I think it's a combination of both. The earth is warming up by itself (and has done so in the past) but our influence is probably increasing it greatly.
Of course it had to become a fashion term, making it a lot less credible in the process...
We do have to minimize our own influence in my opinion, but certainly not in the way it's going now. Concentrating everything on CO2 is not the way to go about it. Nevertheless, it's better than doing nothing at all against it.
This is what I've been tryin' ta tell ppl for a long time here at home. This trend in our earth's activity has been happenin' for Billions of years and it's a little hard ta swallow that humanity is the cause of this. I do agree that humanity is contributin' ta this phenomenon, but I believe that it's more than a little arrogant ta think that we're the cause. Although we're not the cause, we can do somethin' ta slow it down in order ta give humanity more time ta prepare for when the earth does go through this inevitable change. We haveta stop and think 'bout what ta do and do it. We can' stop it though, for the earth has been around for a long time and it'll still b here long after humanity is long gone, goin' through it's paces. This is jus' one of them, but we can adapt; it's one of the things that our species is very good at.