Extinction Rebellion, organisers of the civil disobedience protests in the UK and elsewhere - https://rebellion.earth
Client Earth, a legal charity aiming to repeatedly take governments to court on climate matters. Became much more visible when Dave Gilmour made a major donation recently: https://www.clientearth.org
They're not quite as established in the US yet as they landed there later, but are active and growing. The whole movement is only around a year or so old. They've achieved remarkable visibility and growth the time since the first UK demos.
Client Earth has a US arm: https://www.clientearth.org/usa/ though it doesn't look terribly active. I suspect there are better US equivalents that have achieved more stateside. I don't know who though, sorry.
I'll do something today about the climate and planting trees. I'm not physically able to plant stuff myself, so it will have to be something I donate to. I can also push my employer to start sponsoring climate disaster mitigation efforts.