In 2023, Earth experienced its warmest year on record, with nearly 20% of the record warmth believed to be a result of reduced sulfur emissions from the shipping industry. Regulations implemented by the International Maritime Organization in 2020 required a significant reduction in the sulfur content of shipping fuel globally, leading to fewer sulfur aerosols entering the atmosphere. When ships burn fuel, sulfur dioxide is released, which can contribute to the formation of sulfur aerosols and pollution, leading to acid rain. The reduction in sulfur emissions was intended to improve air quality around ports but inadvertently contributed to the warming trend over the northern hemisphere.
The study, led by scientists at the Department of Energy’s Pacific Northwest National Laboratory, found that the decline in sulfur emissions resulted in a decrease in visible ship tracks in satellite images, leading to a rise in temperatures in those areas with reduced cloud cover. The bright sulfate particles from sulfur emissions can reflect sunlight, creating linear clouds known as ship tracks that can cool Earth’s surface. The authors used a machine learning approach to quantify the decline in ship tracks and then simulated the effects of the aerosols in three climate models to compare with observed temperature changes since 2020. The study estimated that half of the potential warming from the changes in shipping emissions had already materialized in just four years, with further warming expected to follow as the climate response continues to unfold.
While changes in sulfur emissions played a significant role in the record warmth of 2023, the authors note that they are not the sole contributor to global temperature changes. Various factors such as greenhouse gas concentrations and climate patterns also influence temperature fluctuations. Despite their cooling properties, aerosols are shorter-lived than greenhouse gases and can have a significant impact on Earth’s climate. When atmospheric aerosol concentrations decrease suddenly, warming spikes may occur, leading to uncertainties in climate projections. Earth scientist Andrew Gettelman, who led the study, suggests that accelerating climate change by cleaning up air quality faster than limiting greenhouse gas emissions could have unintended consequences.
The study highlights the complexities of climate interventions, whether unintentional through changes in sulfur emissions from shipping or deliberate by adding aerosols over the ocean. Climate models need to be refined to better capture potential feedback from the ocean and improve our understanding of the impact of changing aerosol concentrations on global temperatures. The research team, which also includes Earth scientist Matthew Christensen, emphasizes the need for better access to ship position and emissions data to strengthen climate modeling efforts. Funding for the study was provided in part by the National Oceanic and Atmospheric Administration, indicating the importance of understanding the implications of sulfur emissions on Earth’s climate in the future.
