Ships now emit less sulfur, but warming has accelerated

The work, led by scientists at the Department of Energy’s Pacific Northwest National Laboratory, was published today in the journal Geophysical Research Lists.

Regulations introduced in 2020 by the International Maritime Organization required an approximately 80 percent reduction in the sulfur content of marine fuel used worldwide. That reduction meant fewer sulfur aerosols were released into Earth’s atmosphere.

As ships burn fuel, sulfur dioxide is released into the atmosphere. Stimulated by sunlight, chemical mixing in the atmosphere can encourage the formation of sulfur aerosols. Sulfur emissions, a form of pollution, can cause acid rain. The change was made to improve air quality around ports.

In addition, water tends to condense on these tiny sulfate particles, eventually forming linear clouds known as ship tracks that tend to gather along shipping lanes. Sulfate can also contribute to the formation of other clouds after a ship has passed. Because of their brightness, these clouds are uniquely capable of cooling the Earth’s surface by reflecting sunlight.

The authors used a machine-learning approach to scan more than a million satellite images and quantify the decreasing number of ship tracks, estimating a 25 to 50 percent reduction in visible tracks. Where cloud cover has declined, the degree of warming has generally increased.

Further work by the authors simulated the effects of aerosols on ships in three climate models and compared cloud changes to observed cloud and temperature changes since 2020. About half of the potential warming from changes in ship emissions materialized in just four years, according to the new work. More warming is likely in the near future as the climate response continues to unfold.

Many factors — from oscillating climate patterns to greenhouse gas concentrations — are driving global temperature change. The authors note that changes in sulfur emissions are not the only factor contributing to record warming in 2023. The scale of warming is too significant to be attributed to emissions alone, according to their findings.

Because of their cooling properties, some aerosols mask some of the warming caused by greenhouse gas emissions. Although aerosols can travel great distances and have a strong impact on Earth’s climate, their lifetime is much shorter than that of greenhouse gases.

When atmospheric aerosol concentrations suddenly drop, warming can increase dramatically. However, it is difficult to estimate how much warming might occur as a result. Aerosols are one of the most important sources of uncertainty in climate projections.

“Improving air quality faster than cutting greenhouse gas emissions could accelerate climate change,” said Andrew Gettelman, an Earth scientist who led the new study.

“As the world rapidly decarbonizes and reduces all anthropogenic emissions, including sulfur, it becomes increasingly important to understand what the scale of the climate response might be. Some of the changes could happen quite quickly.”

The work also shows that the actual temperature changes could be due to changing ocean clouds, coincidentally to sulfur associated with ship exhaust, or deliberate climate intervention by adding aerosols back over the ocean. But much uncertainty remains. Better access to ship position data and detailed emissions data, along with modeling that better captures potential feedbacks from the ocean, could help strengthen our understanding.

In addition to Gettelman, Earth scientist Matthew Christensen is also an author on the PNNL paper. The work was funded in part by the National Oceanic and Atmospheric Administration.