We know that humans are responsible for the large increase in greenhouse gases over the last few decades, because greenhouse gases from most non-human sources have different isotope ratios than those we produce, and the non-human sources that do look the same as human sources from an isotope ratio viewpoint can be spotted and monitored and accounted for from satellites.

So unless you are disputing that a large worldwide increase in greenhouse gas concentration in the atmosphere affects climate, your position that humans can't affect global climate is untenable.