A significant regulatory change in 2020 aimed at reducing air pollution from shipping worldwide may have inadvertently contributed to higher global average temperatures, suggests a controversial study.
Termed as the “largest change in oil market history,” the International Maritime Organization (IMO) implemented new standards on January 1, 2020, requires shipping to reduce their fuel sulfur content from 3.5% to 0.5%.
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According to a paper published in the journal Communications Earth & Environment on May 30, a team of scientists reported an 80% reduction in sulfur dioxide emissions due to the rule change. They suggest this reduction could partly explain the extreme heat experienced last year.
Lead author Tianle Yuan, a research scientist at the University of Maryland, described the impact of the clean air regulations as “an inadvertent geoengineering event.” This is because sulfur dioxide, a pollutant produced when sulfur-containing fuels like coal or petroleum oil are burned, reacts with water vapor to create aerosols that reflect sunlight, thereby cooling the atmosphere.
While aerosols have a direct cooling effect, their overall impact on global climate when their levels decrease remains a complex topic of study, according to climate scientists.
“Characterizing this as unintended geoengineering and presenting potentially exaggerated figures could lead to incorrect conclusions about policies aimed at reducing future emissions”
The study noted that the sharp decline in sulfur emissions since 2020 lends support to the feasibility of marine cloud brightening, a burgeoning area of scientific inquiry that some researchers believe could potentially mitigate global warming by cooling the planet.
The question of whether reductions in sulfur dioxide emissions might have contributed to global warming is familiar to climate scientists, but the discussion has regained prominence in light of recent severe heatwaves in the North Atlantic and various parts of Europe.
These extreme temperatures are exacerbated by the climate crisis, primarily driven by the combustion of fossil fuels.
“There are three factors being considered to explain why 2023 was exceptionally warm,” said Jim Haywood, professor of atmospheric science at the University of Exeter, in a telephone interview. “The first one that everyone knows about is El Nino. The second, which is less widely recognized, is the explosive volcanic eruption of Hunga Tonga, which was highly unusual. And the third factor is the IMO shipping regulations.”
The El Nino weather phenomenon, a natural climate pattern that contributed to a significant rise in global temperatures, is reportedly showing signs of weakening, according to the U.N. weather agency. A shift towards the cooling effects of La Nina conditions is anticipated later this year.
Jim Haywood, professor of atmospheric science at the University of Exeter, expressed agreement with prominent climate scientist Jim Hansen’s view that global temperatures may not return to previous levels. Haywood suggested that aerosol-cloud interactions might be underestimated in climate models, potentially accelerating warming trends.
He emphasized the difficulty in precisely quantifying these impacts due to variations in how climate models simulate sulfur dioxide emissions. Haywood acknowledged uncertainty regarding the specific influence of IMO regulations on global temperatures.
Regarding the study’s findings, Joel Hirschi, associate head of marine systems modeling at the National Oceanography Centre, acknowledged the likelihood that reducing sulfur in ship fuel since 2020 accelerated global warming. However, he cautioned that the study might have overstated the impact of these reductions on the record-breaking temperatures observed in recent years.
Hirschi emphasized that multiple factors contribute to the unprecedented warmth seen in 2023 and 2024, highlighting ongoing research into these complex interactions. Meanwhile, Laura Wilcox, associate professor at the University of Reading’s National Centre for Atmospheric Science, questioned the study’s assertions about temperature changes and geoengineering, suggesting they may be overstated given current evidence.
Wilcox noted that reducing sulfur emissions from shipping fuels is generally seen as a positive step for both climate and public health, cautioning against characterizing it as inadvertent geoengineering without clearer evidence of its precise impacts.
