See Michael Barone's level-headed response to Al Gore's emotional outburst about global warming before Congress. If I can paraphrase Barone, he says something like "Grow up! Global warming is an issue for science, not religious faith." Even the normally lefty New York Times says Gore is over-selling the issue.
I have visited six of the seven continents and more than 40 countries, seeing a lot of the planet in the process. I conclude that in our normal self-centered way we humans greatly over-estimate our impact on this planet. To be sure, humans have some influence but major players like variations in the sun's output, volcanism, and reversals in the earth's magnetic poles are much bigger movers in the climate change game.
With no help from humans, this planet has experienced warming before - there are petrified trees in Antarctica where none now grow - and experienced cooling before - Ice Ages. Earth will do so again, whether we will it or not, regardless of our efforts.
Along the way, dominant species like the dinosaurs have gone extinct. What we can do to protect ourselves is to spread human colonization across several planets so that a catastrophic event on one of them will not wipe out our species.
