"The patches are an international problem," said researcher Erik van Sebille in a video abstract for Environmental Research Letters (ERL). "It is not that plastic from one country ends up in one particular patch. Quite the contrary, all of the plastic ends up in all of the patches, and the patches are interconnected in a way that we didn't know before."

Van Sebille and colleagues' model of plastic dispersion is the first to include seasonal variations in ocean currents and to vary the amount of plastic entering the ocean according to the number of people living near each coast.

There are currently five garbage patches, formed at a gyre – a system of rotating ocean currents – in each of the world's subtropical ocean basins. A garbage patch is also likely to form in the Barents Sea in the Arctic within the next 50 years or so, the team's model revealed.

Over 10 years or so, the researchers found, the rubbish in one patch can actually move to another patch, assisted by oceanic eddies up to 50 km across. After thousands of years, a large amount of debris released outside the North Atlantic is likely to have moved to the North Pacific garbage patch.

"If we want to prevent, reduce or clean up the patches, we really need to have an international collaboration," said van Sebille.

To achieve their results, the researchers used data from surface drifter buoys, which float around the ocean and relay regular updates on their position. The Global Drifter Program has introduced such buoys into the ocean since the early 1980s. The team used this information to model the motion of plastic rubbish for the next 1100 years using a particle-trajectory tracer approach.

"The garbage starts at the coasts around the ocean where the people live, and within a few months the currents move it into the open ocean and there it forms the great garbage patches, which are a bit like vacuum cleaners of the ocean," said van Sebille.

The action of the sun and the ocean degrades the plastic into small pellets just millimetres across that float in the upper ocean in a "kind of soup structure". According to van Sebille, "plankton grows on the pellets, birds eat them, fish eat them and because these pellets and the plastics can contain quite a lot of toxins, that becomes part of the food chain."

The effect on the ecosystem is unclear and it is also hard to gauge the weight of plastic in the ocean. "What we do know is that in some regions of the North Pacific there is more weight in plastic than there is in life," said van Sebille. "If you go out fishing, you find more plastics in your nets than you do fish or even plankton."

The plastic is likely to stay in place for at least 1000 years – van Sebille said there is really no solution for getting it out of the ocean. "It is too small, it is too diverse, the soup is too thin to get out there with a ship and pick it up," he said. "Of course, the way to go would be to make plastics that do break down. Plastics that even if they get into the ocean don't really have the time to accumulate in garbage patches because they will just disintegrate and will be gone from the food chain."

Now the team plans to study what happens to plastics closer to the coast. "Clearly, by the amount of plastic found on beaches not all of it ends up in the gyres to form garbage patches in the deep ocean," said van Sebille. "We need to find out what happens to the plastics closer to land, where most fishing occurs, and what effect that has on the environment around our coasts."

Van Sebille and colleagues reported their work in Environmental Research Letters (ERL).