Climate change can't kill humanity. It'll destroy our coastal cities and be catastrophic for us all, surely, but only the animals we share this planet with will die out. I don't think nuclear war is truly a human extinction event, either. A lot of the world isn't really a target. Although, nuclear winter and fallout might kill the survivors. Asteroids are the most likely. I believe meteor impacts are why Stephan Hawkings insisted humanity find/create a second habitable world. We can't prevent them or do anything about them - if it happens, we die, plain as that.