Cutting pollution from the shipping industry accidentally increased global warming, study suggests

The shipping industry's attempt to reduce air pollution has inadvertently accelerated global warming in the short term and contributed to record-breaking sea temperatures, according to a new climate model.

Recent global shipping regulations slashed the sulfur dioxide emissions from cargo ships by a dramatic 80%. But this rapid reduction in sulfur pollution may have "created an inadvertent geoengineering termination shock with global impact," a new study has suggested. 

"The warming effect is consistent with the recent[ly] observed strong warming in 2023 and [is] expected to make the 2020s anomalously warm," the researchers wrote. The warming is equivalent in magnitude to "80% of the measured increase in planetary heat uptake since 2020."

And this reduction in pollution "could lead to a doubling (or more) of the warming rate in the 2020s compared with the rate since 1980," the researchers suggested in the new study, published May 30 in the journal Communications Earth and Environment.

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The new shipping regulations, which were implemented in 2020 by the International Maritime Organization (IMO) reduced the maximum sulfur content in shipping fuel from 3.5% to 0.5%, with the aim of improving air quality and preventing an estimated 30,000 premature deaths each year. 

But aerosols such as sulfur dioxide particles are highly reflective, and when they are released they settle in the stratosphere and bounce the sun's rays back into space — sometimes acting as a giant planetary sunblock. 

So when the regulations brought decades of massive pollution to an end, they began an unintended geoengineering experiment. Since March 2023, the loss of the sulfurous fog — combined with accelerating global warming due to burning fossil fuels, the El Niño climate pattern and the 2022 eruption of the Hunga Tonga-Hunga Ha'apai volcano — has sent average sea surface temperatures to record-shattering highs.

However, other climate scientists have disputed some of the study's conclusions. Gavin Schmidt, the director of NASA's Goddard Institute for Space Studies, told Live Science that, while the researchers' estimate for the increased solar energy entering Earth's atmosphere is accurate, "their estimate of the temperature response is not quite right, I think."

Schmidt pointed to an analysis made by Zeke Hausfather, a climate scientist at the Breakthrough Institute, which argues that the study's warming calculation relies on an overly simplified model that misunderstands heat uptake from the ocean, meaning the study overstates the sulfur reduction's warming impact. 

"We are still waiting on updated analyses related to the HTHH [Hunga Tonga-Hunga Ha'apai] volcano, other aerosol emissions, the solar cycle and various aspects of internal variability," Schmidt added. "That is on top of other analyses of the IMO rules that are ongoing."

The findings come at a time when governments are eyeing up controversial solar radiation management (SRM) techniques, which propose to dim the sunlight reaching Earth by intentionally releasing aerosols such as sulfur dioxide into the atmosphere. 

But Schmidt believes that these techniques are not viable. 

"I strongly doubt that this [solar radiation management] will ever be part of any sustainable response to climate change — but the issues that underlie that conclusion have very little to do with the science and almost everything to do with how such an effort is governed and how fragile it will be to economic or geopolitical uncertainties," Schmidt said.