it is arrogance and conciet of the highest order to think they we and we alone are responsible
I don't think that anyone will make that claim.
But nature is about balance, and it doesn't take much to disrupt that balance and we are more then capable of disrupting that balance.
We've done it quite a few times, the Ozone layer is a global issue, and one that had we not changed gears would be a lot worse right now.
We've over hunted and over fished species to extinction. At one time people would have said the same things about it being to massive for us to have an effect on when it came to the idea of polluting lakes and rivers, they where proved wrong. We introduce a species to a new animal and it can have catastrophic effects, look at what rabbits have done to Australia, and they are just a furry little rodent in a very big country.
We are not the only cause, but when the other causes are balanced and we toss a new source of problems in the balance is offset, things accumulate, and we get problems. It's like if you had a funnel, dn you are pouring water into it and it is draining. Someone else comes along and starts pooring water in as well, even if at a much slower rate, now it can't drain fast enough and goes over. Yes, that person was not the only person pouring water, but the addition they made pushed it over the top.
And you are right, the earth has warmed and cooled before, and it is something that it can do without any help from us. But, we also know the effects of those warming and cooling cycles from studying geology and archeology, and the fact that it is looking like it might be entering another should be worrysome, as those effects would be very bad for us.
BUt just because we can't put out the fire, doesn't mean we should keep throwing gas on it.