An estimated eight million metric tonnes of plastic waste ended up in the world's oceans from the garbage of those who live along the coasts in 2010, a new study says, a figure that amounts to five full grocery bags for every foot of coastline.

The team of researchers estimates 9.1 million metric tonnes of plastic will end up in oceans this year.

"And it can get worse," Jenna Jambeck, the lead author of the study, said at a news conference at the annual meeting of the American Association for the Advancement of Science in San Jose, Calif. "If we assume a business-as-usual projection with growing population, increasing plastic consumption, and increased waste generation, by 2025, this number doubles. We may be adding 17.5 million metric tonnes of plastic per year."

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While marine pollution is a well-known problem, the study by the National Center for Ecological Analysis and Synthesis to be published on Friday in the journal Science is the first time researchers have estimated the amount of plastic waste that flowed into the ocean in a given year.

The study looked at the amount of waste generated within 50 kilometres of the ocean in each of 192 countries with coastline in 2010. Based on those findings, the researchers calculated how much of that garbage was properly managed and how much was not.

They factored in how much was plastic and how much of that plastic was likely to make its way to the sea. Their statistical analysis included population density, the sophistication of waste management systems, and overall plastic use in each country.

"We're being overwhelmed by our waste," Dr. Jambeck said in a statement.

Only a fraction of that plastic is on the surface, she said. Much of it is on the ocean floor.

The median estimate of eight million metric tonnes – from a range of between 4.8 million and 12.7 million – did not take into account natural disasters like the 2011 earthquake and tsunami in Japan that washed about five million metric tonnes of debris into the ocean, said Kara Lavender Law, a team member from the Sea Education Association in Massachusetts.

Nor did it include plastic pollution from ships, fishing or aquaculture, she said.

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"Having sailed in the Atlantic and Pacific … and observing the problem first-hand, I knew that number had to be big, but the magnitude is hard to fathom," she said.

China was, by far, the largest polluter, contributing between 1.32 and 3.53 million metric tonnes of plastic marine debris in 2010. For comparison, No. 2 Indonesia added 0.48 to 1.29 million metric tonnes.

Only one high-income country is on the list of top polluters: The United States is No. 20, responsible for an estimated 0.04 to 0.11 million metric tonnes because of its population density along the coasts and high volume of plastic waste.

Canada, with the longest coastline in the world, fared much better.

This country produced almost 8,000 tonnes of mismanaged plastic waste in 2010, compared with more than 275,000 tonnes in the United States and 8.8 million in China. Under the study model, only a percentage would make its way to the ocean.

A 2012 study by the University of British Columbia found plastic pervasive in the stomachs of beached marine birds along the B.C., Washington state and Oregon coasts.

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Necropsies on 67 Northern fulmars found 92.5 per cent had plastics such as twine, Styrofoam and candy wrappers in their stomachs. One bird had ingested 454 pieces.

Max Liboiron, an ocean activist and academic who is launching an ocean plastic monitoring program in St. John's, said previous studies have suggested two-thirds of all marine animals are affected.

"They eat plastics," she said.

The chemicals in plastic are like "poison pills" that bio-magnify as they make their way up the food chain, all the way to humans, she said.

Special to The Globe and Mail