This is a very interesting topic and one that I usually stay away from. I am sort of a man made global warming agnostic. While I would like not to believe it, I also feel that our presence has to have some impact on this earth. If your dog was sick and infested with six and a half billion ticks, it might be safe to say that they were the cause of its health problems.
That is where I think we go wrong, we tend to disassociate ourselves with the ecosystem and nature, when we are very much a part of it. It is irresponsible to assume that we can continue to suck the life blood (so to speak) out of the earth without some negative effect. The issue to me is not whether we have an effect on our surroundings, it is rather, how our surroundings will react to put things back into balance. This earth has been around for billions of years before we were here and has survived worse threats than us.
It is said not to underestimate the will of life, that it will always find a way. You can believe that the earth will not cease to exist due to our actions. Surely we wouldn't be so egotistical as to think that we are the sole purpose the earth exists, that it is here solely for our benefit. We are a very small part of the grand scheme of things in my opinion. If mother earth decides she has had enough of us, she'll brush us off her back like a horse brushes a fly off with its tail.
Sorry to get so philsophical, but when I go out at night and look up at the stars I suddenly feel very small and vulnerable. It is then that I appreciate the sheer immensity of our surroundings. We are very small, and it would do us all well to remember that.
Mark