More than 90 per cent of oil slicks threatening global marine life caused by humans, US and Chinese scientists find

More than 90 per cent of chronic oil slicks in the world's oceans come from human sources, not just half as previously thought, new satellite image analysis indicates.

Ships, oil platforms and pipelines were responsible for as much as 94 per cent of the ecologically damaging phenomenon, while only the remaining 6 per cent came from natural seeps, researchers from China and the United States wrote in a recently published article.

Their findings show a major shift from an estimate by the US National Research Council that found a roughly 50-50 split in human and natural sources by slick area in the 1990s, the team said.

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Oil slicks are thin layers of oil on the ocean surface that are continuously produced by human activities and natural seepages from the sea floor.

Massive oil spills could also cause them. In 2010, a staggering 4 million barrels of oil were released in the largest marine oil spill in history after the Deepwater Horizon drilling rig in the Gulf of Mexico exploded and sank, claiming 11 lives.

And last year, 9,400 tonnes of oil spilled into the Yellow Sea after a tanker collision off the Chinese port city of Qingdao, polluting nearly 800km of coastline, according to an investigation report released in January. Chinese authorities said it would take at least 10 years for fishing resources to recover to the pre-pollution level.

For the latest study, scientists from China's Nanjing University, and the South Florida and Florida State universities in the US analysed more than 560,000 images of global oceans from 2014-19, taken by the European Space Agency's Sentinel-1 satellite.

Their two-year study found more than 452,000 locations of oil slicks, covering twice the area of France when combined.

"Our results reveal the overwhelming contributions of anthropogenic oil slicks compared with those from natural seeps that are probably due to increased human activities," the researchers said in their article published in the peer-reviewed journal Science on Friday.

While oil slicks were mainly distributed along the coasts, with about half of them seen within 38km of coastlines, the top offshore regions for them were the Java Sea, the South China Sea and the Gulf of Guinea, according to the study.

The team also found 21 high-density belts which coincided well with shipping routes, including areas in the Mediterranean Sea and the Black Sea, and ship-induced pollution belts in the Strait of Hormuz, the Bay of Bengal, the Strait of Malacca, the Java Sea and the Yellow Sea.

Almost 83,000 linear oil slicks were also discovered, most probably caused by ship discharges. These slicks and belts together accounted for nearly 20 per cent of the global total, the study indicated.

"Although the International Convention for the Prevention of Pollution from Ships has been in place since 1983, the substantial contribution of the ship discharges still suggests the urgent need for more effective international cooperation and informed regulation," it said.

Study co-author Liu Yongxue, a professor at Nanjing University's School of Geographic and Oceanographic Science, said intergovernmental collaboration was key to preventing the global problem of oil slicks, which threatens marine ecology and food safety.

"While some marine organisms that live near fixed natural seep locations would be used to oil, a sudden and massive oil spill can be deadly for other marine lives," he said.

Whales and sea turtles, for example, can be harmed when they rise to the surface to breathe and come into contact with the floating oil, whereas even tiny amounts of the substance can affect the plankton that make up the base of the marine food chain.

"Oil also contains cancer-causing substances that can enter the marine food chain and take a toll on human health via seafood," said Liu, who specialises in ocean and coastal remote sensing.

The slicks can be hard to track because the thin patches of oil are moved around by the wind and ocean currents, and broken apart by waves as they travel.

He said it took the team more than two years to analyse the satellite images which provided them with snapshots of the global ocean, allowing them to monitor oil pollution, particularly in waters not easily reachable by humans.

This article originally appeared on the South China Morning Post (SCMP).

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