The frequency of Schumann resonances is directly related to the number of lightning events occurring on Earth. When there are more lightning strikes, the intensity of the electromagnetic waves in the atmosphere increases, resulting in a higher frequency of Schumann resonances. Conversely, a decrease in lightning activity can result in a lower frequency of Schumann resonances.
Reference: https://www.physicsforums.com/threads/connection-schumann-resonance-and-lightning-events.922310/
Global lightning monitor:
https://map.blitzortung.org/#1/18.1/8