I think you've totally misread what I wrote... "I think you sorely underestimate the impact a few degrees can have" is not the same as underestimating the impact it is having. People who may be overestimating the impact are studying all possible outcomes (including "nothing to see here, please move along") while those who may be underestimating the impact are refusing to follow the "if this happens/is happening, what could be the effects, and how do we deal with that?" line of thought.
The problem isn't whether local weather is warmer or cooler, or even if global temperature is warmer or cooler -- it's what effect a change in global temperature will have on the climate in which humans can survive comfortably. Things like the polar ice caps melting, global ocean temperatures rising and global weather patterns changing are facts I think almost everyone would agree with. The beginning of the mass extinction/migration event in the NW Pacific is something that is becoming steadily apparent.
Our world is changing, and it makes sense to model that change and try to figure out how we can prevent/adapt so that humans can, in general, have a comfortable existence in the future. Just brushing it all off as a scam helps only the very few in the short term, and nobody in the long term.