Plastic Debris in the Marine Environment Accumulates in the Five Subtropical Gyres

Since the 1950s, the production of plastics has grown exponentially, and annual gross production of plastics reached 322 million tons in 2015 (PlasticEurope). This weight is nearly as much as 7,800 Tokyo Sky Trees.

Every year between 2% and 5% of the plastic waste generated around the world enters the ocean. This is the same as if around 4 to 12 million tons of plastic were being discarded into the ocean each year (Jambeck et al. 2015).This is like a dump truck loaded to capacity with plastic dumping it into the sea every minute.

Plastics that enter the ocean either float or sink according to their specific gravity.  A PET bottle sinks because it has a denser specific gravity than seawater, but the cap of that same bottle floats because it has lesser specific gravity than seawater (Andrady 2017).

About half of the plastic products being manufactured today are lighter than seawater, so they float on the ocean’s surface (PlasticEurope 2015). 

The floating plastics are trapped within the currents of the ocean and are gradually taken to a specific place in the open sea. This specific place is like a dump site, and there are five such primary dump sites in the world’s oceans, with the biggest one in the North Pacific Ocean. 

The figure below shows that there are five massive, circular current systems in the world’s oceans. These are called subtropical gyres because they circulate through the Subtropical Convergence Zone.

The biggest gyre is in the North Pacific Ocean, while the others circulate through the South Pacific Ocean, the Atlantic Ocean, or the Indian Ocean. 

The circular motion of a gyre draws plastic debris into its stable center and, eventually, this plastic is gradually drawn down several hundred meters deep into the sea. 

However, because most of the plastic debris in the sea has a lesser specific gravity than seawater, it does not sink down but tends to remain on the ocean’s surface(van Sebille et al. 2015).

As time goes by, in all five gyres the plastic debris accumulates and becomes a gigantic collection of marine debris (Law e tal. 2010, Law et al. 2014, Cózar et al. 2014, Eriksen et al. 2014, van Sebille et al. 2015).

This large ocean current that moves in a clockwise direction in the North Pacific Ocean is called the North Pacific Gyre. At the western end of this clockwise ocean current is the powerful Kuroshio Current.

Where will the plastic waste go once it has been discarded off the coast of East Asia, such as Japan, China, or South Korea, or Southeast Asia such as Indonesia? 

Look at the picture below. This picture simulates where a garbage is taken when it is discarded off the coast of Shanghai, China. You will find an answer with the yellow duck as a guide.

A plastic waste that is being discarded off the coast of Shanghai travels eastward on the North Pacific Current, then gradually accumulates off the California coast. Plastic waste discarded from East Asia and Southeast Asia may travel to the North Pacific Ocean on the Kuroshiro Current(Yamashita & Tanimura 2007).

This collection of marine debris off California in the North Pacific Ocean is called the Great Pacific Garbage Patch(Moore et al. 2001). It is referred to as a soupy collection of marine debris, and consists mostly of plastics. That is why this place is said to be “plastic soup”.

Let’s look at other places. A piece of plastic that is discarded off the coast of Mexico is also held in the North Pacific Current. 

This waste may catch the North Pacific Current, continue west, crossing the vast Pacific and heading to Asia. Then, it may travel north on the Kuroshio Current and eventually reach the Great Pacific Garbage Patch located off California.

What about plastic garbage from India?

A piece of plastic that is discarded around the Mumbai area may catch the Indian Ocean Current and drift near Madagascar or drift toward the Bay of Bengal, which is probably the most polluted place in the world (Cozar et al. 2015, UNEP 2016).

Using simulation software, scientists have already verified that plastic debris circling around on one current system may transfer to other circulatory systems over several years(UNEP 2016).

If anybody is more interested in this issue, please check out this website→『Plastic Draft』. This site shows where plastic debris ends up over several years.

How it works. The simulation starts when the user taps a yellow duck icon on the map. It then shows how plastic debris spreads across the ocean. 

For example, by placing the duck near Tokyo, we can see how the debris is carried away by the ocean’s current and becomes concentrated off California.   



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