You skipped a step;
1 - Is warming occuring?
1a - Is it a problem?
1b - If so how much?
2 - If yes, is it caused by man?
3 - If yes again, what should be done?
answers;
1 - yes
1a - We don't know
1b - We know some changes and their expected result, but since we don't know how warm it will get, we don't know the whole story. Plus there is bound to be things happening that nobody expected.
2 - Caused, no. Contributed to, yes. Disagreement on how much.
3 - That depends on 1a & 1b, change is not inherently bad.
That's not even it, in a nut shell. Way more complicated than that.