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There’s less Plastic pollution flowing into the ocean from land than scientists previously thought, according to a study published Monday in the journal Nature Geoscience.

The researchers estimated that about 500,000 metric tons of plastic end up in the ocean each year, with about half from land. The other half comes from the fishing industry in the form of nets, ropes, buoys and other equipment.

An earlier, widely publicized study in 2015 estimated that about 8 million metric tons of plastic were entering the ocean each year from rivers alone. The new research might seem like good news, but the full picture is complicated: The amount of plastic in the ocean is still increasing by about 4 percent every year, according to the study.

Even a small increase each year adds up to a huge accumulation over time. Within 20 years, the amount of plastic on the sea surface could double, the authors found.

“We’re accumulating more and more plastics in the environment,” said Mikael Kaandorp, the study’s lead author and a postdoctoral researcher at Forschungszentrum Jülich, a research institute based in Jülich, Germany.

Why It Matters: Plastic Pollution harms marine wildlife, and humans.

Wildlife can become tangled in discarded nets, ropes and packaging. Many animals also become sick or injured from ingesting plastic. Some even starve to death because their digestive systems become blocked.

Very small pieces, called microplastics, can easily make their way up the food web to humans from fish and other seafood. Sometimes, these microplastics have absorbed or become coated in toxic chemicals.

Onshore, plastic that doesn’t enter the ocean still pollutes rivers, lakes, beaches and land.

Background: Early estimates of ocean pollution left open questions.

The 2015 study was one of the first comprehensive research efforts to tally up how much plastic ends up in the ocean. But there was a large discrepancy between its estimate of 8 million metric tons and the amount of plastic observed in the ocean. Newer studies have tried to address this gap.

The paper published on Monday combined data from many earlier studies that sampled smaller plastics in the ocean using net trawls or observed larger plastics from ships and from shore. The researchers fed this data into a computer model of how objects move around the ocean in order to estimate both how much plastic is entering the ocean each year and how much total plastic pollution there is floating on the sea surface.

Most of the total plastic pollution in the ocean is floating plastic, and that’s what’s most problematic for marine life because it can readily be eaten.

The new study estimated that in 2020, approximately 3.2 million metric tons of plastic debris were floating on the sea surface and suggested this offshore pollution remains on the surface for longer than previously thought.

What’s Next: Controlling pollution at the source is key, scientists say.

Plastic pollution is difficult to clean up once it’s in the ocean. Trying to do so would be both a logistical and an ecological challenge: There’s no way to collect the plastic without also sweeping up and harming wildlife in the process. And humans are adding more plastic all the time.

Dr. Kaandorp said his study’s estimates of this continued accumulation highlighted the importance of stopping the flow. “It really shows we need to take measures,” he said. “It’s going to take a really long time before these plastics actually are removed from our seas.”

This year, countries agreed to start writing a global treaty to curb plastic pollution.

“The policy’s not keeping up with the pace of the problem,” said Marcus Eriksen, a co-founder of 5 Gyres, a nonprofit group that focuses on reducing plastic pollution. Dr. Eriksen published a separate study in March that estimated similar amounts of floating plastic in the ocean and found that this type of pollution had rapidly increased since 2005.

“The U.N. treaty could change that,” he said, if it regulates what kinds of plastic products can be made, puts more of the onus of recycling on manufacturers and is legally binding.