Earth Day in 2018 was the first Earth Day event in Pokémon GO.
Mission Blue, an organization dedicated to protecting the oceans, needed the help of Pokémon GO Trainers. So they teamed up with Niantic to host clean-up events around the world.
Niantic and The Pokémon Company collectively donated $250,000 to Mission Blue.
The following rewards were unlocked worldwide, and will be available until April 30th, 2018. The rewards became available at 9:00PM UTC on Monday, April 23rd, 2018 along with shiny Wailmer. 
|| 2× catch Stardust for Grass-, Water-, and Ground-type Pokémon for 48 hours
|| 3× catch Stardust for Grass-, Water-, and Ground-type Pokémon for 48 hours
Following Grass-, Water- and Ground-type, 1st stage Pokémon were reported to appear more frequently in the wild:
Moreover, these Shiny Pokémon were released with the start of the event.