LBlack14 The alleged danger isn't summers being a little hotter, winters being a little warmer. There's a potential for the planet warming enough that it becomes a self-sustaining loop of continuous warming, which would melt ice that has been frozen for a LONG time, raise sea levels to the point that many heavily populated cities become uninhabitable and compromise fresh water sources, etc. It's not that humans couldn't tolerate slightly warmer temperatures, it's that the effects on the environment could be substantial in ways that most wouldn't think of in day to day life.
I'm not saying that will all happen, as I am not informed enough on the science to have a meaningful opinion, but the stakes suggested are much more significant than something like "life on Mars." And even if the world is warming on its own, we might still need to do something about it if we are to maintain our way of life in the future.
If we proved that the warming wasn't because of humans, but the warming was still happening, it would still be a big problem.