As a small example, a rise in sea levels would make unlivable countless coastal cities and destroy an immense amount of infrastructure. Humans will -survive- it, but say good bye to our cushy 21st century lifestyle.
[1] https://www.theguardian.com/environment/climate-consensus-97... [2] http://science.sciencemag.org/content/348/6234/571
None of that is "unclear", I think. It all follows from "temperatures rise" (which the parent concedes as a given). Ice melts. Waters rise. Life changes.
Or we won't learn, and the rise will be slow enough that no one's going to drown. (Other than very low-lying islands, which were precarious to begin with, and which don't usually house complex civilizations.)
Coastal cities will never /not/ be a thing. The ocean and river mouths contain very many important resources for societies to ignore.
---
> rise will be slow enough that no one's going to drown.
It depends on the solution: the cheapest usually is to build seawalls, and this obviously puts the city at greater risk of flooding.
---
The comment you're replying to states rising sea levels as an example. There are, of course, known consequences that you don't address. To name two, it is a very direct consequence that 1) the weather will get more violent, with the additional amount of energy it contains, leading to a more frequent occurrence of natural disasters. Next, a change in climate will cause a massive redistribution and likely destruction of land suitable for agriculture, as well as lower overall quality for agricultural land, thereby 2) threatening worldwide food security.