"Lightning Strikes Twice as Much Over Shipping Lanes"

Wired:

When a lot of aerosol particles are present, each one gets fewer droplets, and can float high enough into the atmosphere to freeze. In the resulting tall clouds, those bits of ice and slush run into each other and transfer electric charges. The differences in charge create an electric field, which results in lightning.

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You just need to know that more particles means more lightning, and burning fossil fuels is a reliable way to make those particles. Ships are especially culpable because they use bunker fuel to get from port to port.