It is hard to reconcile climate activism with modern transport but apart from Greta everyone involved in the field flies a lot to be able to lobby and push the issue in different parts of the world.
The United Nations built its Environment arm headquarters in Kenya! They did it for political reasons - to get the buy in for African nations to the cause and to show that the United Nations was committed to that part of the world by investing a major headquarters there.
But the consequence is that hundreds if not thousands of their employees have to fly back and forth to home countries and the main heads of departments have to fly constantly because so many important meetings happen in the US and in Europe.