It’s arbitrary. There’s no theoretical reason why 1.5 is worrying in a way that, say, 1.4 and 1.6 aren’t.

And there are no tipping points. It’s a gradient, and the higher we go, the worse problems will likely become. There won’t be any substantial difference in the effects of 1.5 degrees of warming compared to 1.49 degrees of warming. But that’s what calling 1.5 a tipping point predicts.

The danger with focusing on tipping points in public communication is if we pass them, it’ll be obvious to everyone they weren’t tipping points.