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

A sweeping regulatory change in 2020 to slash air pollution from the world's ships could have played a role in boosting global average temperatures, according to the findings of a controversial study.

Described at the time as the "biggest change in oil market history," the International Maritime Organization (IMO) enforced new standards on Jan. 1, 2020, to cut their fuel sulfur content to 0.5%, down from 3.5%.

The rule change resulted in an 80% reduction in sulfur dioxide emissions, a team of scientists said in a paper published by the Communications Earth & Environment journal on May 30, and may help to explain why last year's record-breaking heat was so extreme.

Tianle Yuan, a research scientist at the University of Maryland and the study's lead author, said via social media that the impact of the clean air regulations could be described as "an inadvertent geoengineering event."

That's because the sulfur dioxide, a pollutant which forms when sulfur-containing fuel such as coal or petroleum oil is burned, reacts with water vapor to produce aerosols that reflect sunlight back into space.

The aerosols have a direct cooling effect, though climate scientists note that their contribution to global cooling or warming when they are reduced remains a complex area of research.

The study said the abrupt fall in sulfur emissions since 2020 supports the viability of marine cloud brightening, a growing area of scientific interest which some researchers say could be used to help cool the planet.

The question over 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 across the North Atlantic and many