In that case, I think still harping on the 1.5 degree number is a communications mistake. It is obviously impossible at this point (see the edit in my original comment), and so I think focusing it risks encouraging a "well, this is obviously too late, might as well enjoy our bread and circuses while they last" attitude. I think it would be much better if scientists said "Remember when we warned you about that 1.5 degree limit? Well, y'all f'd that up, so now a lot of these dire predictions are going to come true. Oh, and here is a whole host of even more dire predictions that will occur for every .1 degree you miss the limit, so you better try to limit carbon emissions as much as you can to prevent things from becoming more screwed than they are already guaranteed to be."

I just think that any messaging that talks about things eventually being "too late" is bad from a public motivation standpoint.