Over the last 30 to 40 years, millions of tonnes of plastic have entered the oceans. Global production of plastic now stands at 288 million tonnes per year, of which 10% ends up in the ocean in time. Most of that - 80% - comes from land-based sources. Litter gets swept into drains, and ends up in rivers - so that plastic straw or cup lid you dropped, the cigarette butt you threw on the road… they could all end up in the sea.
The amount of plastic being discarded into the marine environment is such that we could eventually see an ocean where the amount of plastics is roughly one third the total biomass of fish - 1lb of plastic for every 2lbs of fish, according to Nicholas Mallos from Ocean Conservancy, which organizes coastal clean-ups.
According to the UN Environment Programme there are on average 13,000 pieces of floating plastic per square kilometer of ocean - but that goes up to millions of pieces in the gyres. Many of these particles end up being accidentally ingested by marine animals, which can die of starvation because of the plastic filling their stomachs.
Plastics can act as a sponge and soak up chemicals in the water. "There are a lot of pollutants in the oceans now, things like DDT," Nancy Wallace, director of the US National Oceanic and Atmospheric Administration's Marine Debris Program, told the BBC. "Those chemicals adsorb on to the plastic and we know birds and fish are eating those pieces of plastic - so the question is, how does that transfer up the food chain and what is the impact?"
It is a grave situation - so when Slat came along with a seemingly simple solution, he began making headlines across the world. Could a teenager save the world's oceans? His enthusiasm fired up millions of people, but along with the offers of help and donations, came criticism. It wouldn't work, some said. Others argued that it would be better to collect litter from beaches, where it gets deposited by waves.
In June, a month before his 20th birthday, Slat re-emerged with a 530-page feasibility report, the cover of which was made out of recycled ocean plastic. The report, based on extensive testing and computer simulations and authored by 70 scientists and engineers, answered many of the questions which had been leveled at him by his critics. It was followed by another crowd-funding campaign which swiftly reached its target of $2m. This will fund a larger pilot next year and Slat hopes the North Atlantic platform could be a reality in 2020.
But if Slat expected all experts in the field of ocean plastics to welcome his concept this time, he was wrong.
One problem is that plastic isn't just floating on the surface, but found throughout the water column, even in sediment at the bottom of the ocean. Dr Kerry Howell, a deep-sea researcher at the University of Plymouth, told the BBC that she has found rubbish in the deepest parts of the ocean. "You're going to a place no-one's ever been to before, you're going to the last frontier on earth, exploring new places, and you find that our litter's got there first," she says. "It's like going to the moon and finding a crisp packet."
Another issue is the potential effect on wildlife. "In terms of biological damage the concept is flawed," says van Franeker. "They say anything alive will be able to swim under the curtain, but some, like the fish eggs, will be trapped with the plastic which means they will still be there to be eaten by albatrosses - and in 10 years' time you will take away all the fish eggs along with the plastic."
Aside from the question whether the Ocean Cleanup technology could work, there is also the question whether it should be a priority.
"It seems a foolish strategy to focus on approaches to take litter out of the oceans, when we could prevent it from getting there in the first place," says Prof. Richard Thompson of Plymouth University.
"The risk is that people think there is one device that will solve our problem in a few years - this grossly over-simplifies the problem," says Nicholas Mallos.
"He has bright ideas about how to get plastic of various sizes out of the water, but it would be better if he directed his efforts to smaller-scale projects in river outlets," says Dr. Jan Andries van Franeker from the Institute for Marine Resources and Ecosystem Studies (IMARES) in the Netherlands. "Only a fraction of the money would be necessary and it would be more effective."
Several other companies are now emerging with clean-up technology designed to capture plastic in rivers and streams, like the Plastic Visser ('plastic fisher') which is being trialled in the Netherlands, or the Trash Wheel - a solar-and water-powered barrier being used in Baltimore harbor.
Slat, too, is looking to develop spin-off technologies for use in rivers. "It is difficult to adapt something that works in rivers to the sea, whereas it's actually quite easy to adapt something that was developed for the worst conditions in the world - the sea - to work in rivers," he says. "That is why we're approaching it in this order."
At this point he is planning to stay in the Netherlands. "A lot of big names are here that I have to work with so it's a very suitable place to be - it's like the Silicon Valley of the off-shore industry," he says. "Perhaps [the Dutch] are under the impression that through engineering everything can be solved, and we're pretty good at mastering the ocean."
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