In about 5 billion years, Sol, our star, will enter its red giant phase, and engulf the orbit of the earth. After this event, the universe will continue for at least 10^100 years. Well prior to that, Earth's atmosphere will boil away. Earth will be entirely, 100% destroyed, down to the last molecule, taking humanity with it.
The only way to avoid that fate is to attain star faring capability and get off this rock.
I submit therefore, that while stewardship of our resources is a laudable and necessary thing, that ultimately, our planet is 100% expendable, down to the very last molecule towards the goal of our ultimate escape.
Humanity needs exactly two game changing things: a compact, self contained, potent clean energy source along the lines of the fictional ZPM, and a star drive.
Such items will be the product of freedom, prosperity, and the material application of the intellect of man, and won't be dropped out of the sky as a result of granola munching Gaia worship.
Nature is pleased to eat us, or kill us in any of a number of lingering, nasty, painful ways.
(Inevitably, I am reminded of one of my favorite gags from the graves of academe:
Droning Prof: "...and in approximately five billion years, the sun will swell into a red giant, destroying the earth..."
Inattentive student: "Huh!? What?!?"
Droning Prof: "I said 'in about five billion years, the sun will swell into a red giant, destroying the earth.'"
Inattentive student: "Oh, whew! I thought you said five million years.")
(H/T to Kevin.)