Our plastic bags seem to end up floating like waterlogged jellyfish in the sea.
Discarded bottles spoil pristine beaches around the world.
Now, scientists have a glimpse of what happens when tiny fragments of plastic break off and end up on the ocean floor.
In what researchers called the first such global estimate, Australia’s national science agency says that 9.25 million to 15.87 million tons of microplastics — fragments measuring between five millimeters and one micrometer — are embedded on the sea floor.
That is far more than on the ocean’s surface.
It is the equivalent of 18 to 24 shopping bags full of small plastic fragments for every foot of coastline on every continent except for Antarctica.
It is an issue that activists have long warned about even as the fight to clean up the ocean has focused largely on the eradication of single-use plastic products like shopping bags.
The findings were published on Monday in a new study by the Commonwealth Scientific and Industrial Research Organization, or C.S.I.R.O., highlighting how humankind’s use of plastics has seeped into the depths of the world’s oceans.
“It really points to the ubiquity of the problem. It is really everywhere all the time and increasing,” Britta Denise Hardesty, a principal scientist for C.S.I.R.O. and an author of the study, said in a phone interview on Wednesday.
Microplastics are not confined to the ocean. They are also found in air particles and can be spread by wind. A variety of microplastics was even detected in the human gut.
Using a robotic submarine, the scientists collected 51 deepwater samples of sand and sediment in the Great Australian Bight in 2017, hundreds of miles from the shore, and determined the global estimate based on the average number and size of the particles.
The study found zero plastic particles in some deep-ocean sediment, but up to 13.6 particles per gram in others, a figure up to 25 times larger than what had been found in earlier deep-sea studies of microplastics.
The scientists said they made conservative estimates to take into account the full range of samples. They also eliminated fibers or other materials from their count to rule out the potential contamination of the samples.
Dr. Hardesty said that it was important to prevent plastic from ending up in the ocean in the first place. She said she was hopeful that awareness about the pollution would lead to more sustainable policies and shifts in behavior.
“Most of what ends up in oceans are in people’s hand,” she said. “They can see that their behavior — their actions and purchasing power — is very powerful and that can result in change.”