The examples of saving my favorite tree or my mother are a little different than saving the world. My favorite tree is nearby and thus within my sphere of influence. See, I don't believe I can effect change beyond the range of my voice and my projectile weapons. I have no persons under my control, who might effect change for me, nor do I wish to. I write my state and local representatives about issues that are important to me but I have little faith in this process.
You might argue that in this information age my sphere of influence can extend well beyond earshot and stone throw, but I don't know about that. All my bitching about a particular issue, even in the format of professional-like photo journalism, can hardly compete with mass media. All a cyber activists' cries are fruitless until enough people are driven to action in the real world.
To effect global change you need bloodshed and/or brainwashing. Bloodshed is important. Even Ghandi's movement against the empire required violence. In this case it was martyrs blood, violence against peaceful demonstrators that effected change.
"Brainwashing" is also key. It takes a great deal of money. Advertising dollars.