The UN has declared war on the largest source of pollution faced by Mother Nature: yes, our enemy is “marine plastic” (UNEP Newscentre).
Every year, more than 8 million tons of plastic waste is carried into the ocean (Jambeck et al. 2015). The economic loss caused by this pollution is estimated to reach 8 billion dollars in a year (UNEP 2014).
If we keep discarding plastics into the sea at this pace, it is expected that the amount of plastic will exceed the weight of all the fish in the oceans in 2050 (World Economic Forum 2016), while 99% of species of sea bird will end up accidentally eating plastic (Wilcox et al. 2015).
In addition, the number of tiny pieces of microplastic in the ocean is increasing. It is estimated that 51 trillion pieces of microplastics are floating in seas all over the world (van Sebille et al. 2015); this number is equal to 500 times as many stars as there are in our galaxy (UNEP Newscentre).
But, with the advances made by many researchers, it has been determined that this incredibly large quantity of plastic is actually just the tip of the iceberg.
Among the plastic waste that enters the sea, the pieces with a specific gravity lighter than seawater float while the rest sinks into the sea. This plastic subsequently accumulates in various parts of the sea, from the shallow coast to the deep sea, from vast stretches of the sea surface to the sediment at its bottom.
Plastic is a very stable substance and cannot really be decomposed by organisms (Andrady 1994). It takes an overwhelmingly long time, in the hundreds or even thousands of years, for plastics in the sea to physically decompose (Barnes et al. 2009).
Therefore, among the plastics that have been manufactured since the 1950s, most of those that were carried into the sea are presumed to still exist (UNEP & GRID-Arendal 2016).
How much plastic has accumulated in the sea since the 1950s?
Suppose that the proportion of discarded plastic that is carried into the ocean is between 1.4% and 2.8%. This means that the amount of marine plastic debris still existing on Earth is somewhere between 86 million and 150 million tons (Jang et al. 2015, Ocean Conservancy & McKinsey Center for Business and Environment 2015).
But not all the plastic waste remains floating on the sea: some of its sinks. Of the plastic products these days, more than half have a specific gravity that is lower than seawater and, therefore, they float (PlasticEurope 2015).
For example, the caps of plastic bottles are made of polypropylene or polyethylene. Their specific gravity is so low that they keep floating. In contrast, the plastic bottles themselves are made of PET, polyethylene terephthalate. Their specific gravity is large and, therefore, they sink.
In North America, it is said that 66% of disposed plastic waste is floating, and the rest, or 34%, is sinking (Engler 2012). We can make an estimation using this ratio of 66:34, namely floating plastics to sinking plastics.
Let us suppose that the weight of all of the accumulated plastics in the sea from 1950 would be 86 million tons, at the very least (Jang et al. 2015). According to the ratio, the amount of plastic floating on the surface of the sea would be 57 million tons; the rest of the plastic, which has sunk to the bottom of the sea, would be 29 million tons.
Some of the floating plastic remains along the coastlines, such as bays and beaches, but most is carried by the waves and eventually delivered to the open sea.
In actual fact, it is estimated that 60 to 64% of light plastic waste is carried from the coastline to the open sea (Lebreton et al. 2012).
If we apply this ratio, 60-64%, to the 57 million tons of floating plastic, then at least 34 million tons of plastic waste are calculated to be floating in the open sea (UNEP & GRID-Arendal 2016).
However, researchers who have investigated the amount of plastics on the open sea have determined that there is definitely not 34 million tons of plastic floating there.
Many scientists have tried to estimate the amount of plastic waste floating on the surface of the open sea, using observations from boat studies, computer simulations, and so on.
One research team estimated there to be 7,000 to 35,000 tons (Cózar et al. 2014), while another team estimated 66,000 tons (Eriksen et al. 2014). The research team that reported the fact of the 51 trillion microplastics estimated that there were between 93,000 and 236,000 tons (van Sebille et al. 2015).
The reason why these estimations vary so much between research teams is because each team uses a different method of simulation, such as the standardisation of data, the scale-up of amount of the discharged plastic, etc.
Let us suppose that the amount of microplastic that is now floating all over the sea would be 236,000 tons, which is the highest estimated value.
However, this value doesn’t include the amount of large plastic waste. If we include large pieces of plastic, with a size greater than 20 cm, then we have to add another 203,000 tons to that estimation of 236,000 tons (Eriksen et al. 2014).
Then, the total amount of floating plastic on the sea would be approximately 440,000 tons, yet this value represents only 1% of the 34 million tons of plastic that are supposed to be floating in the open sea. The remaining 99% of floating plastic waste seems to have disappeared from the surface of the sea.
These 99% must be hiding somewhere in the sea. Although it is a very controversial topic, most of it has probably been delivered to the deep sea (Woodall et al. 2014). Floating plastic cannot float forever. Various kinds of creature tend to stick to the surface of floating plastic, eventually making it sink due to their combined weight.
Then, they are eaten by creatures such as zooplankton and turn into marine snow and sink into the deep sea. Some of them may land on the beach once more, or some may be eaten by marine animals by accident and remain in their bodies.
Some scientists worry that microplastics are getting tinier and tinier, being refined into something like powder; they become so tiny that even sampling with plankton nets can no longer detect them.
A lot of people are interested in the plastic debris that is floating on the surface of the open sea. However, the important point is that this occupies only 1% of the plastic that has accumulated in the sea (UNEP & GRID-Arendal 2016).
Scientists are working hard to find out where the rest of the 99% has gone.