Energy and Atmosphere Revisited
Thermostat Quote
Dai Davies, 181117

Figure 21: Earth's temperature and CO2 levels over its geological history

I don't know how Figure 21 was composed or how accurate it is, but it seems to be given credence. Given what it's trying to achieve, it's remarkable that it exists at all.
As my added orange line shows, on long time-scales there has been a general trend to lower CO2 levels. There is no consistent correlation between temperature and CO2 levels, which have been ten times greater than present levels. They are recently rising from a low that was just above the minimum necessary for sustaining most life.

A feature relevant to climate, which becomes apparent on the timescale of Figure 21, is that temperatures hit a limit indicated by my added green line. The Earth has a thermostat. In (1) I review relevant literature and discus how it's likely to be produced by nanocrystalline properties of liquid water which cause water molecules to be more strongly bound to each other over small distance scales. These disintegrate as the temperature rises from 0˚C up to 30˚C.

Figure 22: Atmospheric water column and Saturated Vapour Pressure against temperature

The satellite data in Figure 22 shows the rapid rise in evaporation as water surface temperatures approach 30˚C (303˚K). (The unit used, water column centimeters, is the depth of liquid water evaporated into an air column.) At this temperature the ability of air to hold water vapour (the vapour pressure at saturation) can also be seen to rise rapidly. Increased evaporation of liquid water at this temperature and the increased nucleation and precipitation of water vapour to liquid in air cooled below it fit neatly together.

As evaporation suddenly increases, surface temperature hits a limit with evaporative cooling soaking up heat as latent heat of vaporisation, just as sweat cools our skin. Water vapour is lighter than air, so the vapour rich air rises. As rising moist air cools, the relative humidity reaches saturation level and water starts to precipitate as clouds. In doing so it dumps the latent heat into the air of the upper troposphere where it's radiated to space. Completing the thermostat effect, the production of cloud shades the surface below.
After all the discussion of the basis for our current mean temperature of 288˚K, it could be that whatever processes have raised our mean temperature it was bound to settle somewhere just below 300˚K.

Sea surface temperatures around 30˚C are largely tropical, but much of the Earth's land surface is also wet, and can more readily reach 30˚C since it captures the sun's heat at a shallow depth rather than penetrating for hundreds of metres in the sea. And the sea's surface waters mix with cooler sub-surface waters. I give a personal perspective in note (D), showing that this is an effect that most of us may be intuitively aware of.