‘Inadvertent geoengineering’: Researchers say low-sulfur shipping rules have made climate change worse

A sweeping regulatory change in 2020 aimed at reducing air pollution from ships around the world may have played a role in rising global average temperatures, according to the findings of a controversial study.

Described at the time as “the biggest change in the history of the oil market”, the International Maritime Organization (IMO) imposed new standards on January 1, 2020 to reduce the sulfur content of fuels to 0.5%. , compared to 3.5% previously.

The rule change led to an 80% reduction in sulfur dioxide emissions, a team of scientists said in a paper published May 30 by the journal Communications Earth & Environment, and could help explain why the world’s record heat he last year was so extreme.

Tianle Yuan, a research scientist at the University of Maryland and lead author of the study, said via social media that the regulation’s impact on air quality could be described as “a geoengineering event inadvertently “.

This is because sulfur dioxide, a pollutant that forms when sulfur-containing fuels like coal or oil are burned, reacts with water vapor to produce aerosols that reflect sunlight back into space .

Aerosols have a direct cooling effect, although climatologists note that their contribution to global cooling or warming when reduced remains a complex area of ​​research.

The study says the sharp drop in sulfur emissions since 2020 supports the viability of marine cloud brightening, an area of ​​growing scientific interest that some researchers say could be used to contribute to global cooling. .

The question of whether a reduction in sulfur dioxide emissions may have contributed to global warming is not new to climate scientists, but the debate has recently resurfaced following extreme heat waves in the North Atlantic and many regions of Europe.

Extreme temperatures are fueled by the climate crisis, the main cause of which is the burning of fossil fuels.

“There are three interesting things that people are trying to understand to explain why 2023 has been so hot and the first one that everyone has heard of is El Nino,” said Jim Haywood, a professor of atmospheric sciences at the University of California. ‘Exeter in the United Kingdom, at CNBC. by telephone.

“The second point is that people don’t really know about Hunga Tonga, a very unusual explosive volcanic eruption… And the third is the IMO maritime regulations,” Haywood said.

The El Nino weather phenomenon, a natural climate phenomenon that helped fuel a rise in global temperatures, recently showed signs of ending, according to the United Nations weather agency. A return to the cooling influence of La Niña weather is expected later this year.

“You have these stellar climate scientists like (former NASA scientist) Jim Hansen saying we’re not going to get a rebound and the temperature trace isn’t going to come back into the pack – and I think I’m kind of with him on this one,” Haywood said.

“I think there are many ways to underestimate the interactions between clouds and aerosols in climate models, which could have had an accelerated impact,” he added.

“It’s very difficult to quantify exactly how much. All climate models will give you slightly different answers because of how they emit sulfur dioxide,” Haywood said. “We are therefore uncertain about the impact the IMO regulations will have on global average temperatures.”

The scientists, who were not involved in the paper, largely praised the study as timely, but some said the research could exaggerate the impact of the IMO regulations.

Joel Hirschi, associate head of marine systems modeling at the UK’s National Oceanographic Centre, said the study showed that the reduction of sulfur in ship fuel since 2020 would likely have accelerated global warming.

However, Hirschi said the authors likely overestimated the impact of reducing sulfur in ship fuel on the record global temperatures seen last year and in 2024.

“The record temperatures we have seen in 2023 and 2024 are remarkable and cannot be explained by a single factor. Research into why recent temperatures have been so high is ongoing and reduced sulfur content in ships’ fuel is only one contributing factor. ” Hirschi said.

Separately, Laura Wilcox, associate professor at the National Center for Atmospheric Sciences at the University of Reading in the United Kingdom, said the study “makes very bold claims about temperature change and geoengineering that seem difficult to be justified on the basis of evidence.

“For many people, switching to a low-sulfur transportation fuel, which causes less air pollution and reduces aerosol emissions, constitutes a move away from anthropogenic impacts on the climate, as well as a move that reduces health impacts of air pollution,” Wilcox said.

“Describing this as accidental geoengineering and presenting figures that may overestimate impacts could lead to incorrect assumptions about policies intended to reduce future emissions,” she added.