That’s my point. Sure, if there are visible cycles in there, you can make comments based on those cycles. We can say that days are warmer than nights, for example.
But that means nothing either way about a possible gradual increase due to CO2. Without a visible cycle, what are you going to use as your “boundary conditions” to allow you to forecast the future? That’s saying “we can predict if there are visible cycles, so that means we can predict if there are no visible cycles”. It’s a false analogy.
You say “So for June vs December we have reason to be confident, whereas for 2050 vs 2010 we don’t.” I’d restate that as “for 2050 vs 2010 our winter/summer analogy is useless.”
w.