The amount of plastic that reaches the sea is ten times less than previously thought and it is not good news

The good news is that some 500,000 tons of plastic find their way into the sea every year, according to a new study. That means reducing previous estimates by more than 10 times. The bad news is that that plastic lasts longer than it was supposed to, sometimes decades. The new work, published in the scientific journal Nature Geoscience, estimates that there are 3.4 million tons of plastics floating in the oceans, but there must be many more at the bottom or ingested by marine animals.

A good part of the 8.3 billion tons of plastic that humans have made since they learned how to do it has ended up in the oceans. The difficult thing is to give figures that are no longer exact, even approximate, of the problem. It is known almost to the gram how many tons of these polymers are produced per year: 461 million in 2020. The approximate amount that is recycled is also known. But here the certainties end: it is unknown how much plastic ends up burning in the landfill or in the rivers, and from there to the seas.

For a little over a decade, several organizations between environmentalists and scientists, such as 5Gyres, the Tara Ocean Foundation or Surfing for Science, have carried out expeditions or campaigns in which they collect the plastic they find on their journeys. Then they extrapolate based on the area and what was collected, and thus estimate the quantity and mass of plastic materials.

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Supported by a new mathematical model fed by 22,000 measurements of beaches, sea surfaces and depths, a group of scientists has now recalculated the key figures for the flow of plastic. Their goal was to put an end to the mystery of missing plastic, according to Mikael Kaandorp, from the Institute for Atmospheric and Marine Research at the University of Utrecht in the Netherlands, and first author of the new study. “Based on studies from 2014 and 2015, it was thought that between 4,000 and 12,000 kilotonnes (one kt. is 1,000 tons) of plastic reach the ocean each year, with only about 250 kilotonnes floating around,” he recalls. “This would mean that an incredible amount of plastic disappears every year, so we believe that our new numbers, 500 kilotonnes of input and around 3,400 kilotonnes of plastic in the water, make much more sense,” adds the researcher at the Institute of Bio and Geosciences IBG-3 from Jülich (Germany).

In the open sea, plastic floats ending up on islands of waste that are formed in ocean gyres thanks to ocean currents. In the image, a net from The Ocean Cleanup expedition full of plastics.the ocean cleanup

The new calculations detail the origin of marine plastic. Around 40% enters from the coast. “We can think of plastic waste that is poorly managed near the coast, as leaky landfills. Or maybe waste thrown into the sea by rain runoff in coastal cities or blown into the ocean by the wind,” Kaandorp details. But what stands out is the distribution of the rest. Almost half of the waste of these petroleum derivatives comes from fishing, mostly from their nets. And only 12% would come from the rivers.

As for the plastic that floats in the sea, they estimate that there must be between 3 and 3.4 million tons on the surface. The vast majority of this plastic is relatively large, more than 25 millimeters, and tends to be concentrated in the so-called plastic islands, collections of garbage that are formed by the play of sea currents in the so-called ocean gyres. The figure is more than double that estimated by other previous studies, which left it at a quarter of a million. But its size and concentration make it feasible, the authors of the paper believe, to remove all this artificial material from the sea.

But the worst must be at the bottom. According to the new work, almost half of the accumulated plastic from other years ends up on the seabed, forming part of the sediment at a rate of 220,000 tons per year. If they go back to the time when mass production began, in the 1950s, the researchers calculate that there must be 6.2 million tons down there that floated into the sea. It is in this transit between the surface and the bottom of the sea where the greatest unknowns remain. The problem is that, although water bottle caps still bearing the manufacturer’s name have been found in the arctic, it is not easy to trace the plastic.

José María Alsina, from the Maritime Engineering Laboratory of the Polytechnic University of Catalonia, uses the example of the water bottle: “It is not the same if it comes from the river or if it has been thrown from a boat. It is necessary to take into account where it ends, if on the shore, in the oceanic gyres or if it sinks. Degradation by solar radiation or erosion by sand are also involved. And there is the biofouling, the incrustation of microalgae in the plastic (which affects its buoyancy)… Everything complicates knowing where that bottle of water will end up”.

Despite its greater visibility, the percentage of plastic that ends up on the coastline is negligible when compared to that which goes to the bottom of the sea. In the image, cleaning work on the beach of Marunda, in Jakarta, Indonesia.Future Publishing / Getty

Previous studies have estimated that only around 3% of the plastic in the sea is floating on the surface. The vast majority would be at the bottom. The problem, as Alsina recalls, is that “more than 99% of the measurements have been made on the surface, with the floating plastic, there are hardly any data in depth.” Tracking the bottom requires special vehicles and technologies that are not in abundance. For example, the current work is supported by more than 20,000 measurements, but only 120 are data from the sea floor.

Another factor complicates the calculations: between the surface and the bottom there is a huge mass of water, thousands of meters deep, and nobody knows how much plastic is there. In principle, this material floats. But not always. Some of the modern plastics like PVC or PET containers can have a greater mass than salt water and sink slowly. A work published last year, reviewing previous research, estimated that there would be 170 million tons at the bottom of the sea, interspersed with the sediment (the estimate range is between 25 and 900 tons, which already shows the uncertainty that exists). .

neither floats nor sinks

The oceanographer from the King Abdullah University of Science and Technology (Saudi Arabia) Carlos Duarte led that work on plastic on the seabed. They then concluded that more than 95% of the mass of plastics that has entered the ocean is buried on the seabed, “which denotes very efficient sedimentation processes, the nature of which remains to be resolved and which this article does not clarify.” comments Duarte, who is surprised by some of the figures they have obtained. Duarte’s study also mentioned processes about which hardly anything is known and that cause there to be plastic that neither floats nor sinks: in their cycle in the sea, the little pieces of plastic can sink under the weight of the biofilm that forms on its surface. But as it sinks, the amount of light it receives decreases, which would kill the algae. In addition, these biological materials are formed mainly by salicates and carbonates, which dissolve in cold water. So there could be a huge amount of plastic dancing up and down without counting.

One of the greatest experts in this matter of plastic in the sea is the biologist from the University of Cádiz Andrés Cózar. A pioneer in counting this material in the oceans, he was the first to raise the mystery of the missing plastic. For Cózar, the problem with Kaandorp’s work is the usual one: “There are two ways to make these numbers fit together. One is by making the residence time of the plastic on the ocean surface very short, that is, the plastic that enters is quickly removed to the coasts and the bottom. This has been common in previous studies. The second way to adjust numbers is that the inputs from rivers are much less than those estimated up to now. And this is what this new study uses. Mathematically, both options are valid.

The problem, according to Cózar, “is that we still do not have calculations of plastic load on the surface, nor of inputs from rivers, precise enough to opt for one option or the other.” “In other words, the range of uncertainty is so wide that it allows both mathematical adjustments,” he summarizes. But if the latest calculations from Kaandorp’s work are valid, this implies that they “indirectly locate large amounts of plastic inland,” according to Cózar.

The arrival of new plastic into the sea could be much less than previously believed, but the contribution, according to the calculations of the Kaandorp group, is growing at a rate of 4% per year. If nothing is done to reduce or remove it, the amount of plastic floating in the sea, be it a lot or a little, will double in two decades. And if they are right in their numbers, they will remain as macroplastic, microplastic or simple plastic on the surface, on the beaches, at the bottom or dancing in medium depths for years.

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