A large proportion of plastic waste in our oceans comes from ships that are dumping their litter at sea – in contravention of international law. This is according to new research from the University of Cape Town (UCT). This research contradicts the widely held assumption that most plastic in our oceans comes from land-based sources.
The researchers found that Inaccessible Island – a small, isolated Southern Atlantic Ocean island thousands of kilometres away from any continent – is riddled with plastic waste, most of which does not come from land.
Fishing tackle and shipping waste comprise much of marine plastic
“Recent studies examining litter in the North Pacific garbage patch and remote islands in the Pacific Ocean have shown that fishing tackle and other equipment related to shipping makes up much of the plastic mass at sea,” says UCT professor Peter Ryan, who led the research and is an expert on marine plastics. “The challenge is understanding where ‘general’ litter originates – items such as domestic products and food packaging. These are waste items which could come from both ships or sources based on land,” he says.
When they are near a gyre, isolated oceanic islands can become collection points for ocean waste. Despite being far from major plastic-waste sources, the shores of these islands collect a disproportionate amount of plastic pollution. Although it is situated more than 2500km from any mainland, Inaccessible Island’s shores are covered with plastic debris. About one-third of this waste is made up of bottles, especially soft drink and plastic water bottles.
Inaccessible Island and the litter it collects have helped scientists, led by UCT, piece together clues as to where the ‘general’ plastic in our oceans originates from. This is because the island has been acting as a sample net for South Atlantic Ocean litter.
Plastic bottles useful tracers of pollution sources
“Bottles can be very useful tracers because they often have marks that indicate where and when they were made, unlike much general household litter. This provides us with an estimate of the maximum amount of time these bottles could have been adrift at sea. The presence of marine animals – like goose barnacles – on the bottles is another indicator of the amount of time they had been in the ocean,” Ryan explains.
The research team started monitoring litter on Inaccessible Island in the 1980s, enabling them to chart long-term changes in the origins of bottles over the last three decades. “Most litter drifted 3000km from South America when we first visited the island,” says Ryan. “By 2009, Asia just surpassed South America as the main source of bottles, and by 2018, 74% of bottles came from Asia,” he adds.
84% of the bottles that arrived during the researchers’ three-month stay on Inaccessible Island were from Asia. Of these bottles, at least two-thirds came from China. The majority of these bottles were manufactured within one to two years of washing ashore. It would take a minimum of four to five years for bottles to drift from Asia via the Indian Ocean. Most litter from China has a bigger chance of ending up in the North Pacific garbage patch than at Inaccessible Island.
Knowing litter sources is essential to combating pollution
The global production of plastics has grown by about 8% per year since the 1950s and now stands at more than 300 million tonnes per year. Knowing the source of litter is essential to devising ways to solve the problem. The dumping at sea of plastics – and other forms of waste that are slow to degrade – was banned in 1989.
It has since been widely assumed that some 80% of litter in the sea comes from land. There has, however, been very little concrete evidence to confirm the presence of land-based sources for plastic in remote areas. Over the last few decades, there has been a vast increase in merchant shipping between Asia and South America.
More than 2400 ships now pass the archipelago of Tristan da Cunha, of which Inaccessible Island is a part, annually. The total number of bottles drifting ashore at Inaccessible Island has increased over the past 30 years – twice as fast as any other kind of litter. Of the drink bottles the research team found, at least 61% were water bottles. Unless there is a change in the way water is packaged, this proportion can be expected to continue growing.
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