I think we are saying the same thing. Let me rephrase: As a simple approximation, you would expect water vapor to increase cloud formation, raising albedo and acting as a negative feedback. However, in reality it’s not that simple as differences in where the clouds form, and at what altitude, makes all the difference. And this is still one of the least understood aspects of climate change.
True, but CO2’s effect on climate diminishes with quantity exponentially, so there is a limit to how much it’s going to affect climate change. This is why we refer to CO2 climate sensitivity ‘per doubling’. So increasing CO2 by 280 PPM will double it over pre-technology levels, and our best estimate of climate sensitivity now is that this would increase temperatures somewhere between 1.5 and 4 degrees C over pre-industrial levels. But to increase it by a similar amount over that would require another 560 PPM, and to increasse it again by a similar amount would require an additional 1120 PPM. And so on.
Now that you’ve gone further with “diminishes rapidly”, you’ve moved into the realm of flat-out wrong.
A simplified approximation for CO2 forcing as a function of incremental CO2 increase is:
F = 5.35 \times ln \frac {C_{new}} {C_{ref}}
So assuming a present value of around 400 ppm CO2 (for convenience – it’s actually quite a bit more than that) and using 300 ppm as the baseline, we get a forcing of about 1.5 W/m2, which is a little less than the current estimate.
For CO2 at 1000 ppm, we get 6.44 W/m2.
For CO2 at 2000 ppm, we get 10.15 W/m2.
While this is not linear, it would hardly be described as “diminishes rapidly”.
The reason this happens is shown in this article, and it’s basically because the part of the logarithmic curve that we care about is pretty close to being a straight line, as you can see from my approximate calculations – not linear, but not that far off.
I’m not sure how one can dismiss the cooing effect of clouds blocking sunshine. All you have to do is go outdoors on a cloudy vs sunny day. So what is the proposition - that the solar energy is being absorbed by the shiny white clouds more than being reflected back into space? Yes, the clouds do trap heat from leaving the ground (as cloudy nights are warmer than clear nights). And clouds do have a level of latent heat, as water can hold a lot of calories.
But deserts will have more hotter days than jungles, they just cool off a lot at night where jungles don’t.
As for complexities - for example, the melting glaciers about 15,000 years or so ago created a giant lake in the Canadian prairies as the melt water could not escape northward. At one point, the glaciers meted enough that this ice dam gave way and flooded the Artic with fresh water. This flood making its way down between Greenland and Labrador disrupted the Gulf Stream, causing a mini ice age to return to northern Europe for about 500 years. The fear is that a quick melting of the Greenland glaciers may have a similar effect to disrupt the current that keeps northern Europe warmer than the Canadian arctic. (Buffalo, which got 3 feet of snow in one blast, is about level with Barcelona. Ireland, UK, Scandinavia are all about he same latitude as Hudsons Bay/James Bay, which freezes over for almost half the year.)