I think, in broad strokes, human origin for the temperature change is pretty simple to estimate and test? We know how infrared scatters on CO_2 from quantum/EM

I think, in broad strokes, human origin for the temperature change is pretty simple to estimate and test? We know how infrared scatters on CO_2 from quantum/EM physics, so from that it should be possible to take the difference in global concentration due to humanity and incident solar radiation, and derive as a result an expected temperature rise. This can then be matched against historical average temperature records. I'm being a classic physicist and assuming everything is a first-order approximation, so someone do correct me if I'm wrong.