The worst-case scenario of global warming (which might be inevitable given the current CO2 levels) is a 216 foot rise in sea level [1]. This will cause mass migration and be extremely expensive and unpleasant for society as a whole, but poses little threat of ending humanity.
NASA's JPL did a great lecture a few months ago talking about the current scientific consensus on the consequences of global warming - it's well worth the watch: https://www.youtube.com/watch?v=ZJYs8L84L4s&t=3714s
[1] http://www.nationalgeographic.com/magazine/2013/09/rising-se...