A new atlas of the world’s oceans points the finger at countries responsible for giant floating patches of garbage, say researchers.

By redefining the boundaries of the world’s oceans, the new model, reported today in the journal Chaos, also has implications for our understanding of ocean ecosystems.

The infamous Great Pacific Garbage Patch between Hawaii and California is made up of plastics, chemical sludge and other marine debris drawn in by a vortex formed by rotating currents (gyres) in the North Pacific.

Scientists believe there are four other gyres and associated garbage patches in the Indian Ocean, North Atlantic, South Atlantic and South Pacific.

Common sense suggests that garbage in a particular area, for example the North Pacific, comes from countries that border that area.

But, says oceanographer Dr Erik van Sebille from the University of New South Wales, it’s more complicated than that, especially in the Southern Hemisphere.

“It’s a bit artificial how the oceans have been defined,” he says. “Geographers have just put up these boundaries in the oceans not thinking about how the ocean circulation actually works.”

New atlas of the oceans

Van Sebille and two mathematician colleagues developed a new model of ocean flows drawing on what’s known as ‘ergodic theory’, used to analyse interconnected systems such as the internet and human society.

“We’ve looked at the catchment area for each of the garbage patches,” says van Sebille.

They found garbage on the sea surface flows in unexpected ways because the boundaries of the oceans are different to where we think.

For example, says van Sebille, while traditional atlases define the ocean west of Tasmania as the Indian and the ocean to the east as the Pacific, the new model shows otherwise.

“What we’ve found is that that makes no sense at all,” he says. “If you throw plastic in the ocean at the Great Australian Bight it is much more likely to end up the Pacific Ocean rather than in the Indian Ocean.”

Also, even though Madagascar and South Africa and Mozambique are in the Indian Ocean, their garbage flows into the South Atlantic, says van Sebille.

He says the new model also has implications for ocean ecology since plankton, the base of the food chain, drift passively on currents.

“So these catchment areas are biogeographic regions and confine where plankton is likely to live,” says van Sebille.

Pointing the finger

But, while the new model can point the finger at which countries contribute to each garbage patch, it cannot yet identify the relative size of each country’s contribution, says Van Sebille.

“This paper is a very important step in that direction but we don’t know what the relative impact of Australia versus Peru is for the South Pacific, for example,” he says.

Van Sebille says this is because there is still not enough data on the amount of garbage floating in different areas of ocean.

“There have been a whole lot of people measuring inside the garbage patches but not so many people measuring outside of them,” says van Sebille. “And that information is just as important if you want to understand the pathway of the plastic.”

He says the new model may help priorities areas for garbage monitoring so that the model can be fine-tuned.