In January 2020, the International Maritime Organization (IMO) enforced new emissions standards intended to cut global maritime vessel pollution. A major undertaking that had been years in the making, the IMO had pledges from countries across the developed and developing world, rendering the initiative perhaps the greatest collaborative environmental achievement to date.

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The most commonly used marine fuel at the time was estimated to have approximately 2.7% of sulfur content. The IMO’s objective was to ban those vessels with a sulfur content higher than 0.5%, and over 170 countries enthusiastically agreed.

Yet, a recently published paper in Communications Earth & Environment suggests that the impact of the clean air regulations could very likely be an “inadvertent geoengineering event.” In short, it could have aided in boosting, not lowering, global average temperatures over the past 2 to 3 years. 

The shipping industry has long been under pressure to reduce sulfur emissions. The pollutant is an element of acid rain, which contributes to the acidification of the oceans, harming wildlife and vegetation. Yet, the same pollutant reacts with water vapor to create aerosols that reflect sunlight back into space. These aerosols, at high levels, have a cooling effect, and some researchers are positing that the pre-IMO enforced emissions could have helped mitigate the warming effect. 

Last year’s record-breaking heat was extreme. Most of the world’s scientists cite 2023 as the hottest year on record, and the IMO rule change resulted in an 80% reduction in sulfur dioxide emissions. While much research remains, a decrease in sulfur emissions is just potentially one factor of why 2023 was so hot. The El Nino weather phenomenon caused a spike in global temperatures and the undersea volcanic eruption of Hunga Tonga was a highly unusual event estimated to have had an impact on the warming of the Earth. 

The study is being treated with great caution. The positive health impacts alone from less pollution are undoubtedly improved with a cutback of sulfur. However, should this end up being an “inadvertent geoengineering event,” the ripple effects on the larger environmental movement will be noticeable.