Article: Ninety-nine percent of the ocean's plastic is missing
A scientific expedition was sent to various "gyres" in the major oceans. These were long thought to be places that plastic debris would accumulate in the ocean. They expected to find lots of plastics, and expected to find lots of grocery bags and balloons. They found little of the former and none of the latter.
The million ton figure was an estimate made in the 1970's. The number has been endlessly recycled since then and is now fully entrenched. [See the "Woozle Effect," below.]
No study has been made since then despite the following: littering is now considered despicable (less trash and more people picking up), trash and trash disposal sites are a lot more tightly managed in general, and recycling of plastics is now widely practiced. It may also been an over-estimation at the time.
By-the-by, I was around in the 1950's and 1960's; littering was a huge problem and it was considered normal to throw things way outside. And litter is, by the way, an excellent example of dealing with a problem in a conservative manner. Significant fines for littering were passed (and enforced). However, the real change change came as a result of First Lady, Lady Bird Johnson's education campaign to "Beautify America."
The researcher who came up with the original, million ton estimate revised his numbers. "His latest estimate ranges from 7,000 to 35,000 tons, and even most of that has biodegraded into granules."
Possible reasons for the small amount: the original number was an over-estimation, the plastic is actually biodegrading completely, it is being consumed and sinking to the ocean bottom in feces, and/or it is providing a microhabitat for sea creatures and their weight is causing the particles to sink.
My guess is all of the above.
His million ton number was also used to estimate that 1.5 million marine animals were killed each year due to ingestion of plastics. That number has been revised downward, drastically, as well.
The original numbers were then used to justify banning plastic grocery bags in many California communities. Even to attempt to reverse this ban would be denounced as "anti-environment," despite that the science says that the problem is a lot smaller than originally thought.
Using a rough, back-of-the-envelope calculation, 35,000 tons is less that 25 ppb (parts per billion).
The "Woozle effect".
The Woozle effect, also known as evidence by citation, or a woozle, occurs when frequent citation of previous publications that lack evidence misleads individuals, groups and the public into thinking or believing there is evidence, and nonfacts become urban myths and factoids.The Woozle effect is a variation on the "appeal to authority" logical fallacy.