You can't, however, use the change as proof of AGW, because that would be circular reasoning.
No, it would be called science.
(A) We've known the mechanism since the 1800s when Fourier et al first raised warnings about CO2s spectral absorbsion lines and the implication the coal spewing industrial revolution might have on atmosphere. This is validated science and underwrites so much physics that we'd have to turn the clock back on at least a century of scientific understanding in multiple fields if it wasn't true.
(B) We have a solid graps of how much CO2 is being put into the atmosphere from both economic modelling and satelite and terrestrial telemetry.
(C) This permits us to do a back of the napkin calculation as to how much energy (thermal and kinetic) is being added to the climate system from human intervention (its a lot).
(D) This in turn gives rise to more complicated modelling that can tell us how much of that energy goes to warming, how much to increased kinetic activity (cyclones/etc), how much gets absorbed by the ocean and how much radiates back out.
(E) The end result both matches up with observation (And *n o* natural process can account for what we are seeing. Volcanic activity is incredibly insubstantial. Even krakatoa hardly put a dent in it. And solar activity is also quite minor).
Occams razor says we *must* conclude humans are causing substantial climate change, because if they are not we have to find a mechanism that (A) Prevents physics from working as it is known, and (B) Makes it look like physics is working as it is known. Should this be found, it would be Nobel prize level monumental. However, as they say, big claims require big proof, and that proof is not remotely forthcoming.