Ocean plastic is still a relatively new problem. Scientists only began studying it about 40 years ago, and the first major ocean "garbage patch" wasn't discovered until the 1990s. It's now common knowledge, but there's still a lot we don't know about it. How much plastic actually ends up in the ocean during a given year? How exactly does it get there? And what, if anything, can we do about it?

Many of these mysteries are now clearer thanks to a new study published Feb. 13 in the journal Science. It offers the best estimate yet of the plastic influx into Earth's oceans, along with insight into where all the trash is coming from and how it escapes land. And by revealing the pathways plastic takes to the sea, the study's authors may also be shedding light on how we can start to stem the tide.

Between 4.8 million and 12.7 million metric tons of plastic entered the ocean in 2010, according to the study, which traced plastic waste from 192 coastal countries around the world. That suggests the oceans take in roughly 8 million metric tons of plastic in a typical year, says lead author and University of Georgia environmental engineering professor Jenna Jambeck in a statement about the research.

"Eight million metric tons is the equivalent to finding five grocery bags full of plastic on every foot of coastline in the 192 countries we examined," she adds.

While another recent study concluded the oceans now contain more than 5 trillion pieces of plastic — totaling about 250,000 metric tons — the annual pace of this pollution has remained unclear. A 1975 study estimated about 0.1 percent of global plastic production finds its way to sea every year, but Jambeck's study suggests that number is actually between 1.5 and 4.5 percent.

"For the first time, we're estimating the amount of plastic that enters the oceans in a given year," says co-author Kara Lavender Law, a professor at the Massachusetts-based Sea Education Association. "Nobody has had a good sense of the size of that problem until now."

ocean plastic

Illustration: Lindsay Robinson/University of Georgia

The main culprit behind ocean plastic is mismanagement of plastic waste in coastal areas, the researchers found, generated by the 2 billion people who live within 50 kilometers (30 miles) of a coastline. Part of the problem is that waste-management infrastructure has lagged behind the planet's booming plastic production, especially in developing countries. Some of the 192 countries studied have no formal waste-management systems, and Jambeck notes that dealing with solid waste often takes a backseat to more urgent public-health priorities like clean water and sewage treatment.

"The human impact from not having clean drinking water is acute, with sewage treatment often coming next," she says. "Those first two needs are addressed before solid waste, because waste doesn't seem to have any immediate threat to humans. And then solid waste piles up in streets and yards and it's the thing that gets forgotten for a while."

Eleven of the top 20 countries for plastic pollution are in Asia, the study found, with China at No. 1. Other countries in the top 20 include Brazil, Egypt and Nigeria — and the U.S. at No. 20. The U.S. has well-developed infrastructure for managing solid waste, but it also has dense coastal populations who use a lot of plastic. About 40 percent of the total U.S. population lives in coastal counties, according to a 2010 government report, with an average density of 446 people per square mile. Overall, Americans generate 2.6 kilograms (5.7 pounds) of trash per capita every day, 13 percent of which is plastic.

plastic pollution

Photo: Martin Bernetti/AFP/Getty Images

It's helpful to know how much plastic is flowing into the oceans, but this is still just the tip of the iceberg. While plastic can "photodegrade" in sunlight and crumble amid churning waves, it doesn't truly break down like more biodegradable materials do. And with roughly 321 million cubic miles of ocean on Earth, researchers are still struggling to assess the scope of our plastic problem.

"This paper gives us a sense of just how much we're missing," Law says, "how much we need to find in the ocean to get to the total. Right now, we're mainly collecting numbers on plastic that floats. There is a lot of plastic sitting on the bottom of the ocean and on beaches worldwide."

Any plastic in seawater can endanger wildlife, including large items like fishing gear that entangles dolphins or plastic bags that clog sea turtles' stomachs. Tiny pieces known as "microplastics" are especially insidious, absorbing a variety of ocean pollutants and then passing them on to hungry seabirds, fish and other marine life. This can become a "frightfully efficient mechanism for corrupting our food chain," Marcus Eriksen of the 5 Gyres Institute told MNN last year.

Ocean plastic is going to get worse before it gets better. A 2013 study warned that Earth's garbage patches will be around for at least 1,000 years, even if all plastic pollution stopped immediately. And Jambeck expects the cumulative impact of ocean plastic will equal 155 million metric tons by 2025. According to a report by the World Bank, humanity won't reach "peak waste" until next century.

"We're being overwhelmed by our waste," Jambeck says. "But our framework allows us to also examine mitigation strategies like improving global solid waste management and reducing plastic in the waste stream. Potential solutions will need to coordinate local and global efforts."

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.