Well, .5 C degree increments in average temperature are big enough to be qualitatively different, so that's why we use them. Hitting 1.5 C includes some things

Well, .5 C degree increments in average temperature are big enough to be qualitatively different, so that's why we use them. Hitting 1.5 C includes some things things like "All the ice in the arctic melts in the summer which really sucks for local wildlife" but nothing with huge negative effects on humans. 2.0 C includes "Sea level rise wipes some small island nations off the map", though, which made it of particular concern to said island nations during negotiations. Probably, the climate is a complex system and we've never had a ringside view of CO2 levels rising this rapidly so there's really a large degree of both upside and downside uncertainty in all of this. Probably hitting 2C just means a few refugee crises and no disasters that'll effect most of humanity but we can't be sure.