The more waste in an area that wasn’t disposed of properly, the more ends up in rivers, the team discovered. And large rivers carry more plastic per cubic metre of water than small ones.

Indeed, the 10 river systems with the highest plastic loads – eight in Asia and two in Africa – are responsible for around 90% of the global input of plastic into the sea, according to the researchers, who analysed a number of other studies to come up with their results.

"Halving the plastic input from the catchment areas of these rivers would already be a major success", said Christian Schmidt of the Helmholtz Centre for Environmental Research. "To achieve this, it will be necessary to improve the waste management and raise public awareness for the issue. We hope that our study will make a contribution to a positive development so that the plastic problem in our oceans can be curbed in the long run."

Next Schmidt and colleagues plan to find out how long it takes plastic debris to travel down a river to the sea. "It is important to be aware of this as the impact of a measure becomes apparent only with a corresponding time delay as existing pollution has yet to be washed into the sea," Schmidt explained. "Only when we are aware of roughly how long plastic debris remains in the respective river system will it also be possible to assess a measure to improve the waste management system in the catchment area."

The team reported their findings in Environmental Science and Technology.