Here's a thought, C3 plant life goes extinct if CO2 falls below ~200ppm. No more rice, no more wheat, no more potatoes, no more evergreen trees. Life on earth would probably suffer a good deal more in the long run (in geological terms) if we left all fossilized carbon in the ground.
Do you have any convincing arguments for why a wetter, warmer, carbon-rich world would be a downside for humans on that scale? If I were betting on us surviving, I'd take that over an ice-age any day of the week.
Parasites and diseases, much more common in warmer, wetter places. You'll notice that on the whole tropical civilizations have been less successful than those in colder climes; there's a wide variety of possible reasons for that, but the effects of parasite load on health and general intelligence are real. Our technology/economy/etc. has so far been much better at helping us live in inhospitable cold places (Norway) than at dealing with tropical parasites and diseases.
Not to mention that a climatic shift on this timescale, in either direction, has historically meant about 20% of extant species going extinct. How lucky do you feel?
Not nearly as difficult as living on Mars.
In the near term, it's going to cause tremendous dislocation of human civilizations. In poorer areas, climate change will cause millions to die and tens of millions to suffer. That's the undeniable moral component.
Ecologically, climate change will hasten the mass extinction of species. A world covered in kudzo, algae, and roaches is technically "teeming with life", but I'm not sure it's the aesthetic vision you have in mind.
The major threat of climate change is not to things so expansive as atmospheric viability, but only to present geography and weather cycles. There will still be breathable oxygen in the atmosphere, there will still be water, animals, and vegetation on the surface. As dystopic as a post-climate-change world may look, it undoubtedly looks better than Mars.
This is probably even true after massive nuclear winters, etc. I don't think there's anything humans can do that would so disadvantage Earth as to make another planet in our solar system better suited to human life. The only thing that may require external planetary colonization would be an extinction-scale geological or astronomical event (i.e., something totally outside of human control).