New research reveals a decrease in ship tracks, lower cloud cover, and an increase in warming since the implementation of emissions regulations for ships in 2020.
The previous year was the hottest on record for Earth. A recent study indicates that approximately 20% of the extraordinary warmth observed in 2023 can be attributed to lower sulfur emissions from the shipping sector, especially noticeable over the northern hemisphere.
This research, conducted by scientists at the Department of Energy’s Pacific Northwest National Laboratory, has been published in the journal Geophysical Research Letters.
In 2020, the International Maritime Organization introduced regulations mandating about an 80% reduction in the sulfur content of shipping fuel worldwide. As a result, fewer sulfur aerosols were expelled into the atmosphere.
When ships burn fuel, they emit sulfur dioxide into the air. Under sunlight, chemical reactions in the atmosphere can lead to the creation of sulfur aerosols. These sulfur emissions, recognized as a form of pollution, can subsequently result in acid rain, with the primary aim of enhancing air quality near ports.
Additionally, water vapor tends to condense on these minute sulfate particles, creating linear clouds referred to as ship tracks that are typically found along major shipping lanes. Sulfate can also assist in forming additional clouds following a ship’s passage. Because of their reflective nature, these clouds play a pivotal role in cooling the Earth’s surface by deflecting sunlight.
The researchers employed machine learning techniques to analyze over a million satellite images, revealing a 25 to 50% decrease in the number of visible ship tracks. Where fewer clouds were identified, the level of warming usually increased.
Moreover, by simulating the impacts of ship-related aerosols in three climate models, the authors compared cloud changes with documented observations of cloud patterns and temperature shifts since 2020. Their findings suggest that about half of the potential warming resulting from adjustments in shipping emissions has already been realized within a span of just four years. It is anticipated that more warming will occur in the future as the climate continues to respond.
Numerous elements—including shifting climate phenomena and greenhouse gas levels—impact changes in global temperatures. The researchers emphasize that the changes in sulfur emissions are not the only factor responsible for the record warmth experienced in 2023. Their findings indicate that the extent of the warming is too great to be solely ascribed to the alterations in emissions.
Some aerosols, due to their cooling effects, can obscure part of the warming caused by greenhouse gases. Despite their ability to travel significant distances and strongly influence the climate, aerosols have a much shorter lifespan compared to greenhouse gases.
When the concentration of atmospheric aerosols drops rapidly, there can be an increase in warming. However, predicting the extent of potential warming resulting from this change is challenging. Aerosols represent a major source of uncertainty in climate models.
“Improving air quality at a quicker pace than reducing greenhouse gas emissions might be propelling climate change forward,” stated Earth scientist Andrew Gettelman, who led the research.
“As the world aggressively shifts to cleaner energy and reduces all types of man-made emissions, including sulfur, understanding the potential magnitude of the climate response will become increasingly crucial. Some changes may manifest quite rapidly.”
The study also highlights that real-world temperature variations could arise from alterations in ocean clouds, whether unexpectedly due to sulfur from ship emissions or through intentional climate interventions that would reinstate aerosols over the ocean. Nonetheless, many uncertainties still exist. Improved access to ship location and detailed emissions data, along with more comprehensive modeling that considers potential ocean feedback, could enhance our understanding.
Alongside Gettelman, Earth scientist Matthew Christensen also contributed to this PNNL research. This work received partial funding from the National Oceanic and Atmospheric Administration.
