The world’s oceans might be vast, but they’re far from pristine—floating plastic debris is a common sight not only near beaches but also on the open sea. Roughly 500 million kilograms’ worth of discarded water bottles, fishing nets, and other plastic debris is making its way into the ocean each year, according to a new study. That’s far less than previous estimates, but it doesn’t change the reality that plastic pollution is increasing, researchers have suggested.
A Perplexing Discrepancy
In 2014, researchers estimated that more than 250 million kilograms of plastic debris was floating at sea. That’s about the weight of a fully loaded container ship. However, far, far more plastic pollution—anywhere around 5–50 billion kilograms—was likely entering the ocean every single year, other teams have estimated.
Even after accounting for some debris sinking over time or breaking into small pieces that are undetectable, that’s still a perplexing discrepancy, said Mikael Kaandorp, a physical oceanographer at the Forschungszentrum Jülich research center in Germany. It’d be kind of like a person earning a huge amount of money each year but having only a very little bit of savings, he said. “That doesn’t really make sense.”
Kaandorp and his colleagues set out to investigate that mystery. “We wanted to figure out why there was this huge mismatch,” Kaandorp said.
The Plastics We Know
The researchers focused on plastics that would initially float when they entered the water. Common examples include polyethylene, polypropylene, and polystyrene, which are familiar to most people, Kaandorp said. They’re used in everything from water bottles to packaging materials, he said. “They’re mainly consumer-based plastics.”
The team considered three primary sources of those plastics: coastlines, rivers, and fishing activity. To estimate how much debris was flowing into the ocean from coastlines, Kaandorp and his collaborators used data from a study that tabulated so-called mismanaged plastic waste produced in coastal areas. The researchers also amassed data from a different investigation that tabulated plastic pollution transported by rivers. Finally, Kaandorp and his colleagues scaled published estimates of fishing activity by different factors to assess how much fishing-related plastic was entering the ocean.
In total, the data sets that Kaandorp and his team relied on were based on more than 22,000 measurements of plastics recorded on beaches, on the surface of the ocean, and from the deep ocean.
Next, the researchers estimated a size distribution for all that plastic. Getting a handle on the debris’ dimensions was important because size affects, for instance, how a piece of plastic is transported in the water. The team assumed a size distribution based in part on measurements of plastic made at waste facilities and focused on items as small as 0.1 millimeter and as large as 1.6 meters.
Kaandorp and his collaborators then numerically modeled how all that plastic debris would move throughout the water column. “There are a couple of processes that we looked at that might remove plastics from the ocean surface over time,” Kaandorp said. For instance, marine algae might colonize the debris and weigh it down. Or ultraviolet light and wave action could cause the debris to fragment into smaller and smaller pieces that eventually become undetectable.
Less Debris, but It Sticks Around Longer
On the basis of observational data of the amount of plastics entering the ocean each year and model predictions of the rate at which that debris disappeared over time, Kaandorp and his collaborators concluded that about 500 million kilograms of initially buoyant plastic debris are entering the ocean each year. That’s less than a tenth of some of the most extreme previous estimates, the researchers noted. “We come up with quite different numbers,” Kaandorp said.
And though that might sound like good news, Kaandorp and his colleagues cautioned that that’s a whole lot of plastic in the ocean. The new calculations furthermore suggested that plastics tend to reside far longer in the environment than previously thought.
One earlier study estimated that if plastic pollution could somehow be stopped instantaneously, more than 95% of the plastic mass that’s currently floating would be removed from the surface within 1–2 years as the debris either sank or fragmented. But Kaandorp and his colleagues found a far less optimistic result: Only about 10% of the plastic mass would be removed over that same time interval.
The researchers also estimated that the amount of buoyant plastics entering the ocean is increasing by 4% each year. And thanks to the same principle behind compound interest, that growth rate has the potential to translate into significant change over time: In the absence of mitigation, the mass of plastics on the ocean’s surface will double by the 2040s. These results were published in Nature Geoscience.
This work was an impressive undertaking, said Kara Lavender Law, an oceanographer at the Sea Education Association in Woods Hole, Mass., not involved in the research. “It’s a much more sophisticated piece of modeling than has been done before.” Even so, uncertainties remain simply because our knowledge of ocean-borne debris isn’t perfect, said Law. “We don’t have nearly enough data on what’s actually out there in the ocean.”
There is a silver lining to these new findings, however, Kaandorp and his colleagues suggested. More than 90% of the plastics on the ocean’s surface by mass are relatively large—bigger than 25 millimeters, the team estimated. That offers some hope for cleanup efforts, Kaandorp said. “Big items are just a lot easier to clean up than small microplastics.”
—Katherine Kornei (@KatherineKornei), Contributing Writer