As for the cost of adaption, floods and hurricanes are the most costly disasters we face. In addition to that, moving infrastructure around on a short timescal

As for the cost of adaption, floods and hurricanes are the most costly disasters we face. In addition to that, moving infrastructure around on a short timescale (50 years) to deal with changing climate involves a massive, massive output of effort. In short, it is far more inefficient to move a large part of our infrastructure and deal with increasingly damaging weather than it is to clean up our emissions. Also, what makes human generated forcings special is their outsized impact on climate: they appear to be moving the climate an much faster that than other forcings. The earth is currently in a cooling cycle, but is now heating rapidly as a result of industrial emissions of greenhouse gases. Because we have such an outsized impact, we need to regulate our behavior. It would be just as terrible if a thousand new volcanos suddenly opened up and started spewing out carbon, but that's not what we're dealing with.