Previous studies had found frequent lightning along shipping routes over the Bay of Bengal before a 2020 International Maritime Organization rule capped sulfur in fuel used by oceangoing ships, leading to a roughly 70% drop in sulfate emissions in the Bay of Bengal.
"I think there are two reasons for this," said lead author Qinjian Jin, assistant teaching professor of geography and atmospheric science at KU. "The first is the shipping activity is so frequent that it releases a lot of sulfate aerosols, more than other oceanic regions. The second is that the Bay of Bengal is an area where we see lots of strong convection that is required for lightning to occur. I think both reasons contribute to the observed frequent lightning activity over this region."
Jin said these two ocean regions best revealed the connection between shipping emissions and lightning. The KU researcher and his colleagues found lightning-stroke density - the number of individual lightning discharges, or "strokes," per square kilometer - to be about 36% lower than before the 2020 IMO sulfur cap.
"The drop in sulfates from ships can cause fewer cloud condensation nuclei, larger cloud drops, weaker convection and storms, and thus fewer ice crystals and less frequent lightning," Jin said.
The findings were published in the peer-reviewed journal Climate and Atmospheric Science. Jin's co-authors were Jianping Huang of Lanzhou University in Lanzhou, China; Jiangfeng Wei of Nanjing University of Information Science and Technology in Nanjing, China; and Bing Pu, KU associate professor of geography and atmospheric science.
A similar drop in lightning strokes was detected along other busy shipping routes, according to their findings.
"There is another region very close to the Bay of Bengal, which is the South China Sea," Jin said. "We see very similar enhanced lightning there. Another region is a very small area in the Red Sea. We see a very weak signal there, not as strong as in the Bay of Bengal and the South China Sea."
The lightning data used come from a network called the World Wide Lightning Location Network, produced by the University of Washington.
"That network has been developed for more than a decade, starting in the early 2000s," Jin said. "It provides very high-resolution lightning data. It is global data. Based on this dataset, the phenomenon of lightning decrease has been observed in those regions."
According to Jin, sulfate aerosols in the atmosphere, emitted naturally or from human activities, have two major climatic impacts.
"One is scattering solar radiation, causing a cooling effect on Earth's climate," Jin said. "The other is modifying cloud microphysical properties, such as cloud droplet size and cloud droplet number concentration. By changing clouds, they can also influence radiation."
Jin explained that once sulfate aerosols are released into the atmosphere, they can act as cloud nuclei, which is the mechanism by which sulfates released in ship exhaust can boost lightning strokes.
"When we have more sulfate aerosols, or more cloud nuclei, the cloud droplets become smaller," he said. "When they're smaller, it's harder for precipitation to occur. Clouds can last longer in the atmosphere. With a longer lifetime, they have a higher chance to develop into high clouds, where ice clouds form. When we have more ice clouds, we have a higher chance of lightning. That is how sulfate aerosols can be connected to lightning."
While the 2020 regulations on shipping were intended to clean up the air, the reduction in lightning can be seen as a side benefit, as lightning can be dangerous to mariners and equipment, and can hinder visibility and normal operations at sea. Jin said another consequence of the shipping regulation might be warmer global temperatures.
"Due to the 2020 emission regulation imposed by the International Maritime Organization, we observed a decrease in sulfur emissions from ships after 2020," he said. "With less sulfate aerosol emitted from ships, we observed darker clouds over the North Atlantic Ocean and the Pacific Ocean. Because clouds become darker, they absorb more solar radiation. Our previous studies imply that the decrease in shipping sulfate aerosols could be responsible for the record-breaking global warming temperatures in 2023 and 2024."
Jin said his future research will aim to better understand this potential effect.
"This is an even bigger impact of changes in ship-emitted sulfate aerosols," he said. "Most current studies use modeling simulations with global climate models. One drawback of global climate models is their coarse spatial resolution, which have limited ability to resolve boundary layer cloud processes in the atmosphere. That affects the accuracy of aerosol impacts on radiation and clouds. In the future, I plan to use regional climate models with high spatial resolutions. These can resolve cloud formation, especially stratocumulus clouds, which are low clouds and very reflective. With high-resolution regional climate models, we can have a better estimation of the impacts of the decrease in ship-emitted sulfate aerosols on global temperature increase."
Further, Jin plans to study the effects of regulations by nations in Asia that cut sulfate emissions beginning about 15 years ago.
"Based on our results, we know that with less sulfate aerosol, we could have less lightning activity," he said. "This is a very important conclusion. During the past decade, many Asian countries implemented clean air acts to improve air quality. Our study in 2021 shows that starting around 2010, we observed a decreasing trend in sulfate aerosols and other aerosol species. This could result in less lightning in Asian countries. We have a plan to look at the long-term trends in lightning in Asian countries."
Research Report:Observational evidence of reduced Bay of Bengal lightning since 2020 linked to cloud responses to shipping emission regulations
Related Links
World Wide Lightning Location Network
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