Greenhouse Gases Can Cause Cooling !
I thought that would attract attention. It’s the opposite of the general opinion so how can it be so?
The answer lies in the differing responses to greenhouse gases that can be observed over land and over sea. There is no reason why that could not have been realised 30 years ago.
Should the current consensus really be one of Man Made Global Cooling?
1) Over Land
Not much problem here. More greenhouse gases in the air such as water vapour or CO2 absorb more of the incoming solar energy than the Oxygen and Nitrogen in the air and then radiate it back and forth, up and down to increase the time it takes for that solar energy to depart the Earth system again as outgoing longwave radiation. It is the increase in that length of time that results in warming.
So far so good. All in accordance with the findings of Arrhenius, Tyndall and modern climatology.
2) Over Oceans.
This is where the problems arise and there are two.
i) The absorption capabilities of water.
Water is penetrated to depths of 100 metres or more by incoming solar energy in the ultraviolet and visible wavelengths but is not penetrated by infrared wavelengths.
Thus a proportion of incoming solar energy (in the infrared) never gets into the oceans to affect the ocean heat content (I prefer the term ocean energy content and will use it from now on).
The thing is that greenhouse gases absorb incoming solar energy at several wavelengths of the solar spectrum but only radiate it out again in the infrared.
Consequently increased greenhouse gases intercept some of the solar energy that would otherwise reach the ocean surface and penetrate it to add to ocean energy content.
To that extent greenhouse gases in the air effect a cooling of the oceans and it is the temperature of the oceans that controls the temperature of the air above.
The net effect of this process is global cooling caused by an increase in greenhouse gases.
There is some debate about the significance of that effect given the limited wavelengths of incoming solar energy that are intercepted but then one has to consider how sensitive the climate system is to changes in solar input and that is currently hotly debated in some circles.
Any extra energy added to the ocean surface will result in a faster rate of evaporation. Other factors are relevant but they need not concern us here. It is sufficient for present purposes that on average globally there is ongoing evaporation from all oceanic water surfaces most of the time.
That brings us back to that infrared radiation which reaches the ocean surfaces either directly from the sun or alternatively from what is known as downwelling infrared from the gas molecules in the air. That downwelling happens constantly even at night and worldwide and it has been theorised by global warming proponents that extra downwelling infrared from more greenhouse gases is at the heart of the proposed climate effect from human sources of CO2.
However the effect of downwelling infrared is always to use up all the infrared in increasing the temperature of the ocean surface molecules whilst leaving nothing in reserve to provide the extra energy required (the latent heat of evaporation) when the change of state occurs from water to vapour. That extra energy requirement is taken from the medium (water or air) in which it is most readily available. If the water is warmer then most will come from the water. If the air is warmer then most will come from the air. However over the Earth as a whole the water is nearly always warmer than the air (due to solar input) so inevitably the average global energy flow is from oceans to air via that latent heat of evaporation in the air and the energy needed is taken from the water. This leads to a thin (1mm deep) layer of cooler water over the oceans worldwide and below the evaporative region that is some 0.3C cooler than the ocean bulk below.
If there were no evaporation then there would simply be a smooth temperature gradient to the topmost molecules of the water surface with a temperature discontinuity at the surface where the water and air interface is located. The existence of that cooler layer with the temperature discontinuity 1mm below the surface is entirely a result of the fact that the evaporative process is a more effective energy transport system than the separate conductive and radiative processes. Indeed, the faster the rate of evaporation the deeper the level of temperature discontinuity will go and/or the larger the temperature differential will be between the cooler layer and the ocean bulk below.
Sarmiento and Sundquist (1992) and Robertson and Watson (1992) raise the issue of ocean ``skin'' temperature.The temperature of ocean water within 1 mm of the ocean-atmosphere interface is known to be about 0.3C cooler than water in the bulk mixed layer due to thermal radiation and evaporation."
I have been engaged in a discussion with a young climatology professional who thought that evaporation from the ocean surface left behind a residue of surplus energy to warm the oceans by reducing the rate of energy release from the oceans and thus justifying the AGW scenario.
The idea itself is set out in more detail here:
My response is as follows, and I think this issue is at the heart of the reality or otherwise of human induced climate change:
The process of evaporation has a net cooling effect. That means that it takes more energy from the surrounding environment than is required to initiate it. A basic physical principle of thermodynamics. If that were not so then the effect of sweat evaporating would be neutral. One’s skin would feel no cooler.
Air above water is very rarely near saturated and then not for long so in global terms we can ignore the issue of humidity. High humidity generally leads to cloud rain and wind anyway which are cooling effects and so the warming effect from high humidity is generally self cancelling.
