Caption: Oil tanker discharges are responsible for 20 per cent of all oil slicks. Their contributions to marine oil pollution have previously been grossly underestimated.
Credit: Felix Cesare/Getty
Ocean oil slick map proves human activity almost completely to blame
Naturally seeping oil from seafloor reservoirs was thought to contribute as much to marine pollution as tankers, pipelines and drilling platforms. But new satellite data paint a very different picture.
When the Deepwater Horizon oil platform in the Gulf of Mexico exploded in 2010, it killed 11 people and released more than 3 million barrels of oil into the ocean. Images of animals foundering in the slick were beamed worldwide. But most oil spills go unnoticed, lost in the vastness of the Earth’s oceans, which cover 70% of the planet.
A Nanjing University team has now taken a significant step towards mapping oil slicks and exposing illegal and accidental spills across the Earth’s seas.
By scanning hundreds of thousands of satellite images that employed microwaves to capture details of the planet's surface through cloud, they discovered that previous surveys had grossly underestimated the oil slicks discharged from anthropogenic sources, such as tankers, pipelines and drilling platforms.
The team's inventory of oil slicks also revealed some worrying trends. “It was alarming to see the coincidences of oil slicks with shipping routes,” says Yongxue Liu, a specialist in geographic information science at Nanjing University, who led the work published in Science1.
Getting on the radar
When Liu was analyzing satellite imagery to compile an inventory of the locations of offshore oil and gas platforms, he often noticed dark patches around the platforms that suggested oil pollution.
An opportunity to test his hunch came with the launch of the Sentinel-1 twin satellites by the European Space Agency in 2014. Dedicated to monitoring changes on the Earth's surface, these satellites orbit the planet at 180 degrees to one another, snapping a photo of every location on Earth every few days. But these are not your usual optical photographs. The Synthetic Aperture Radar (SAR) on board the Sentinel-1 satellites emits invisible microwaves that penetrates clouds and receives backscattered signals from the Earth's surface.
“Sentinel-1 data can be freely accessed, has global coverage, and is immune to clouds and rain — making it a very suitable sensor for mapping oil slicks,” Liu explains.
For more than two years, Liu and five colleagues analyzed 560,000-odd SAR images collected between 2014 and 2019. The team first analyzed the images at low resolutions, eliminating the ones that obviously didn’t have oil slicks, before zooming in on the rest at the highest resolution for closer analysis.
Even so the researchers could only identify oil slicks under specific conditions, when there were winds of between 1.5 m s−1 and 10 m s−1 which stirred up the uncontaminated water but not the water covered in oil. Stronger winds broke the oil up into smaller droplets, making the slicks impossible to detect. In complete calm, the difference between the still water and the smooth oil was also indistinguishable.
The researchers found 452,057 locations with oil slicks, most of them close enough to the coast to come under the jurisdiction of coastal states. They covered a total area of 1.5 million km2, more than twice the size of France.
“The toxicity of high-concentration oil negatively affects marine biodiversity and threatens food safety and human health,” Liu says.
Anthropogenic vs natural
To work out whether the oil slicks originated from natural seeps of oil under the ocean floor, the researchers analyzed how they changed over time.
Slicks that moved, especially in straight lines, suggested leaks from ships. For recurrent oil slicks, the researchers cross-referenced inventories of oil platforms and pipelines. If there were no platforms, pipelines or aquaculture net cages, which can produce slicks from fish food, in the vicinity, the researchers attributed the slicks to natural seeps.
Through this process, they estimated that 94% of the observed oil slicks could be blamed on anthropogenic activities — much higher than an earlier estimate that found just 54%2 of marine oil slicks worldwide were due to human activity.
Caption: Oil pollution in the ocean comes mainly from human sources not from natural seepage from undersea reserves, according to a comprehensive assessment of oils slicks by Nanjing University researchers. The results were published in Science.
Ship discharges comprised almost 20% of all oil slicks. Dense oil slick belts, made up of 21 severe slicks, coincided with shipping routes in the Mediterranean Sea, Black Sea, Strait of Hormuz, Bay of Bengal, Strait of Malacca, Java Sea and Yellow Sea.
“We expected to find emissions from ships, but if a clear, high-density belt has already formed, that suggests it’s common practice for ships to discharge oil,” Liu says. A further 83 thousand smaller, linear slicks were also probably from ship discharges.
Safeguarding oceans
Liu believes the findings are almost certainly an underestimate, because their method ignored oil slicks smaller than 0.05 square km.
Since 1983, the International Convention for the Prevention of Pollution from Ships (MARPOL) has sought to reduce accidental and deliberate pollution from ships. Despite this directive, “better regulations and enforcement are clearly needed,” Liu says.
Liu hopes that their research could be translated into a real-time warning system to enable rapid spill responses — but such an effort would require international cooperation and a larger set of satellites orbiting Earth. Meanwhile, Liu is training a deep learning model to rapidly identify oil slicks from satellite data — an effort that could help with enforcement of regulations against oil spills in the future.
Reference:
1. Dong, Y., et al, Science 376, 1300-1304 (2022). DOI: 10.1126/science.abm5940
2. National Research Council, Oil in the Sea III: Inputs, Fates, and Effects (National Academies Press, 2003)
