Stricter international limits on sulphur in ship fuel are reducing lightning activity along some of the planet’s busiest shipping routes, according to new research from the University of Kansas.
The study links a sharp drop in sulphur emissions from oceangoing vessels to a significant decline in lightning stroke density over heavily trafficked routes in the Bay of Bengal and the South China Sea. The findings were published in the peer-reviewed journal Climate and Atmospheric Science.
Researchers found that lightning strokes – individual electrical discharges – fell by about 36% in these regions after a 2020 rule by the International Maritime Organization (IMO) capped the sulphur content of marine fuel. Earlier studies had shown that the regulation led to roughly a 70% reduction in suphate emissions over the Bay of Bengal.
Lead author Qinjian Jin, an assistant teaching professor of geography and atmospheric science at the University of Kansas, said: ‘I think there are two reasons for this. Shipping activity is so frequent that it releases a lot of sulphate aerosols, more than other oceanic regions. And the Bay of Bengal has strong convection, which is required for lightning to occur.’
Sulphate aerosols, whether from natural sources or human activity, play a key role in cloud formation. When released into the atmosphere, they act as cloud condensation nuclei, helping clouds form smaller droplets that persist longer and are more likely to grow into tall, ice-rich clouds capable of producing lightning.
Jin explained: ‘The drop in sulphates from ships can cause fewer cloud condensation nuclei, larger cloud drops, weaker convection and storms, and thus fewer ice crystals and less frequent lightning.’
Using data from the World Wide Lightning Location Network, the researchers tracked lightning activity before and after the 2020 sulphur cap. Similar, though weaker, declines were also detected along busy shipping routes in the Red Sea.
While the IMO regulation was designed to reduce air pollution and protect human health, the reduction in lightning may be an added benefit. Lightning poses risks to ships, crews and onboard equipment and can disrupt visibility and normal maritime operations.
There is, however, a downside to the eradication of sulphur in the atmosphere. Sulphates help to cool the planet by reflecting sunlight and making clouds brighter and more reflective. With fewer sulphate aerosols in the atmosphere, clouds can become darker and absorb more solar radiation.
Jin said earlier research by his team suggests that the reduction in ship-emitted sulphates may have contributed to record-breaking global temperatures observed in 2023 and 2024.
Future research will focus on better quantifying this effect using high-resolution regional climate models, which can more accurately capture cloud formation processes than traditional global models. Jin also plans to examine whether long-running clean air policies in Asian countries have led to similar declines in lightning activity over land.
The full research can be read here
Photo: Luca Carrà
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