I think that depends on how you define 'civilisation'. My inclination is that most people would say civilisation has ended if life is drastically different to how they perceive their life/world they live in. Think 'civilisation as we know it' rather than a dictionary definition.
However, I disagree that it's not an existentisl threat, if only on the basis of possible crop failiures on a massive scale (reduced crop yields are a global issue already). Don't underestimate the impact of food shortages on everything else, we in the west have become accustomed to easy access to food.
Indeed, in terms of sudden impact and method of impact, no they are very different, and climate change probably won't go so far as to make the human race extinct, at least not for a very long time. However, whether or not it will be catastrophic for the human race within the next 100-200 years no-one can accurately predict, given we do not know how much we'll do to stop it before it's too late (bare in mind that some scientists already believe the tipping point beyond which we can no longer stop it is well upon us).
As mentioned, the collapse of farming may well undermine any efforts to stop climate change given the big knock on negative impact on the world economy. Though that could also save us as there'd be a sudden massive drop in fossil fuel use and carbon emissions in such a scenario. There's a lot of variables, but a catastrophic collapse is definitely a possibility. I think the human race is capable of saving itself from this, but capitalism and the corporate economy I fear stand in its way.