Finding the ocean’s missing plastic

Millions of tonnes of plastic is trapped along the world’s coastlines and will eventually find its way into the ocean, according to new research.

And even if we stop dumping it today, the amount of plastic in the marine environment will continue to grow because our shorelines are working like a holding facility for plastic already on its way to sea.

That’s the grim finding from modelling carried out for the Netherlands-based Ocean Cleanup Foundation and reported in the journal Scientific Reports.

The researchers wanted to find an explanation for the huge discrepancy between the volume of plastic that is estimated to have found its way into the ocean, and the amount that is floating on its surface.

Most estimates put the quantity of floating plastics at more than 250,000 tonnes. That’s a lot, but only a tiny fraction of the tens of millions of tonnes of plastic believed to have been released into the ocean since the 1950s.

About two-thirds of all plastics has a density lower than sea water, and thus should be floating. It isn’t.

That gap has usually been attributed to the breakdown of macroplastics (bits more than five millimetres long) into microplastics, which do not float. In other words, it has been assumed that the missing plastic has gone under, perhaps down to the ocean bed and into the ecosystem.

That would be bad enough.

More recent work analysing material gathered from the ocean’s surface shows a more complicated picture, however. Much of the floating plastic recovered was decades old, suggesting that the process of degradation takes far too long to account for the missing tonnes.

So the Foundation fed field research data into modelling that also weighed the probability of debris becoming stranded in coastal environments, rather than getting out to sea. This can happen if the plastic becomes entangled with other matter, or because of wind and ocean currents.

The researchers say their model supports the theory that plastic floating near the shoreline is being recaptured by the landmass and trapped in sediment until it eventually is freed and floats away – or breaks down into microplastics.

Their modelling estimates that 66.8% of all the buoyant plastic released into the marine environment since the 1950s is being stored by the world’s shorelines as stranded, settled or buried debris. This would translate to between 46.7 and 126.4 million tonnes of plastic.

Another third of all marine plastics may already have degraded into microplastics. Disturbingly, the model predicts that most of today’s microplastic contamination is from material dating back to the 1990s and earlier – so huge quantities of more recent plastics haven’t even started to break down yet.

Looking ahead, the Foundation plugged different scenarios into their model to find out what this vast coastal storage of plastic might mean in the future.

If we continue to dump plastic as we have done to date, the quantity floating on the surface could quadruple by the year 2050, and up to 231.6 million tonnes will have polluted the ocean as microplastics.

The picture is better if we stop releasing any plastic into the ocean from 2020. In that scenario, the volume of floating plastic would drop to 59% of its current levels. But the quantity of microplastics in the ocean would still more than double by 2050 as material already trapped in the environment degrades.

The problem will be with us for decades, the researchers write.

“Mitigating microplastic pollution in the global ocean requires two major components: (1) drastically reducing emissions of plastic pollution in the coming years and (2) actively engaging in removal operations of plastic waste from the marine environment to reduce further generation of secondary microplastics.

“Without proper handling and management of accumulated plastic waste, the legacy of the last 70 years of throw-away society will live on through the generation of ever smaller synthetic polymer fragments in soils, freshwater ecosystems and eventually the ocean.”

Related reading: Revealed: the mystery of missing ocean plastic