Skies studied over two of the world’s busiest shipping lanes experienced twice as much lightning as the open ocean around them, according to researchers from the University of Washington and the American Geophysical Union. Their findings provide some of the best evidence yet that human activity is altering cloud activity on a continual basis. ‘It’s one of the clearest examples of how humans are actually changing the intensity of storm processes on Earth through the emission of particulates from combustion,’ says Joel Thornton lead author of the study and atmospheric scientist at the University of Washington.
Clouds are formed when water condenses around aerosols – tiny airborne particulates. Fewer aerosols in the air will create larger, warmer cloud droplets that turn into rain, while more aerosols have the effect of shredding water into smaller cloud droplets. These smaller droplets create lightning by being lifted to higher altitudes, freezing, and then generating a positive electric charge. As the ships pass, it is thought they are pumping small aerosol particles into the air, ‘seeding’ cloud droplets that are more prone to lightning production. ‘These particles act as the nuclei on which cloud drops form and can change the vertical development of storms, allowing more cloud water to be transported to high altitudes, where electrification of the storm occurs to produce lightning,’ writes Thornton.
The data was collected from lightning storms experienced in the Indian Ocean and the South China Sea between 2005 and 2017, and showed that lightning occurred twice as often in the skies above the ships. By comparing the skies above shipping lanes to non-shipping lanes that were directly adjacent and running parallel to them, the team was able to rule out other factors that might have accounted for more lightning. Mapping the difference, it was found that increased lightning followed the ‘same angular paths’ of the shipping lanes, often with a correlation that was so clear, the lightning maps directly resembled the shipping maps. ‘All we had to do was make a map of where the lightning was enhanced and a map of where the ships were travelling and it was pretty obvious just from the co-location of both of those that the ships were somehow involved in enhancing the lightning,’ says Thornton.
The team also found that lightning strikes seem to increase over the years, especially in the South China Sea. By looking at port activity records, they found this increase correspondended to an increase in shipping - the tonnage coming in and out of Singapore had almost doubled over the same time period.
Because shipping lanes inject aerosols to otherwise remote areas of open ocean, the maps are a clear-cut example of how human combustion can be directly altering the weather. The team concludes that: ‘our findings suggest that even small absolute increases in remote marine aerosol particles due to human activities could have a substantial impact on storm intensity and lightning.’