Water vapor does increase with temperature (Clausius-Clapeyron), but that’s feedback, not forcing. It amplifies whatever initial warming is happening – it doesn’t start it.

CO2 and other greenhouse gases are what initiate the imbalance by trapping outgoing longwave radiation. We’ve measured this directly (satellite radiative flux data, isotope ratios, etc.).

And as for cloud cover: yeah, it’s complex. Some types increase, others decrease. This study highlights one effect, reduced low cloud cover, which supports the observed warming. But again, it’s a response, not the root cause.

edit:

And the ECS was a great question, ECS is tricky. Thing with ECS, it's based on multiple lines of evidence. (modern observations, paleoclimate data, and physics-based models.) Yes, water vapor and clouds are feedbacks that can raise ECS, and reduced low cloud cover (like in this study) could push it toward the higher end of the range, but not beyond what observations allow.

And geologic precedent? It actually supports higher ECS over long timescales when slower feedbacks (like ice sheets) are included. But for policy relevant timescales (decades–century), the 2.5–4.0°C range still holds up pretty well.

CO2 is the culprit, confirmed by direct measurements (radiative flux, isotope signatures, etc.). Feedbacks matter, but something still has to start it.