New Research: Shipping's Reduced Sulfur Emissions Behind Increased Global Warming

The IMO's sulfur directive implemented in 2020 has cleared the air of sulfur dioxide but has also thinned the cloud cover that reflects the sun's rays back into space. According to new research from the University of Maryland, this has significantly contributed to accelerating global warming. However, the research is facing criticism.

The summer of 2023 is the hottest ever recorded. Heatwaves, droughts, and wildfires plagued Asia, Africa, Europe, and North America, with headlines detailing their impact on economies, ecosystems, and human health. Could this be linked to the IMO's sulfur directive implemented in 2020, which has drastically reduced global shipping emissions since then? Yes, says a research team at the University of Maryland. Their explanation, proven with mathematical models, is relatively simple: sulfur dioxide (SO2), a major pollutant, also provides protection against global warming by forming aerosols that thicken and brighten clouds, reflecting the sun's rays back into space. Since the IMO reduced the allowable sulfur content in marine fuel from 3.5 percent to 0.5 percent, shipping's sulfur dioxide emissions have decreased by 80 percent, creating a geoengineering shock with global impact, according to the researchers. They estimate that reduced shipping sulfur emissions account for up to 80 percent of the planet's increased warming since 2020.

"If our calculation is right, it would suggest this decade will be really warm. Combined with background warming caused due to rising greenhouse gas concentrations, the added heat could mean that the record temperatures of 2023 will become 'the norm' in the coming years," commented lead researcher Tianle Yuan in New Scientist last week.

The drastic results published in the journal Communications Earth & Environment have received mixed reactions in the scientific community. Stuart Haszeldine, director of the Edinburgh Climate Change Institute at the University of Edinburgh, believes the results highlight a worrying trend. "This cooling effect is well understood – and documented episodes have occurred as consequences of several major volcanic eruptions emitting SO2 during the past 2000 years”, he noted in Interstellar.

However, Deliang Chen, professor of physical meteorology at the University of Gothenburg, is skeptical that reduced shipping sulfur emissions could have such a significant impact. "It's a short time period, and there's a lot of uncertainty around such studies. I haven't seen the article, but claiming that shipping accounts for 80 percent of the increase sounds unreasonable. There are other causes as well. For example, El Niño has been seen as a major cause of the 2023 heat records. We have had many El Niños in history and know how much it can worsen the situation," he told Lighthouse.

Laura Wilcox, associate professor at the National Centre for Atmospheric Science (NCAS) at the University of Reading, also questions the researchers' conclusions and told the Science Media Centre that the researchers are making very bold statements about temperature changes and geoengineering based on the limited material they have. Furthermore, she argues that the upper and lower bounds for the expected warming lie within the range of variability of the global average temperature, but this is not discussed at all in the study.

"For many people, a shift to a lower-sulphur shipping fuel that causes less air pollution and reduces aerosol emissions is a move away from human-induced impacts on climate, as well as a move that cuts health impacts from air pollution. Describing this as accidental geoengineering, and presenting figures which may overestimate the impacts, could lead to misguided assumptions about policies intended to curb future emissions," she said.

To the article