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.