About fifteen years ago I read my first report of climate change. It described the greenhouse effect but then added the liklihood of the tundra melting from the heat of the greenhouse effect and releasing methane. There is a lot of tundra and methane is 30 times as effective a greenhouse gas as carbon dioxide. This a positive feedback loop. Because methane traps heat the "loop gain" is high. (Ever hear a sound system squeel, that's positive feedback.) Sometimes positive feedback is useful but in this case it is a disaster. It is a significant part of what makes these future graphs so terrifying.
However, at 1.0°C we already have many more disasterous weather events. The Paris target is 1.5°C which we do not have a prayer of making. It is difficult and relies on self policing. So then 2.0°C becomes the target and it is considerably worse than 1.5°C. The NASA report Why a half-degree temperature rise is a big deal gives the details.