The good news is that around 500,000 tons of plastic enter the ocean every year, according to a new study. This means that previous estimates have to be reduced by more than tenfold. The bad news is that this plastic lasts longer than intended, sometimes even decades. The new work, published in the journal Nature Geoscience, estimates that 3.4 million tons of plastic are floating in the oceans, but many more must be lying on the bottom or being ingested by marine life.
A good chunk of the 8.3 billion tons of plastic that humans have produced since we learned how to do it has ended up in the oceans. The difficult thing is to come up with numbers that no longer accurately or even approximate 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 this is where the certainties end: It is not known how much plastic ends up in landfills or in rivers and from there into the seas.
For a little over a decade, several environmentalist and scientific organizations such as 5Gyres, the Tara Ocean Foundation or Surfing for Science have been running expeditions or campaigns collecting the plastic they find on their travels. They then extrapolate based on the area and the amounts collected, estimating the amount and mass of the plastic material.
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Using a new mathematical model based on 22,000 measurements of beaches, sea surfaces and depths, a group of scientists has now recalculated the key figures for plastic flow. According to Mikael Kaandorp of the Institute for Atmospheric and Marine Research at Utrecht University in the Netherlands and first author of the new study, their goal was to put an end to the missing plastic mystery. “Based on the 2014 and 2015 studies, it was thought to be between 4,000 and 12,000 kilotons [una kt. son 1.000 toneladas] About 250 kilotons of plastic end up in the sea every year,” he recalls. “That would mean that an incredible amount of plastic disappears every year, so we think our new numbers, 500 kilotons of input and around 3,400 kilotons of plastic in the water, make a lot more sense,” adds the researcher at the Institute of Bio and Earth Sciences IBG- 3 from Julich (Germany).
In the open sea, plastic swimmers end up on islands of garbage that are created by ocean currents in ocean eddies. In the picture, a net full of plastic from the Ocean Cleanup expedition
The new calculations provide information about the origin of marine plastic. About 40% come from the coast. “We can think of plastic waste that is poorly disposed of near the coast as leaky landfills. Or maybe rubbish thrown into the sea by rainwater runoff in coastal cities, or blown into the sea by the wind,” Kaandorp elaborates. But what stands out is the distribution of the rest. Almost half of the waste from these petroleum derivatives comes from fishing, mainly from their nets. And only 12% comes from the rivers.
They estimate that there must be between 3 and 3.4 million tons of plastic in the ocean at the surface. The majority of this plastic is relatively large, over 25 millimeters, and tends to be concentrated on the so-called plastic islands, accumulations of garbage that are created by the play of the sea currents in the so-called ocean eddies. The number is more than twice the estimate of a quarter of a million in other previous studies. But its size and concentration make it possible, according to the paper’s authors, to remove all this artificial material from the sea.
But the worst has to be at the bottom. According to the new work, almost half of the plastic accumulated in other years ends up on the seabed and becomes part of the sediment at a rate of 220,000 tons per year. Going back to when mass production began, in the 1950s, the researchers reckon there were 6.2 million tons floating in the sea down there. In this transition between the sea surface and the sea floor, the greatest unknowns remain. The problem is that while water bottle caps with the maker’s name have still been found in the Arctic, it’s not easy to trace the plastic.
José María Alsina, from the Laboratory of Marine Engineering at the Polytechnic University of Catalonia, cites the water bottle as an example: “It’s not the same whether it comes out of the river or it’s thrown off a boat.” You have to take into account where it lands, whether on the shore, in the sea eddies or whether it sinks. Degradation by solar radiation or erosion by sand also play a role. And then there is biofouling, the encrustation of microalgae in the plastic [que afectan a su flotabilidad]… Everything makes it difficult to know where the bottle of water will end up.”
Despite the greater visibility, the proportion of plastic that ends up on shore is negligible compared to that that ends up on the ocean floor. Pictured cleaning work on Marunda Beach in Jakarta, Indonesia.Future Publishing/Getty
Previous studies have estimated that only about 3% of the plastic in the ocean floats on the surface. The vast majority would be at the bottom. The problem, Alsina recalls, is that “more than 99% of the measurements were taken at the surface, with the floating plastic there is little data at depth.” Tracking the bottom requires specialized vehicles and technologies, which are not available in abundance are. For example, the current work is supported by more than 20,000 measurements, but only 120 of these are seafloor data.
Another factor complicates the calculations: Between the surface and the ground is a huge mass of water, thousands of meters deep, and no one knows how much plastic is in it. In principle, this material floats. But not always. Some of the modern plastics such as PVC or PET containers can have a greater mass than salt water and can sink slowly. A paper reviewing previous research published last year estimated that there would be 170 million tons on the sea floor, interspersed with sediment (the estimated range is between 25 and 900 tons, already showing the existing uncertainty).
neither swims nor sinks
King Abdullah University of Science and Technology (Saudi Arabia) oceanographer Carlos Duarte led this work on plastic on the sea floor. They then concluded that more than 95% of the mass of plastic that entered the ocean is buried on the seabed, “indicating very efficient sedimentation processes, the nature of which has yet to be elucidated and which is not covered in this article.” be clarified,” she comments Duarte, who is surprised by some of the figures obtained. Duarte’s study also mentioned processes about which little is known that lead to the existence of plastic that neither floats nor sinks: as they cycle in the sea, the small pieces of plastic can, under the weight of the biofilm that forms on their surface, sink. But as it sinks, the amount of light it receives decreases, which would kill the algae. In addition, these biological materials consist mainly of salicates and carbonates, which dissolve in cold water. So there could be a huge amount of plastic bobbing up and down uncountable.
One of the greatest experts on the subject of plastic in the sea is the biologist Andrés Cózar from the University of Cádiz. As a pioneer in counting this material in the oceans, he was the first to solve the mystery of the missing plastic. For Cózar, the problem with Kaandorp’s work is the usual one: “There are two ways to put these numbers together. On the one hand, the residence time of the plastic on the sea surface is kept very short, ie the plastic that has penetrated is quickly transported to the coasts and to the seabed. This was common in previous studies. The second way to adjust the numbers is that inputs from the rivers are much lower than previously estimated. And that’s exactly what this new study uses. Mathematically, both options are valid.
The problem, according to Cózar, “is that we still don’t have calculations of surface plastic pollution or river inputs that are precise enough to make us choose one option over the other.” “In other words, the uncertainty range is so large that it allows for both mathematical fits,” he summarizes. But if the latest calculations from Kaandorp’s work are accurate, it means they are “indirectly locating large amounts of plastic inland,” according to Cózar.
The amount of new plastic entering the sea could be much lower than previously thought, but the contribution is growing at 4% a year, according to calculations by the Kaandorp Group. If nothing is done to reduce or remove it, the amount of plastic floating in the ocean, be it much or little, will double in two decades. And if their number is right, they remain for years as macroplastics, microplastics or simple plastics on the surface, on the beaches, on the bottom or dancing at medium depths.
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