I don't really want to relitigate the entire AGW thing again.
Josh is skeptical of CO2 being the primary driver of the change. Or, perhaps more concisely, that increases in CO2 drive up water vapor consistently to create an effective feedback of ~3.
You then say, well what about the observed warming then???
Josh then picks from a potpourri of explanations of varying falsifiability: the sun, clouds (possibly due to cosmic rays), decadal ocean or atmospheric processes, experimental measurement change/error.
My point is that you're not going to convince him by saying the TSI is flat - even if that particular point is denied, another hydra head will spring up.
This is why Steve and I like the Santer paper josh linked:
https://pennstate.pure.elsevier.com/...pheric-warming
Understood properly, it shows that any simplistic model error (e.g. CO2/H2O feedback being of the wrong magnitude) would not reproduce the historical record. It's a good confirmation of the positive hypothesis that CO2 is "in the driver's seat", as opposed to swatting individual arguments.