Another way to look at (b) is "We're only being saved from a mini ice-age by CO2-induced global warming, therefore it's urgent to avoid cutting emissions and plunging into it for real". The Earth has seen warmer and colder than now, but the warm extremes have been better for life than the cold extremes.
Which life? Industrial age humans? That's who we care about. Our goal is (or at least I hope it is) to enable humans to thrive, we don't really care about other life (at least if we had to pick between humans and other life) as long as it is not instrumental to our success.
The problem is that without the other life there can be no human life. We are on the top of the food chain and as such we depend on all other animals for our survival. Hopefully it makes sense what I'm saying.
oddly enough some climate scientists (james lovelock, in this case) take it as a given that most animal life is already doomed, and that humans will have to engineer a food source in order to survive. green goo might be in your future
Eventually there will be some extinction event anyway. Is it more important to maximize number of humans who ever lived, or more important to maximize the age of the species? Your stated desires could mean either, but they prescribe very different strategies.
