Methane leaks from ships using liquefied natural gas as fuel make most of the vessels dirtier than ones using diesel or heavy fuel oil, a new analysis shows. 

The findings, by Brussels-based nonprofit Transport & Environment, call into question a key plank of the European Union’s proposed regulations aimed at decarbonizing the emissions heavy shipping industry. The study was based on data from the European Commission and IHS Markit. 

Nearly 80% of ships that burn LNG use a type of engine that leaks 3.1% of the fuel into the atmosphere, according to Transport & Environment. The group also used an infrared camera to document methane leaking from LNG-powered ships in Rotterdam. Their footage shows methane plumes leaking from the LNG-powered Louvre container ship, which is owned by the French shipping company CMA CGM Group, and from the EcoDelta, a dredger vessel.  

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CMA CGM said it was hard to answer questions without seeing the report. The company is “actively working” to reduce the leakage of unburned methane by improving its engines and using software, a spokesperson said, resulting in a “significant reduction” in emissions. Bloomberg data showed EcoDelta is owned by Netherlands-based Van der Kamp BV, which didn’t immediately respond to an email seeking comment. 

LNG has long been promoted as cleaner than other fossil fuels because it produces relatively less carbon dioxide when burned. But methane, which is the primary component of natural gas, traps 84 times more heat than CO₂ over its first two decades in the atmosphere and scientists say eliminating unnecessary releases is crucial to avoiding the worst impacts of climate change. 

“Most LNG ships on the market today are more damaging for the climate than the fossil ships they’re supposed to replace,” Delphine Gozillon, a shipping officer at Transport & Environment, said in a statement. “In promoting LNG ships, European policymakers are locking us into a future of fossil gas.”