The concern about plastic being discarded into the oceans of the world -- and its threats to aquatic life and the environment -- just got more intense with a first study to provide an estimate of just how much plastic is dumped each year.

Analyzing densities of coastal populations, our consumption levels of plastic and current waste-handling practices, scientists have come up with the worrying number of 8 million metric tons of plastic being dumped every year.

That's as much as 4.5 percent of the world's yearly plastic production, they say.

Even more worrying, they add, is that we don't know exactly where 99 percent of that plastic pollution is ending up, so it's hard to analyze or even predict what risks it presents to marine life and the human food supply.

With plastic consumption continually increasing, by 2025 the cumulative amount of plastic dumped in the ocean since the beginnings of its widespread use in the 1950s could reach 155 million tons, the scientists say.

Even today there is enough plastic floating in the world's oceans to cover an area more than 30 times the size of Manhattan with an ankle-deep layer of plastic refuse, say industrial ecologist Roland Geyer from the University of California, Santa Barbara, home of the National Center for Ecological Analysis and Synthesis, which led the study.

Or put another way, the weight of plastic being dumped in the world's seas every year is about equal to the tonnage of tuna caught annually by the world's fishing fleets.

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

Asia is the leading source of marine plastic pollution, the research found, due to dense coastal populations, rampaging economic growth and poor or nonexistent collection and disposal practices for plastic waste.

China was the worst offender, with between 1.3 and 3.5 mission tons of plastic; other offenders include the Philippines, Indonesia, Vietnam and Sri Lanka.

The U.S., with somewhere between 40,000 and 110,000 tons of plastic waste, ranks 20th.

"This is a significant study," says Nancy Wallace, director of the marine debris program at the National Oceanic and Atmospheric Administration, who was shown the paper before it was published.

"Of course we know these aren't absolute numbers, but it gives us an idea of the magnitude, and where we might need to focus our efforts to affect the issue," she says.

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