Oil slicks from ships and offshore oil platforms cover nearly 1.6 million square kilometers (618,000 square miles) of the ocean surface each year, an area slightly larger than the nation of Mongolia.
The estimate comes from a recent report by Skytruth, a U.S.-based nonprofit that used satellite imagery, machine learning and vessel tracking data to trace global oil pollution in the oceans between 2023 and 2025.
According to John Amos, CEO of Skytruth, this kind of pollution was largely invisible before satellite imagery made it possible to track.
“Any vessel operator could be pretty confident that nobody was going to notice what they were doing out in the middle of the ocean,” Amos told Mongabay by phone. “So that’s changed. We can see it now.”
Much of the vessel oil slicks come from bilge waste, a mix of oil and water that collects in the bottom of a ship’s hull during operation. Tankers sometimes also wash out their cargo holds and flush crude oil residue into the sea.
“It’s nasty oily dirty gunk that you just want to get rid of,” Amos said. “And the easiest thing to do with it is just pump it out into the ocean while you’re at sea.”
According to the report, most of these discharges are illegal under MARPOL, the international treaty on pollution from ships. A MARPOL treaty adopted in 1973 allows vessels to discharge bilge water containing no more than 15 parts of oil per million parts of water.
SkyTruth used modeling to estimate ocean oil pollution between 2023 and 2025. They found vessels were responsible for 81% of the slick area, mostly cargo ships and tankers, while oil infrastructure such as offshore oil platforms accounted for 18%.
Indonesian waters had the most verified slicks in the report, with 823, followed by Malaysia, which had 214. Both countries share the Strait of Malacca, one of the busiest waterways in the world.
“The sheer volume of large cargo ships and oil tankers transiting their waters could be largely responsible,” Amos said.
SkyTruth could only find ownership records for 9 of the 20 vessels flagged as repeat polluters.
Two of those belonged to Indonesia’s PT Waruna Nusa Sentana and another two to Nigeria’s Sea Transport Services Nigeria Ltd. Mongabay reached out to both companies for comment but did not receive a response by publication.
According to Amos, the global estimate is likely an underestimate. A ship that keeps its oily waste on board has to detour to a port with proper disposal facilities, which is expensive. A day at sea can cost a large cargo ship tens of thousands of dollars, and sometimes even more.
“So you can either do that, or you can just wait until you get out in the ocean where you think nobody’s looking, and then discharge it directly overboard,” he said.
Banner image: Oil spill in 2024 off Tibaguin Island, Philippines. Image © Noel Celis/Greenpeace.
