Most plastic marine litter is the result of poor waste management. Continental plastic waste is thrown into the ocean by storms and river systems or is discharged directly into coastal waters. It has been estimated that at least 60% of the plastic that floats in the ocean is exported from the coasts to open ocean waters. It is estimated that between 1.15 and 2.41 million tons of plastic waste enter the ocean each year from rivers.
While photodegradable plastics (plastics capable of decomposing with light) can decay from their first state (or created state), these plastics never fully degrade, but are actually divided into small pieces called microplastics. Climate, terrain, land use, and distances within river basins affect the likelihood that poorly managed plastic waste will be released into the ocean. The global legal and illegal trade in plastic waste can also damage ecosystems, where waste management systems are not sufficient to contain plastic waste. The main sources of plastic waste found in the ocean come from land and come from urban and storm runoff, sewer overflows, garbage, inadequate waste disposal and management, industrial activities, tire abrasion, construction and illegal dumping.
There are methodologies for identifying, measuring and addressing sources of marine plastic pollution and plastic leaks, including those from the IUCN. Since consumers in rich countries tend to use much more plastic, people expect them to contribute much more to plastic pollution than they actually do. Third, river basins had high rates of precipitation (meaning that plastics reached rivers and the flow of rivers to the ocean was high). They can come from large plastics that break down, or they can be produced as small plastics, such as microbeads, which can be found in products such as toothpaste and facial cleanser.
Plastic is found on the coasts of every continent, and more plastic waste is found near popular tourist destinations and densely populated areas. The Copernicus Marine Service provides information on the oceans, including current data in near real time that can help institutions create coherent plastic management strategies. Numerical models estimate the routes of plastic, which sheds light on how plastic is distributed and shipped across the global ocean, thus revealing the areas of distribution and accumulation. Marine wildlife, such as seabirds, whales, fish and turtles, mistake plastic waste for prey; most die of hunger when their stomachs are filled with plastic.