We've been hearing for a while about the increasingly dire problem of discarded plastic that's going into the world's oceans, where it poses a threat to the environment and aquatic life. But now, the urgency is driven home by a new study published in the journal Science that reveals just how much plastic humans are putting into the world's waters. 

Based on coastal population densities, plastic consumption and waste-handling practices, researchers calculate that it comes to 8 million metric tons of plastic trash.

To get your head around your number, think of it this way. At the annual meeting of the American Association for the Advancement of Science in San Jose, Calif., the study's lead author, Jenna Jambeck, an assistant engineering professor at the University of Georgia, held up a plastic trash bag. "Think of five bags filled with plastic for every foot of coastline in the world," she said.

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That's enough plastic to cover an area 34 times the size of Manhattan in an ankle-deep layer, according to Roland Geyer, an industrial ecologist from the University of California-Santa Barbara, whose National Center for Ecological Analysis and Synthesis orchestrated the study.

Or maybe this is more surprising: On the low end of the estimate of between 4.8 and 12.7 million metric tons -- the amount of plastic dumped in the ocean would be equivalent to the tonnage of tuna that fishing boats catch each year, worldwide.

"We're taking out tuna and putting in plastic," said co-author Kara Lavender Law, a research professor at the Massachusetts-based Sea Education Association.

Worse yet, with plastic consumption on the rise, by 2025, the cumulative amount of plastic that's been dumped in the ocean -- some of it washing up on shorelines -- since its widespread use began in the 1950s could reach 155 million tons.

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Nicholas Mallos, director of the marine debris program at the Ocean Conservancy in Washington, D.C., who was not involved in the study, said it was important because it calculated how much plastic is ending up in the oceans, rather than relying on the portion that can be observed on the surface of the water.

"It's one to three magnitudes greater than we had expected," he said.

The study found that the top five countries in terms in terms of aquatic plastic waste were Asian countries with a combination of dense coastal populations, rapid economic growth and inadequate waste collection and disposal practices. China topped the list, with between 1.32 million and 3.53 million metric tons of plastic annually. Indonesia, the Philippines, Vietnam and Sri Lanka rounded out the list.

The United States ranked 20th, with between 40,000 and 110,000 metric tons of plastic waste.

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The world produces about 275 million tons of plastic waste per year, but much of it is buried in landfills. Even in the United States, only 9 percent of the plastic waste is recycled, according to the Environmental Protection Agency. Only a few percent of the total waste is tossed into rivers and other waterways and ends up in the oceans, but that's enough to become a major environmental problem, the study shows.

Exactly what happens to all that plastic once it gets into the ocean remains unclear, the scientists said. Another study, published in PLOS ONE in 2014, concluded that as much as 250,000 tons of plastic -- some 5.25 trillion pieces of various sizes -- is floating on the ocean surface.

Some believe that much of the rest is sinking to the ocean bottom or being broken down by microorganisms. But an even bigger worry is that some of the waste breaks down into tiny -- sometimes even microscopic -- pieces called microplastic, which is being consumed by aquatic animals ranging from worms to whales. The scientists said that some of it eventually will make its way into the human food chain and there is some evidence that it's already happening.

What to do about the plastic is an even more troubling dilemma. Mallos said that the most important priority is to reduce the amount of plastic that's being discarded by improving collection and recycling practices in countries that produce the most waste. Once that input is reduced, he said, it might be possible to gradually collect much of the waste that's already in the water.

One group has proposed the use of floating barriers to trap and remove the plastic on the surface. But as Geyer noted, removing plastic that's already sunk thousands of feet to the ocean floor would be difficult and undoubtedly costly. Removing microplastic from the water without harming plankton and other marine life is another challenging problem.