There is far more plastic in the oceans than previously estimated: according to Markus Eriksen, a researcher at the 5 Gyres Institute, an organisation dedicated to monitoring and mapping plastic waste at sea, the figure stands at around 270,000 tonnes.

How is this possible? The millions of pieces of waste and debris produced every day by the world's population — from plastic bags to bottles to disposable cutlery — if poorly managed, end up in the sea via rivers, where they form more or less large heaps and islands: for example, the one discovered around 15 years ago off the Pacific coast, also known as the Pacific Trash Vortex, is the size of Texas and weighs just over 21,000 tonnes.

According to the study by the institute's research group, published in the scientific journal Plos One, the current weight of oceanic plastic is around five times that recorded in the 1970s and ten times that of the Pacific garbage patch.

The new model used by the research group produced an estimate of oceanic plastic mapped during 24 expeditions carried out across the world, from Iceland to the Bay of Bengal. The team then combined visual surveys with the (assumed) net weight of the plastics, using mathematical models to calculate how ocean currents might also transport plastic, and compared the results according to categories of debris size.

The new estimates — which some other scientists, such as ecologists at the Spanish University of Cádiz, consider excessive because they rely too heavily on visual mapping of waste — indicate the presence of around 5.25 trillion pieces of plastic debris, with a combined weight of approximately 269,000 tonnes. Almost 75 per cent of that weight would be accounted for by large objects, such as buoys, buckets and other fishing equipment.

According to the author, it would not be possible to clean up the sea using dedicated robots, precisely because of the size of the plastic objects involved. "Instead, an incentive programme — for example, offering fishermen a dollar per pound of rubbish collected — could encourage people to clean up the oceans themselves," Eriksen added.