– Xochielt Sanchez
Answer by Jacob VanWagoner:
It's bad for humans because we've built our infrastructure to fit certain places that have certain climatic characteristics. We've chosen building materials, put up energy infrastructure, and designed all our systems around the climate as it was when we built it.
Change that, and all the infrastructure has to change in order to stay useful. Places built expecting lots of rainfall will have very little water collecting resources, so decreasing that will have drastic consequences on the water supply. Places built around very little rainfall will find themselves quite literally in deep water if their rainfall increases — they don't have effective draining infrastructure, and the concrete and asphalt they use are not blended for constant soaking. (in high-rain places, they sometimes blend latex into the concrete to repel water, etc).
We built our agriculture around expected climates AND soil conditions. Change the climate, and the crops don't grow so well. Move them to somewhere else that newly has a favorable climate, and the soil conditions will be awful and they still won't grow well. It means having to move a lot of infrastructure, and then having to re-do a ton of work to figure out how to efficiently do everything again.
I'm leaving out anything about natural disasters, because that's either fishing for MOAR UPVOTES from one crowd or just asking for "Yeah, right, prove it" from a different crowd, and quite frankly I don't like either crowd very much. I'll leave it at this statement: Change the climate, and the probabilities of various undesirable events will change. Some will become more likely, others less likely. I'm not too keen on finding out experimentally how the balance shifts.
Sea level rise is also an interesting beast. Old geological records indicate that sea level is pretty close to the earth-age low, so having built anything near the coast without accommodating for sea level rise was pretty stupid on our part, but then again we started doing so before we knew anything about where sea level used to be.
There's a lot more, but I thought this explanation might hit home for more people than images of those poor, poor Polar Bears.