Study: Shipping's Sulfur Cut Caused Worldwide Heat Wave

Average temperatures have soared off the charts over the lastr year, catching climate scientists by surprise. Climate models have long predicted an accelerating pattern of warming, but a sudden jump of 0.16 C wasn't in the forecast, and experts have been debating the cause. One suspect is the shipping industry's sudden switch to low-sulfur fuel, effective in January 2020. With the exception of the Russian "dark fleet," the overwhelming majority of shipowners have adopted IMO2020-compliant solutions, like very low sulfur fuel oil, exhaust scrubbers or LNG dual-fuel power. Cumulatively, this has reduced hazardous sulfur emissions from shipping by 80 percent, improving air quality and health outcomes for coastal residents.

That same reduction in sulfur from shipping has had an unintended impact on the climate, climatologists say. Sulfur particles reflect light and seed cloud cover, which reduces the amount of heating at the ocean surface. It's effective enough that deliberate sulfur pollution has been suggested as a potential way to cool the planet. Conversely, less sulfur means more sunshine on the surface, and more heating. 

Scientists disagree over just how meaningful the drop in sulfur emissions might be; the El Nino-La Nina transition and an increase in solar activity could also be contributing factors to sudden warming. But a new study suggests that IMO2020 could be the leading cause of the unexpected increase in temperature. 

According to Dr. Tianle Yuan of the University of Maryland and co-authors, the sudden imposition of IMO2020 created a shock to the climate system through a substantial reduction in suspended pollutant particles (aerosols) and cloud density. The highest impacts were found in the busiest shipping lanes, like the North Atlantic - where ocean surface temperatures have soared - and the South China Sea and Caribbean. The authors calculated that IMO2020 alone was responsible for 80 percent of the increase in heat energy retained on Earth from 2020-2023. 

The authors also warned that the effect is a small-scale version of what might happen in a geoengineered future. If humanity ever decides to put sulfur into the atmosphere as a way to cut down on warming, and continues emitting carbon at the same time, a much larger "shock" could occur if sulfur geoengineering ever stopped. 

Others are not so sure that shipping's sulfur cut bears the blame. "The [sulfur reduction] is certainly a contributing factor to the recent warmth, but it only goes a small way toward explaining the 0.3C, 0.4C, and 0.5C margins of monthly records set in the second half of 2023,"  Dr. Zeke Hausfather, at analysts Carbon Brief, told The Guardian.