Study: Cleaner shipping fuel accelerated global warming

An international effort to improve air quality by requiring ships to use less-polluting fuel caused a spike in global warming, according to research published Thursday on this unintended climate "shock".

Global shipping's switch to low-sulphur fuels starting in 2020 "could lead to a doubling (or more) of the warming rate" this decade and has already contributed to record-breaking heat over the past year, the study said.

That is because the tiny particles in sulphur pollution reflect and absorb sunlight and make clouds more mirror-like, creating a temporary cooling effect on the planet.

Scientists had anticipated that switching to the cleaner fuels would reduce this reflecting effect and accelerate warming, even if they debated by how much.

The study suggests that deliberately brightening clouds could put a brake on global warming, even if such measures do not address the underlying driver: pumping greenhouse gases into the atmosphere.

Scientists warn that such "geoengineering" approaches to climate change may have unwanted side-effects, and say that caution is needed.

For the new study, researchers combined satellite observations and model simulations to estimate the climate impact of slashing sulphur in shipping fuel under an International Maritime Organization regulation that took effect in January 2020.

Introduced to limit airborne pollution, this ruling reduced sulphur dioxide emissions from the global shipping industry by 80 percent, said Tianle Yuan, lead author of the study published in the journal Communications Earth and Environment.

- Substantial effect -

But the rapid drop also had a "shock" effect on the planet, he added, dimming the capacity of clouds to reflect some of the Sun's energy back into space, resulting in a "substantial warming effect".

"It would essentially double the warming rate of the 2020s," the senior research scientist at NASA's Goddard Space Flight Center told AFP.

This effect was uneven across the globe but it appeared there was a stronger impact in the North Atlantic, leading to warmer sea surface temperature, Yuan said.

"It contributed to the anomalous warming we experienced in 2023 and 2024. However, we cannot attribute precisely how much" and they were unable to say it was the main cause, he added.

Last year was the hottest year on record and that trend has crashed over into 2024, with temperatures on land and sea hitting new monthly highs.

Human activities are the dominant cause of warming but scientists have also looked at other possible contributors to this extraordinary heat, including the role of El Nino, a periodic natural weather pattern over the Pacific.

- Open question -

Nicolas Bellouin, a climatologist from the University of Reading, told AFP that this latest study was "more scientifically sound" than previous research and "should be taken seriously".

"But I think the contribution of this warming to the 2023 anomaly and future warming rates remains an open question, even after this study," said Bellouin, who was not involved in the research.

Other scientists said the study offered evidence that seeding clouds with particles to boost their heat-reflecting capacity could help partially slow warming if greenhouse gases keep rising.

For some, geoengineering -- which includes technologies to dim incoming sunlight or reduce acid levels in oceans -- is a distraction from tried-and-tested climate solutions already at hand.

Currently, there is no formal global governance for the development or deployment of such technologies and an incomplete understanding of the risks they carry.

Edward Gryspeerdt, an expert on the physics of clouds from Imperial College London, said there remained large uncertainties around the side effects of geoengineering, such as changes to rainfall patterns.

"These risks must be better understood to make informed decisions about any future geoengineering strategies," he said.