that this has on animals, but at the very least it is going to waste the animal's energy," said Erik van Sebille of Imperial College London, UK.

Using 20 times more data than previous studies, van Sebille and colleagues estimated the amount and distribution of floating microplastic (pieces less than 20 cm wide). Compiling more than 11,000 observations of plastic collected in surface-trawling plankton nets, the team removed sampling biases and used ocean circulation models to extrapolate into regions with a paucity of measurements.

Due to a scarcity of data in most of the world's oceans there is large variability in the results, but van Sebille and his colleagues estimate that in 2014 between 15 and 51 trillion particles of plastic accumulated across the surfaces of the world's oceans, weighing between 93 and 236 tons.

"This is even more plastic waste than we had envisaged, but still it accounts for only 1% of the global plastic waste estimated to have entered the ocean in 2010," said van Sebille, whose findings are published in Environmental Research Letters (ERL).

More than half the plastic was in areas where the density of plastic was not that high, the scientists found, implying that plastic particles are more evenly distributed than previously assumed. As for the missing plastic, much of it is likely to be washing up on coastlines, sinking down to the seafloor, or ending up inside the stomachs of animals.

In a connected study, also published in ERL, van Sebille and student Peter Sherman used both surface and satellite-tracked buoy observations to simulate floating microplastic waste up to 2025. They assessed the sources of plastic and calculated the best places to focus on clean-up.

"We showed that China and South East Asia have the greatest plastic waste output at the moment, and that the optimal waste removal locations are off the coast of China and in the Indonesian Archipelago," said van Sebille. Around one third of the small plastic waste could be removed by 2025 using 29 floating plastic collectors operating at a 45% capture efficiency in these places, the pair’s model suggests. By contrast, the same plastic collectors focused on the Great Pacific garbage patch would only collect 17% of the total plastic waste by 2025.

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