"All a planet's life?" It's a false choice.
The Earth's biosphere has weathered a hell of a lot worse than humanity, and will again. In the Devonian Period, the "Age of Fish," CO2 levels were twenty times what they are now. The Permian mass extinction after this killed 95 percent of the species on Earth. Gaia got better. Where were the polar bears swimming during the Ice Age?
Even the current global warming issue mostly revolves around how damage will be done to human living areas, and a few noticable species. If the Earth got five degrees warmer, nature would just move some borders around like it always has. Will this be convenient for human farmers and coastal dwellers? Nature doesn't give a shit.