Plastic litter in oceans is overestimated, study says

There is much less plastic littering the ocean than scientists previously thought, according to a new published in the journal Nature Geoscience.

The study is from scientists from the University of Utrecht in the Netherlands, who developed a new numerical model to estimate the amount of plastic in the ocean. The study found that around 500,000 metric tons of plastic end up in the sea every year, with half coming from land and the rest coming from the fishing industry’s equipment.

This total is much less than the four to 12 million tons per year that some studies have estimated.

The results are based on a 3D model of the ocean using a huge amount of observational data and measurements taken from surface water, beaches, and the deep ocean from 1980 to 2020.

The modeling study estimated that pieces of plastic bigger than 25 millimeters (one inch) make up over 95 per cent of plastic floating on the ocean; and that, while most plastic particles in the ocean are very small, the total mass of these microplastics – defined as less than five millimeters (0.2 inches) – is relatively low.

The study found that the largest fraction of plastic mass, between 59 per cent and 62 per cent, is located at the ocean surface. This is in stark contrast with the widely held assumption that only about one per cent of the total amount of plastic in the oceans floats on the surface. That theory posits that there is a “missing sink” of plastics pollution in deeper waters.

The new 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 per cent to 98 per cent, and that these constitute most of the total buoyant plastic mass.

The fact that most of the waste plastic is floating around in large pieces could with help clean-up efforts, the study noted. “Large, floating pieces on the surface are easier to clean up than microplastics,” the study’s co-author Erik van Sebille of Utrecht University in the Netherlands said in a statement.

“Recent estimates of the oceanic input of plastic are one to two orders of magnitude larger than the amount measured floating at the surface,” the study said. “This discrepancy could be due to overestimation of input estimates, processes removing plastic from the surface ocean or fragmentation and degradation. Our model estimates an ocean plastic input of about 500 kilotonnes per year, less than previous estimates.”

The study focused only on polyethylene, polypropylene, and polystyrene, which 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 PVC and PET, estimated to make up between 35 per cent to 45 per cent of the plastic mass entering the marine environment.

The new study can be read at this link.