Redo of post (now complete):
More and more #ClimateJustice groups and activists are joining the #Fediverse and they have more instances available than ever before:
https://forfuture.social https://climatejustice.global
https://funk.climatejustice.global
https://climatejustice.rocks https://climatejustice.social
https://climatejustice.video/ https://social.rebellion.global https://tube.rebellion.global
https://mobilizon.rebellion.global
Here is a long list of accounts:
https://joinfediverse.wiki/The_Climate_Justice_Movement_in_the_Fediverse
Currently we're trying to get Greta Thunberg here as well. #JoinTheFediverseGreta.
I hope Greta will, if she joins, end up on a well moderated instance, which blocks at least the worst trolls.
Having her join and leaving shortly after, because of the harassment would be most unfortunate.
Although not specifically #ClimateJustice oriented, I feel those of us over at 
https://sunbeam.city could safely be included in this list.
@PaulaToThePeople this is a great list, thank you. I must check, is it fair to publicly-and-with-hashtag badger Thunberg to do anything? She has done much more than maybe any of us had at that age (/in our lives) - but that doesn't imo make her public property, like a brand or politician. Maybe when she's not spearheading school strike she just needs to be a teenager, and not manage another social profile and the 1000s of interactions it could generate. I know you mean good, but wanted to check.
@PaulaToThePeople Thanks for your research! :-)