One of the big issues was the NZ data being 'massaged' by the scientists. Looked bad that the early (pre 1920s) data had been upwardly adjusted nearly 1 C.
Until you understood that the recording station was moved in the 1920s...(and that this had previously been explained to the same people now accusing the NZ scientists of deception).
What happened to the temps in the US in the 3 days following 911 (while no airliners could fly)?
I would call that a clearly measurable 'effect of human activity' on the climate.