Oil slicks blanket an estimated 1.58 million square kilometres of the world's oceans every year — an area roughly three times the size of Spain — according to satellite analysis that quantifies, for the first time at global scale, the chronic pollution generated largely by commercial shipping and offshore energy infrastructure.

Conservation technology nonprofit SkyTruth published the findings on 23 September during Climate Week NYC. The analysis draws on satellite observations collected between 2023 and 2025 through Cerulean, the organisation's oil pollution monitoring system. Researchers modelled annual pollution from vessels and offshore facilities, adjusting for gaps in satellite coverage and differences in vessel density.

The headline number for the shipping industry is stark: ships were linked to around 81 per cent of the oil slicks whose likely source could be identified. Oil tankers and cargo ships were the most common culprits. Offshore oil and gas infrastructure accounted for another 18 per cent.

"Oil pollution is a constant, global issue that happens far more often than the spills that make headlines," says Karl Wuster, chief technology officer at SkyTruth.

That framing matters for carriers and charterers. Unlike the Deepwater Horizon disaster, most of this pollution comes from smaller, routine slicks — the operational discharges, tank washings and bilge dumping that rarely trigger headlines but now, increasingly, leave a satellite-detectable trail tied to individual vessels via AIS data.

The geography of the pollution tracks the world's busiest trade lanes and hydrocarbon corridors with uncomfortable precision. More than 90 per cent of the estimated pollution occurs within national waters, often close to coastlines. Concentrations run highest in the Red Sea, the Persian Gulf, the Mediterranean, the Black Sea, waters off West Africa and around Indonesia — chokepoints and loading zones that already face elevated security and insurance risks.

The analysis also identified repeat offenders. Researchers repeatedly observed the same 16 vessels alongside oil slicks. Three of them — a Palau-flagged cargo ship, a Russian oil tanker and an Indonesian oil tanker — appeared near slicks in more than one in five satellite images. For tanker operators and cargo owners, that level of persistent, vessel-level attribution signals a growing exposure: charterers vetting counterparties and insurers pricing hull and pollution cover now have a third-party monitoring layer that regulators and NGOs can equally access.

Fixed infrastructure shows the same pattern. Oil slicks appeared in nearly 29 per cent of satellite images of one facility off the coast of Indonesia and in about 24 per cent of images of another off Nigeria — both countries where offshore production feeds export terminals serving commodity trades.

Marine protected areas offer no refuge. Around 75 per cent of them had lower oil slick concentrations than surrounding waters, but pollution was still detected in sensitive habitats, including the critically endangered Rice's whale habitat in the Gulf of Mexico, a region criss-crossed by tanker traffic serving US crude exports and Gulf Coast refining.

The researchers caution that the true picture remains incomplete. Satellite coverage over the high seas is limited, leaving large expanses of ocean poorly monitored — meaning the 1.58 million square kilometre estimate likely understates pollution on the open ocean corridors where most deep-sea trades run.

Beyond routine operations, conflict and accidents are adding to the slick inventory. In August, oil from a carrier struck during the war in the Gulf washed ashore on Iran's Qeshm Island and reached its protected mangroves. A separate spill from a Russian shadow fleet tanker off Oman spread across more than 2,000 square kilometres — a data point that will do little to ease port states' scrutiny of ageing, opaque tanker tonnage moving sanctioned barrels.

Accidents contribute their own toll. A mystery vessel that capsized off Tobago in 2024 covered beaches in oil and triggered a national emergency. A passenger ferry that ran aground in Sweden a year earlier left a five-kilometre slick of fuel in the Baltic Sea that eventually reached the coast — a reminder, for operators in the Baltic and North Sea, that even non-tanker traffic carries pollution liability.

The legacy fleet beneath the waves compounds the risk. An estimated 8,500 potentially polluting shipwrecks, mostly dating from the two world wars, lie on the ocean floor, and some are already leaking oil as their hulls corrode.

For shipowners, the commercial implication is straightforward: continuous satellite surveillance is converting what was once an invisible operational externality into attributable, vessel-level evidence. As monitoring coverage extends beyond national waters, expect closer port state attention to discharge records in the Red Sea, Mediterranean and West African corridors — and higher compliance and insurance costs for operators whose vessels show up repeatedly in the data.