Ability to use a keyboard...
Hmm, to think I once believed it required intelligence to type coherently...
Look, do humans burn up fossil fuels - you know, forms of carbon previously buried - on a massive scale? Yes. Pessimistically I rather think the entire store of such fuels will be used sooner or later, at rate which may well be higher than can be naturally regulated.
Does carbon dioxide - among other things - trap heat in the atmosphere? Yes.
Does a runaway greenhouse effect eventually make a planet inhospitable to water based life? Oh, yes, think of Venus (not caused by humans, in that case!).
So, the question is how significant is the effect of human burned fossil fuel - so far, it appears to have raised CO2 levels by 10-25% - and are there feedback effects which decrease or increase the warming caused thereby (hmm, it is only here that I become a skeptic - but gradually less so). It may be that human fuel burning causes a lot of warming, or maybe not so much as would be dangerous...
However much I believe in free speech, it is safe to ignore any output from those who deny AGW as a hypothesis, just as it is safe to ignore those who deny evolution or the round earth theory. Saying that climate change happens naturally on a large scale is no better an argument against AGW than saying that a catastrophe wiped out the dinosaurs means they didn't get a chance to evolve, thus disproving evolution... Or that a similar asteroid is perhaps as likely as AGW to end human existence. The question remains, is this threat sufficient to take seriously? Yes.
Finally, are there reasonable steps that can be taken to mitigate fossil fuel burning? Wind mills don't bother me; radioactive waste is however likely to be dangerous for longer than human history. Solar energy - moving from burning the stored version to using and temporarily storing the freely available energy hitting earth anyway (where conditions permit!) seems a no-brainer good thing.
My parking space with the electric car!