How bad is it?
The Problem
Climate change is a problem of Climate System, a problem exacerbated by humans (hence Anthropogenic) by disturbing the delicate balance of creation and absorption of GHGs. The IPCC defines climate change as “a change in the state of the climate that can be identified (e.g. by using statistical tests) by changes in the mean and/or the variability of its properties and that persists for an extended period, typically decades or longer”.
Earth is Sick, and we have caused the sickness
The Earth has experienced much higher carbon levels in the past, and has survived far more severe heat than we could possibly inflict upon it. The atmospheric giant will survive this, and will certainly survive us. That's not the issue here. The last time carbon concentrations were as high in the atmosphere as they are now was during the Pliocene, some three million years ago. During this time, global temperatures were 3 or 4 °C warmer, and sea levels twenty metres higher than they currently are! Sea level rise of 20Metres is Game over scenario for the current inhabitants on Earth (we weren't there 3 million years ago!).
At present, approximately half a billion people live within twenty metres of the sea, you can do the maths.