I think Carlsen's natural playing strength right now is the strongest ever seen in the history of human chess. He's getting to the point where the rest of the world's best players are a full class lower than he is.
That has happened before twice in my lifetime. Fischer and Karpov were both light years ahead of the rest of the world.
I've seen quite a few moves that Carlsen found which weren't in Houdini's top three at 20-ply! I was surprised that Be6 wasn't though, because I found that one myself unaided, and it's kind of obvious when you see all the threats it sets up and notice that the pawn is pinned.
If Carlsen wins the Candidates tournament and defeats Anand in the WC match, then I think there won't be much of an argument that he isn't the strongest player ever.
The real question is how much further he can go. He's still improving, and if he wants it enough, I think he could keep improving for at least a few more years, and possibly longer.