WE — the human species — have arrived at a moment of decision. It is unprecedented and even laughable for us to imagine that we could actually make a conscious choice as a species, but that is nevertheless the challenge that is before us.

Our home — Earth — is in danger. What is at risk of being destroyed is not the planet itself, but the conditions that have made it hospitable for human beings.

Without realizing the consequences of our actions, we have begun to put so much carbon dioxide into the thin shell of air surrounding our world that we have literally changed the heat balance between Earth and the Sun. If we don’t stop doing this pretty quickly, the average temperature will increase to levels humans have never known and put an end to the favorable climate balance on which our civilization depends.

--Al Gore, from "Moving Beyond Kyoto." The New York Times, July 1, 2007.