Another guest post from Dr Robert Brown, Physicist at Duke University.
I make beer. On part of making beer is boiling the wort for some hours to reduce the fluid volume of the barley-sugar-water to the right specific gravity to ferment to the desired target alcohol level (and do things to proteins and sugars and at the right point to bitter and flavor it with the hops). Big pot, lots of fluid, hot on the bottom, cool on the top (even before the boil). The otherwise reasonably clear liquid is full of little chunkies of coagulated proteins as well, so the liquid has a clearly visible “texture” that lets you see the movement of the fluid.
As any good fluid physicist should understand, the heating on the bottom relative to the top creates instability. Warm water is much less dense than cold water, from 4C right up to 100+ C (beer/syrup boils a bit over 100C). Conduction is slow. Radiation is very slow. Rather than heating the water in a stratified way, bottom to top, as the bottom water heats, it expands and experiences a buoyancy force from the denser cooler water above and around it. It is pushed up. Of course at the top there is nowhere to go (it’s in a pot, bound by gravity) so it just displaces the cooler water there, which sinks, is heated at the bottom, rises to the top, gives off its heat via evaporation and conduction and radiation, cools a bit, sinks, picks up more heat, iterate indefinitely.
But the rising and falling are not uniform. The wort creates convection rolls of warmer lower pressure lower density rising liquid and cooler higher pressure higher density falling liquid, heating at the bottom, transporting the heat to the top, giving it off there, and going back to the bottom for another load all while the liquid itself gradually warms. When the convection is obstructed, one can quickly build up a much higher temperature differential, and it actually takes MUCH longer to reach equilibrium — you can actually boil off all of the liquid on the bottom in local patches and scorch things in contact there because the bottom of the pan isn’t COOLED by the convection rolls.
In a pot, the convection rolls are clearly manifest — usually it goes up on one half of my pot and down on the other, unless I stir it or have it really perfectly aligned on the heat. On the earth, the same process occurs in a very irregularly shaped, heated, and cooled “pot”. Heat is dumped in from the sun, but in a constantly varying pattern as clouds move around reflecting a large fraction of incident energy from some areas and not others. It is differentially absorbed by the ground and the water. Some of that heat is differentially released immediately into the air (which is itself also directly warmed by the light that passes through it). Some causes evaporation of water, cooling the surface of land or water but carrying away absorbed heat into the air. Air, too, rises when warm and falls as it cools and this creates huge masses of air that are just as trapped as the beer in my pot, rising over here, falling over there, and running along the ground or upper troposphere in between in both vertical rolls and in horizontal cycles as well. This air all moves in a rotating frame that causes it to deflect as it moves, creating large scale patterns of circulation around low and high pressure systems. All of this is driven by thermal differentials that move energy around, carrying it from where there is a lot to where there is less as an approximate rule, carrying it from where it is relatively hot (down low) up to where it is relatively cool (above) as an approximate rule.
Radiation is what ultimately removes the energy picked up from the sun, but it is not all radiation from the solid ground that does it, nor is it all, or even mostly, CO_2 in dry air responsible for obstructing that heat transport. This is why, in the desert where the humidity is very low, on a quiet night it can freeze by morning where the temperature rises to close to 100F during the day. Not much of a “greenhouse” effect there, I’d say (and the best possible measure of true greenhouse effect cooling, one that is unfortunately not generally directly studied).
When the heat is transported up by convection, it goes through the greenhouse reflector. The stronger the greenhouse trapping, the greater the thermal pressure differential, the comparatively stronger the convective transport process becomes and the more efficient the cooling becomes. The “stratified” reflective blanket is penetrated by the cooling holes of convective rolls, by the active transport of heat from where it is trapped to where it is not. All of this favors smaller sensitivities.
If there is a real lesson in this, this is it. It is a simple principle of very elementary thermodynamics that all perturbations away from the simple radiative model of cooling obstructed by greenhouse gases will increase the cooling rate compared to the purely radiative baseline. It will do this because the greater temperature differentials are a source of free energy that is begging to do work. The system will nearly invariably self-organize to do work, and in the process reduce the temperature differential between the lower and upper troposphere. This, in turn, will increase the efficiency of the radiative cooling by lifting the heat to be lost up above the greenhouse blanket. I don’t know why this simple argument is ignored so often in climate studies when it is the source of the very instability that produces the convective rolls in my heating beer, the wind in my hair, the rain on my garden, and the seasons. Heat trapping is never improved by a heat-differential instability — that would just make the system even more unstable!
Hence the full expectation that the climate sensitivity and feedback should be expected to be negative and cool the earth compared to what one might expect from “pure” CO_2 greenhouse trapping even before one looks at its details. This is just the “fluctuation/dissipation theorem”, and the climate models that postulate egregiously high climate sensitivity are egregious because they violate it. Given the existence of multiple modes (e.g. radiation and convection) for non-equilibrium energy transport, blocking one will increase the rate of others, not vice versa. Sometimes so well that it is difficult to see any effect of the blockage.
This argument doesn’t really depend much upon water, but the evaporative cycle in general is a perfect example. It cools down low and warms up high, with a very, very few exceptions brought about by peculiar geography (e.g. Santa Ana winds) because in general the warmer moist air rises, gives off its heat, condenses as (cooler) water, and falls. Yes, the clouds and water vapor modulate albedo and greenhouse trapping, but most of this modulation is random compared to improved transport as the temperature differential increases. Again one expects the overall effect of water to be negative to neutral, not positive feedback, because it will in general consume free energy to move water around relative to static stratified models, energy seeking equilibrium with outer space at 3K far overhead, energy that wants to move vertically (on average) from the warm surface to the cold overhead and horizontally from hot places to cooler places.