(Data source: Dong et al, Science 376, 1300–1304 (2022); Graphic: Javiera Pizarro Osorio / China Dialogue Ocean)

World - Study: Oil Slicks Cover an Ocean Area Twice the Size of Turkey

More than 90% of oil slicks are caused by shipping and human activity on land. Fixing the problem should start with shipping.

Images of seabirds covered in crude oil might bring home the impact of spills on ocean ecosystems, but oil in the oceans doesn’t always come from newsworthy incidents. Regular discharges from land and from ships at sea make up a far bigger proportion – and deserve more attention.

Chinese and US researchers published a paper in Science earlier this year that for the first time sketches out a global map of oil pollution at sea. It found that anthropogenic contributions accounted for around 94% of the cumulative area of ocean oil slicks between 2014 and 2019. The researchers identified 21 oil slick “belts” that match up closely with busy shipping lanes.

The oil in these cases tends to be lighter and more spread out than that from oil spills – and therefore harder to clean up. Like ocean plastic, the dangers of these slicks are not yet fully understood but they may worsen as human activity continues to increase. There is a particular need to toughen oversight of oil discharges from the shipping industry.

What are oil slicks?

An ocean slick is a very thin layer of oil floating at sea. It can be made of crude oil, oil products like fuel oil and diesel, and even vegetable oil.

Unlike big oil spills, slicks tend to drift with the wind, tides and currents. At certain wind speeds they can be spotted on radar. This allowed the research team to process over 560,000 radar images taken between 2014 and 2019 from the Google Earth Engine, and combine these with other data to create a global oil slick dataset and map. They found oil slicks covering a cumulative ocean area of 1,510,000 square kilometers – twice the size of Turkey.

The global distribution of oil slicks. The top map (A) shows how oil slicks are primarily distributed along the coasts. The enlarged views (B–G) of areas marked by white dashed boxes on the top map show high-density belts of oil pollution that coincide with shipping routes. (Graphic © Dong et al, Science 376, 1300–1304 (2022). Reprinted with permission from AAAS.)

While the area covered by oil slicks is relatively small given the size of the ocean, the resulting harm to the environment is significant. The damage depends on how quickly the oil is released, the nature of the oil – in particular, the quantity of toxic compounds it contains – and the vulnerability of the local ecology.

Crude oil forms thick slicks that prevent photosynthesis by phytoplankton, the tiny organisms at the bottom of the ocean food chain. This in turn reduces oxygen levels and causes die-offs.

But more common is long-term exposure to low levels of oil. That can affect survival and breeding rates for seabirds and ocean mammals.

Fish and other forms of ocean life are particularly vulnerable to oil exposure early in their development. Hu Chuanmin, one of the paper’s authors and a professor at the University of South Florida’s College of Marine Science, said: “The eggs of some fish species float on the surface of the ocean and even a tiny amount of oil can be fatal.”

Also, polycyclic aromatic hydrocarbons, which are harmful pollutants that result from burning oil and other fuels, can affect cardiac development in larval fish, as well as accumulating in the food chain and presenting a safety risk to people.

Where does the oil come from?

Ocean oil slicks can result from natural or anthropogenic sources. Some oil naturally seeps from fissures in the ocean floor, much like water from a spring; the rate of release is slow and local ecosystems can adapt, or even thrive. Anthropogenic sources include oil platforms and pipelines, but releases are mainly from activities on land and from shipping.

(Data source: Dong et al, Science 376, 1300–1304 (2022); Graphic: Javiera Pizarro Osorio / China Dialogue Ocean)

In 2003, the US National Research Council (NRC) reported that between 1990 and 1999 natural and anthropogenic sources accounted for 46% and 54% of oil slicks by volume, respectively. The more recent paper, which looks at area rather than volume, finds that natural sources account for only 6.2% of the total, with anthropogenic sources responsible for the rest.

Hu Chuanmin said that the NRC report used a number of assumptions and inferences, while the oil slick map is based on direct measurements of area, and so the two sets of figures cannot be compared: “The two studies used very different statistical methods. But looking at it logically, the increase in human activity over the last 20 to 30 years, particularly in shipping, will have increased anthropogenic source oil slicks.”

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