https://www.techradar.com/news/pokemon-gos-next-global-event-will-have-you-picking-up-trash

Niantic, the developer of Pokemon Go, has just announced that it will dish out rewards on Earth Day 2018 all in an effort to encourage its 20 million active users to get out and clean up their communities. 

Attend a local event, and you could net yourself two or three times the amount of stardust for Earth, Water and Grass-type Pokemon.