When a research team at the University of Maryland claimed a little over a year ago that reduced sulfur emissions from shipping accounted for most of the planet’s increased warming since 2020, the news hit like a bombshell. But what happened afterward? Could shipping really have disrupted the entire climate balance of the Earth?

The summer of 2023 was the hottest ever recorded. Heatwaves, drought, and wildfires plagued Asia, Africa, Europe, and North America, with headlines constantly reporting the toll on economies, ecosystems, and human health. Could this be linked to the IMO’s sulfur directive introduced in 2020, which drastically reduced global shipping’s sulfur emissions?

Yes, argued the University of Maryland team. According to their calculations, the IMO’s lowering of the permitted sulfur content in ship fuel from 3.5 percent to 0.5 percent created what they called a “geoengineering shock” with global impact. Shipping was responsible for up to 80 percent of the planet’s increased warming since 2020!

The explanation was simple: sulfur dioxide (SO₂) is not only a major pollutant, it also protects against global warming by thickening and brightening clouds, which reflect the sun’s rays back into space.

The research results, published in Communications Earth & Environment, received a mixed response in the scientific community. The cooling effect of sulfur dioxide has long been known, noted some researchers who saw the findings as a worrying trend. Others were more skeptical, such as Deliang Chen, professor of physical meteorology at the University of Gothenburg.

“That shipping would account for 80 percent of the increase sounds unreasonable. There are, after all, other causes. For example, El Niño has been identified as a major driver of the record heat in 2023. We’ve had many El Niños in history and know how much they can worsen the situation,” he told Lighthouse in June 2024.

So what happened next? Have these dramatic results been confirmed? The answer is both yes and no. Since the Maryland study, a number of leading institutions – including the Met Office, Scripps, CICERO, Plymouth Marine Laboratory, Max Planck, the University of California, and others – have contributed with both model calculations and observations. All confirm that the IMO’s sulfur directive and reduced shipping emissions have affected clouds and climate – but the estimates of the extent vary. Generally, they point to a more modest impact.

In December 2024, the UK Met Office published a study showing that reduced sulfur emissions from shipping cannot explain the extreme warming in 2023. Rather, they account for a small but climatically relevant extra warming — on the order of a few hundredths of a degree globally (0.03 °C – 0.04 °C). In some regions, particularly the Arctic, the effect may be stronger.

A study funded by the Swedish Research Council, the Research Council of Norway, and the EU showed this year that although the sulfur directive has clearly affected the climate, there is no evidence that shipping’s contribution to warming up to 2023 has been statistically significant.

"Overall, while the IMO regulations may contribute up to 0.16 °C to the global mean surface temperature in individual years during this decade, consistent with some early studies, such a response is unlikely to have been discernible above internal variability by the end of 2023 and is in fact consistent with zero throughout the 2020–2040 period", the researchers write in the article Warming effects of reduced sulfur emissions from shipping.

Other research has shown that the amount of low, reflective clouds has decreased in more places worldwide, not only along shipping routes. This may be due to reduced aerosols (which thicken and brighten clouds, reflecting sunlight back into space), but also to natural variation in the climate system. Moreover, as Deliang Chen points out, a strong El Niño occurred at the same time, temporarily warming the planet.

There is also research that partly supports the Maryland results. In August 2024, a study from the Pacific Northwest National Laboratory showed that satellite data recorded about 25–50% fewer linear cloud formations, so-called “ship tracks,” which correlated with higher temperatures in 2023.

“Cleaning up air quality faster than limiting greenhouse gas emissions may be accelerating climate change,” said Andrew Gettelman, who led the research when it was published.

But Gettelman and his colleagues do not yet know how much. The picture is complex. During the same period, low clouds also decreased in other parts of the world, further reducing Earth’s ability to reflect sunlight.

In summary, the conclusion seems to be: cleaner ship fuel may have made Earth a little warmer, but it is not the whole explanation for the increased warming. At the same time, the development illustrates a difficult dilemma: the measure that saves lives by producing cleaner air has also removed a kind of “protective shield” that previously dampened warming. But, as researchers emphasize, we cannot rely on pollution to cool the planet – the long-term solution must be to rapidly reduce greenhouse gas emissions.