Because we know that the course that this administration does take in lieu of Paris will likely worsen climate change - it's not like they are going to introduce a carbon tax instead. It's symbolically relevant as an indication of the direction we are going.

Also, a significant chunk of liberals who are concerned about this are people who have made often difficult carbon-reducing lifestyle changes consistent with their beliefs on this issues, even at the cost of traditional comforts (large houses, big cars, etc).

Lattes really have nothing to do with this issue, except as some kind of cheap-shot stereotype unrelated to the issue, and last I heard, McMansion-filled exurbia leans pretty strongly conservative:

http://www.dailykos.com/story/2014/2/9/1274795/-Democrats-ar...