Global Warming. The planet gets hot. The ice melts. Everybody dies. Explained that way, the average joes can't wrap their tiny noodles around the idea. "Boy howdy!" they toodle, "We ain't no tability fo breaking the whole wide world, no how!" You can track them up the greasy rungs all the way up to Rush Limbaugh's portly bottom as he swings from the bottom of the helicopter and smiles for his legions. I heard good Rush say: "We arrogant humans think we can affect the planet? Well we can't."
But the thing of it is, and stay with me here, the planet (in this case, Earth) is an ecosystem -- this is a fancy way of saying that a system supports a system supports a system, and so on out into infinity. Plants breathe out, we breath in, and vice versa. The ocean feeds the sky and the great blue above returns the favor.
Ever hear of the Butterfuly Effect? Get something out of whack, watch out.