Once a plastic bag reaches the ocean, how long does it hang around at the surface? And when the bag starts to disintegrate, what happens to it next? Questions like these are notoriously hard to answer, because Earth’s oceans are huge, and plastic rubbish is unevenly spread and comes in a variety of shapes and sizes. However, after modelling ocean plastic a study indicates that the ocean surface could be plastic-free within an average of three years if we were to stop plastic entering the oceans tomorrow.
Recently it was revealed that just ten river systems transport more than 90% of plastic waste to the world’s seas. These large rivers, including the Nile and the Ganges, run through heavily populated regions where littering is common. The rivers act as superhighways, transporting plastic waste swiftly out to sea.
But once this plastic gets to the sea we have very little idea of what happens next. The huge whirlpool of plastic found in the North Pacific Gyre – often called ‘the Great Pacific garbage patch’ – shows that waste concentrates in some areas. Meanwhile, marine surveys indicate that plastic breaks down and sinks at different rates, depending on the size, density and shape of the piece of plastic. Beyond this very little is known about the fate of plastic entering our oceans, or indeed how much of the plastic we see bobbing the waves today is rubbish from the distant past.
Albert Koelmans from Wageningen University in The Netherlands and his colleagues took a systems engineering analytical approach to create a whole ocean mass balance model of plastic in the oceans. Using estimates of the amount of plastic entering the oceans each year, along with mathematical equations to calculate how quickly plastic fragments and sinks, the scientists gained a global perspective of what happens to the plastic in our oceans.
Their results suggest that 99.8% of all plastic that had entered the oceans since 1950 had sunk below the surface by 2016. For the year 2016, the model predicted that of the 0.309 million tons of plastic in the surface layer, 83.7% was macro-plastic, 13.8% micro-plastic and 2.5% nano-plastic – less than 0.335mm in diameter. If all plastic disposal to the oceans was stopped tomorrow, the model predicts that ocean surfaces would be near plastic-free within three years.
“It is important to emphasize that these figures are averages,” said Koelmans. “Small plastics will sink earlier, whereas floating sun-loungers will take much longer to fragment into settleable particles.” The findings are published in Environmental Research Letters (ERL) .
Getting a handle on what happens to plastic waste when it reaches the ocean is important if we want to optimise our strategies for tackling the plastic problem in future. “Our model suggests that the response time for plastic in the ocean is actually quite short, which favours strategies to reduce emissions of plastic, over strategies that aim to clean up the plastic that is already there,” said Koelmans.
However, Koelmans and his colleagues stress that more data is needed to understand plastic behaviour in the water better. They intend to explore the impact of cleaning up ocean plastic in their next version of the model.