There wouldn't be a single factor driving it but a combination of many factors. Loss of coastline (and cities built along it) including greater susceptibility t

There wouldn't be a single factor driving it but a combination of many factors. Loss of coastline (and cities built along it) including greater susceptibility to storms for unaffected areas will obviously have economic costs, highly increased weather and storm variability will be significant (think monsoon rains, "atmospheric river," etc.), increased drought in some areas due to both temperature and weather pattern changes (see western US water rights among the states as the civil portion of this), mass movements of refugees (sure the US can close the southern border, but what happens if you get 50,000 migrants all deciding to come over at once in one area? Are you simply going to shoot all of them?). Human extinction seems very unlikely, but the collapse of the infrastructure that allows creation of the infrastructure that allows modern life? That could be much more likely, particularly when you factor in military conflicts as well as purely climate-based changes and losses.