A team of scientists has conducted what they say is the most comprehensive study to date of the patch’s size and the debris floating in it.
Using a combination of drag netting and visual surveys from boats and an aeroplane, they estimated the patch is 1.6 million square kilometres in area — almost the same size as Queensland.
Packed into this area is more than 78,000 tonnes of plastic, the researchers report in the journal Scientific Reports.
Most of the mass was made up of pieces larger than five centimetres. While microplastics, which account for about eight per cent of the mass, made up a bulk of the estimated 1.8 trillion pieces of plastic floating in the patch.
Lead researcher Laurent Lebreton said the garbage patch was growing exponentially and was boosted by debris washed out to sea during the Japanese tsunami in 2011.
“We show that plastic concentration has been increasing exponentially since the 1970s for different reasons,” said Dr Lebreton, an oceanographer at the Ocean Cleanup Foundation in the Netherlands.
“We found about 30 per cent of the identifiable objects were likely coming from Japan.
“We correlated that with our model and we looked at estimates from the Japanese government in terms of how much they think was washed to sea that day – and we predict that about 10-20 per cent of the materials post-2011 in the larger size class came from the tsunami.”
Previous sampling, which estimated the patch contained around 4800 tonnes of garbage, had primarily involved dragging funnel nets behind vessels to collect surface debris.
But Dr Lebreton said that this method excludes larger debris that cannot be collected by the nets, and that boat surveys can only cover a limited area.
“We saw that the surface area sampled by our trawls was not really large enough to be representative of the contribution of the bigger debris,” he said.
“So we decided to conduct an aerial expedition above the patch. We collected about 7000 images from the plane and that helped us to calculate the contribution of larger debris such as ghost nets.”
By including the larger debris sizes in their study, the researchers knew they’d come up with a bigger figure than previous studies, but they were still surprised by just how much mass the larger debris contributed.
“I wasn’t expecting to find that 92 per cent of the mass would be larger plastics,” Lebreton said.
Almost half the larger debris they identified was commercial fishing gear including nets and fish aggregation devices and other structures set adrift intentionally by fishers to attract fish.
Research scientist Dr Denise Hardesty from the CSIRO said it wasn’t surprising the survey produced a much larger size estimate of the garbage patch, given the different research methods used.
“When you’re comparing aerial surveys that are looking at ghost nets with estimates that are all focused on floating plastics we’re not really making the exact same comparison,” said Dr Hardesty, who was not involved in the study.
“Ghost nets will weigh so much more than all those little tiny bits and pieces and fragments.”
But she says that the new research is still cause for concern.
“Whether you’re focusing on count or mass, I think it is alarming and we all recognise that this is an increasing global project and it’s going to take local solutions as well as hopefully global governance to help resolve the issues,” she said.
“We need to deal with this before it enters the ocean rather than when it’s out in the middle of the ocean.”
Plastic circulating in the garbage patch does eventually get “kicked out” and washes up on coastlines, Dr Hardesty said.
But right now we are feeding waste in at a much higher rate than it can be expelled.
“There’s an increasing source that’s coming from the Austrlian coast. And, yes, shipping and fisheries waste is also contributing, but the lion’s share of mismanaged waste is coming from land,” she said.
The Ocean Cleanup Foundation said they would use the research to develop technologies that, they claim, would be able to “clean up 50 per cent of the Great Pacific Garbage Patch in five years”.