Recent estimates of plastic entering our oceans are one to two orders of magnitude larger than the amount measured floating at the surface. What explains the discrepancy? Researchers from the University of Utrecht, Netherlands, have developed a 3D global marine mass budget model to address this question. They shared their findings in “Global mass of buoyant marine plastics dominated by large long-lived debris,” recently published in nature geoscience.
Their numerical model considered the time period between 1980 and 2020 and measured the amounts of plastic in surface water, beaches, and in the deep ocean. It used an array of variables, including the rate at which plastic washes ashore, breaks up into smaller pieces, and becomes covered in algae, making it heavier and sink to the bottom. The academics said their model stands out from other computer models because of the record number of measurements and observations included.
“Therefore, this model is more accurate," said lead author Mikael Kaandorp. "Until now, scientists were mainly looking at measurements of amounts of plastic in the upper layer of the water surface. We added counts of beach clean-ups in various places around the world, and observations of large floating plastic objects on the water, amongst other things. Those pieces are relatively scarce, but because they are heavy, they make up a large portion of the total amount of plastic in the ocean."
Another novel feature of the model is the wide size spectrum of plastic particles considered, from 0.1 mm to 1,600 mm, with microplastics being classified as less than 5 mm, and larger plastics as more than 25 mm.
Importantly, the study focused only on polyethylene (PE), polypropylene (PP), and polystyrene (PS), which have been shown to make up the majority of items in the ocean’s surface, deeper layers, and beaches. It did not consider polymers denser than seawater such as polyvinyl chloride (PVC) and polyethylene terephthalate, which are estimated to make up about 35% to 45% of the plastic mass entering the marine environment.
Results show that the largest fraction of plastic mass is located at the ocean surface, between 59% and 62%. This is in stark contrast with the widely held assumption that only 1% of the total amount of plastic in the oceans floats on the surface, supporting the theory that there is a ‘missing sink’ of marine plastic pollution.
The study found, instead, that the total amount of buoyant marine plastic litter is much higher than previous estimates, at around 3,000 kilotons to 3,400 kilotons. It also found that the majority of plastic mass is contained in large plastic items, around 90% to 98%, and that these constitute most of the total buoyant plastic mass.
“Our mass estimate for particles between 5 mm and 200 mm, 700 kilotons, is much higher than previously estimated (30.6 kilotons),” the authors wrote. “This is even more so for particles larger than 200 mm, where our estimate of 2,500 kilotons vastly exceeds the previously estimated 202.8 kilotons.” The difference can purportedly be explained by the very broad size of intervals used in previous studies, where large particles are likely to be under sampled, causing an underestimation of the total mass, the scientists claimed.
Moreover, results show that plastic input into the marine environment is around 460 kilotons to 540 kilotons per year, which is at least an order of magnitude less than previous estimates. This means that the new model denies the original assumption that there’s a considerable discrepancy between the amount of plastic entering the ocean and the amount floating on it. Large masses of plastic aren’t sinking into the bottom of the ocean. Rather, around 60% of plastic stays floating at the surface, mostly made up of larger, heavier stock.
“Our finding of a lower plastic input into the marine environment and a higher standing stock means that the residence time of plastics in the marine environment is much higher than previously estimated,” the scientists said. They calculated that in a scenario where there is a sudden stop of new plastics entering the marine environment in 2025, only 10% of the plastic mass would be removed within two years.
"It means that it will take much longer until the effects of measures to combat plastic waste will be visible,” said co-author Erik van Sebille. “It will be even more difficult to get back to the situation as it once was. Also, if we don't take action now, the effects will be felt for much longer,” he urged.