SINGAPORE - Shipping fuel regulations introduced in 2020 have led to a substantial cut in sulphur dioxide (SO2) pollution, but may also have made the ocean warmer by reducing cloud cover, according to a modelling study in a paper published late on Thursday.

International Maritime Organization (IMO) rules to tackle marine pollution forced shippers to cut their fuel sulphur content to 0.5% from 3.5%, leading to an 80% decline in SO2 emissions, according to a research team led by Tianle Yuan at the University of Maryland.

SO2, however, besides being a major pollutant, also masks global warming by forming aerosols that thicken and brighten clouds, reflecting the sun's rays back into space.

IMO fuel standards could have been responsible for 80% of the planet's total net heat uptake since 2020, with the impact particularly pronounced in busy shipping lanes, the researchers estimated in the paper published by the Communications Earth & Environment journal.

Climate scientists identified the reduction of SO2 as a potential contributor to record ocean temperatures last year. Some also suggest cuts in air pollution around the world could have accelerated global warming.

"This cooling effect (of SO2) is well understood - and documented episodes have occurred as consequences of several major volcanic eruptions emitting SO2 during the past 2,000 years," said Stuart Haszeldine, director of the Edinburgh Climate Change Institute at the University of Edinburgh.

Haszeldine, who was not involved in the paper, said while it was difficult to make exact predictions about the impact on global temperatures, the trend was "very clear, extremely worrying and very significant".

Other scientists said the research might exaggerate the impact of the IMO fuel policy.

"Research into why recent temperatures have been so high is ongoing and the reduced sulphur content in ship fuel is only one contributing factor," said Joel Hirschi at Britain's National Oceanography Centre.

The authors said their research showed that "marine cloud brightening" could become a potential geoengineering solution to global warming.

Scientists have been looking into ways to reflect heat back into space, but proposals to inject SO2 into the atmosphere have been controversial. Other experiments have also been conducted to spray seawater into the air to thicken clouds.

(Reporting by David Stanway; Editing by Tom Hogue)