Save humanity? From what global warming? Global warming is nothing compared to Mars. If we can terraform mars into a beautiful landscape then climate change isn't really an issue on earth...
Going to Mars for scientific research would be an incredible step forward for man kind. But the feasibility of space still holds a lot in question. It is exciting, but let's not call it "saving humanity".
Save humanity from an extinction level event. Think asteroid impact or global nuclear war rather than climate change. Musk has been pretty explicit about his intention to make humanity a multi-planetary species. [0]
What nightski is saying is that achieving a truly independent colony on Mars, which can survive the destruction of Earth, is much harder than just putting people there.
Mars might an interesting playground for testing climate engineering, since it'd be hard to mess it up more than it is now. That said though, it's hard to imagine fixing things like the lack of atmosphere without a lot of investment.
Going to Mars for scientific research would be an incredible step forward for man kind. But the feasibility of space still holds a lot in question. It is exciting, but let's not call it "saving humanity".