The difference in lightning activity can't be explained by changes in the weather, according to the University of Washington study. The researchers conclude that aerosol particles emitted in ship exhaust are changing how storm clouds form over the ocean. Water vapor condenses around aerosols in the atmosphere, creating droplets that make up clouds. The particles make cloud droplets smaller, lifting them higher in the atmosphere. This creates more ice particles and leads to more lightning, because storm clouds become electrified when ice particles collide with each other and with unfrozen droplets in the cloud. Lightning is the atmosphere’s way of neutralizing that built-up electric charge.

The results provide some of the first evidence that humans are changing cloud formation on a nearly continual basis, rather than after a specific incident like a wildfire, according to the authors. Cloud formation can affect rainfall patterns and alter climate by changing how much sunlight clouds reflect to space.

“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,” said lead author Joel Thornton, a University of Washington professor of atmospheric sciences.

Co-author Katrina Virts, now an atmospheric scientist at NASA Marshall Space Flight Center, was analyzing data from the World Wide Lightning Location Network, a UW-based network of sensors that locates lightning strokes all over the globe, when she noticed a nearly straight line of lightning strokes across the Indian Ocean.

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