Some 8 million tons of plastic is estimated to have gone into the world’s oceans in 2010, the result of shoddy waste management and littering. It posed significant dangers to marine life, according to a study published in the journal Science.

The five worst offenders were China, Indonesia, the Philippines, Vietnam and Sri Lanka, the study said. The United States ranked 20th.

The study is the first of its kind to measure the amount going in from more than 190 countries with a coastline, instead of the amount actually in the ocean.

The method for determining the amount was a mathematical model based on the per-person waste generation for countries with a coastline.

One percent of the waste was presumed to be plastic, and another 1 percent was presumed to be mismanaged, meaning litter or waste was dumped on land and not properly contained.

“It can get worse. If we assume a business-as-usual projection — with growing populations, increasing plastic consumption and increased waste generation — by 2025 this number doubles,” Jenna Jambeck, a researcher from the University of Georgia, said at the annual meeting of the American Association for the Advancement of Science in San Jose, California.

Middle income countries with rapidly developing economies tended to be the biggest contributors of plastic trash because they were least likely to have developed management systems to keep pace with growth, Jambeck said.

The US was the only wealthy nation in the top 20, its ranking due to the high rate of waste generation, coupled with the nation’s large amount of coastline.

“Using the average density of uncompacted plastic waste, 8 million tons — the midpoint of our estimate — would cover an area 34 times the size of Manhattan ankle-deep in plastic waste,” said co-author Roland Geyer, an associate professor at University of California Santa Barbara’s Bren School of Environmental Science and Management.

“Eight million tons is a vast amount of material by any measure. It is how much plastic was produced worldwide in 1961.”

Co-author Kara Lavender Law, a research professor at the Massachusetts-based Sea Education Association, said the study “is the equivalent of measuring what is coming out of the faucet rather than what is already in the bathtub.”

The amount going in is 20-2,000 times larger than the existing estimates of plastic floating in the ocean, she said.

It remains unclear what happens to all that plastic — though some floats, some is suspended and some falls to the ocean floor — and scientists are concerned about the effect on fragile fish and marine life that consume plastic fragments.

Solutions for the world’s plastic problem include improving waste management and cutting down on consumption, the researchers said.

Since the top 20 countries accounted for 83 percent of mismanaged plastic waste, such efforts should be concentrated on keeping plastic out of the ocean, not trying to clean it up afterward, they said.

“Helping every nation develop a sound solid waste management infrastructure is a top priority,” said Geyer. Increasing the plastic re-use and recycle rates “is equally important,” he added.

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