There is nothing in that statement that implies that he believes in man-made climate change. What he is saying is that we are insignificant.
As I wrote above, he didn't comment on AGW either way. He does not say we are insignificant, he says that we are insignificant in terms of the long term damage we can do to the earth. He also implies on a number of occasions that all the pollution humans are producing might wipe out all of us. Though he also says the earth may need to do that through viruses.
There is a difference between claiming that pollution is insignificant to the planet and claiming that pollution is insignificant to humans.
This is a longer version of the sketch that what I've heard previously. The version I'd heard before ended at about the five minute mark and left a clear impression that Carlin was saying that pollution would wipe out humans.
At the end of this full version of the sketch he says that he thinks AIDS might end humans and says "Well, I can dream can't I?" although it isn't clear to me if he's quoting the earth as saying that or if that is his dream. Most likely it's both.
So, you are correct that he didn't say here that he believed in AGW, but you are also incorrect in suggesting this clip shows that Carlin didn't believe in AGW as he doesn't mention it one way or the other. However, everything else I wrote is clearly backed up by this sketch.