The extra downwelling IR (Infrared radiation) from extra CO2 does warm up the Knudsen layer of the ocean (the topmost few nanometres where all the evaporative action takes place) and perhaps few microns below it. That is what the temperature sensors record as an increase in ocean surface temperature when IR increases such as when a cloud passes over (just like CO2, clouds re-radiate energy downwards to the surface).
The Knudsen layer and a few microns below it (generally down to the penetrative depth of infra red) can become warmer than the rest of the 1mm deep layer of water below the Knudsen layer. The evaporative, conductive and radiative processes combined then set up a thermal gradient causing an upward flow of energy from water to air from where that 1mm layer touches the ocean bulk below, up across the cooler layer then to the Knudsen layer by reversing the normal (warm at the top and cool at the bottom) temperature gradient which exists from that 1mm layer down to the ocean bottom.
Upwards from the discontinuity between the ocean bulk and the 1mm cooler layer the thermal gradient is from warm at the bottom then through the cooler layer and then finally to warmer at the top.
Downwards from the discontinuity between the ocean bulk and the 1mm cooler layer the thermal gradient is from warm at the top to cool at the ocean bottom.
Notwithstanding that reversal of thermal gradient the flow of energy is always upward and it is the sheer power of the evaporative process which causes that phenomenon.
It is that power of evaporation that draws the energy upward from ocean to air whether the thermal gradient goes downward or upward. That is what evaporation does. It more than cancels the initial energy input from above. In the process it prevents the downward energy flow that would otherwise occur between the top and the bottom of the oceans.
The reason for that reversal of the thermal gradient is the power of evaporation in setting up that 0.3C 'cooler' layer at the top of the ocean bulk about 1mm thick and just below the Knudsen layer. Evaporation alone causes that cooler layer because it has a net cooling effect. Conduction and radiation cannot do it however big temperature differentials become. The result of the evaporative process is that energy flows much more readily from the oceans to the air than would otherwise be the case. In effect the evaporation sucks energy from the oceans against the thermal gradient within the ocean bulk and despite the warming of the topmost molecules caused by infra red radiation and then expels it to the air in the form of latent heat carried by water vapour.
The Knudsen layer and a few microns below it may be warmer than that 1mm layer beneath which is in turn 0.3C cooler than the ocean bulk because the temperature of the topmost molecules rises when there is an increase in downwelling IR. That is all that the sensors record. They record more energy in the top few microns but that does not indicate any warming of the rest of the lower 1mm layer above the ocean bulk. In fact that extra energy in the ocean air interface may actually provoke more cooling in the 1mm layer below it as I will now explain.
The higher temperature measured in the Knudsen layer and the few microns below it may not reduce the rate of energy flow out of the ocean as Realclimate and other AGW proponents propose. Any increase in the activity level of evaporation in the Knudsen layer increases the energy flow from below and in the process further cools the 1mm deep layer of water below it which then draws energy faster from the ocean bulk.
If the Knudsen layer warms from increased IR then the resulting increased rate of evaporation should cool the lower 1mm layer even more than the increased IR causes warming of the Knudsen layer so the temperature differential actually INCREASES at the discontinuity between the ocean bulk and the cooler layer above leading to a faster energy flow upward. If that cooler 1mm layer is indeed caused by evaporation drawing energy away upwards then more evaporation should logically cause more cooling of the ocean bulk not less.
Realclimate and others assume that if the Knudsen layer warms then the lower 1mm deep layer stays the same so that the temperature differential DECREASES and the energy flow upward also decreases.
They measure the temperature of the Knudsen layer and the few microns below it which are affected by infra red radiation and a point 5cm lower in the ocean bulk. They ignore the rest of that 1mm deep layer beneath the Knudsen layer.
They should instead measure the temperature of that 1mm layer below the few microns at the top which do become warmer and then measure the point 5cm down. They should find that the temperature gradient between the two points actually increases if the rate of evaporation increases.
They ignore the evaporative effect completely. Increased downwelling IR makes the Knudsen layer and a few microns below it affected by infra red a little warmer AND via increased evaporation further cools the 1mm layer below by an even greater amount. Evaporation has a net cooling effect. Always has, always will and the extra energy required by the evaporative process is drawn mainly or all from the water. Hence the cooling of that 1mm layer beneath.
Of course, if the air were to be warmer than the ocean surface then evaporation would take the extra energy required from the air rather than the water and that 1mm deep layer (0.3C cooler than the ocean bulk) would rise to the surface and dissipate but that doesn’t happen often or for long.
I think the point at the heart of this is that the depth of that 1mm layer and the size of the temperature differential where it meets the ocean bulk is a dynamic interaction dependent upon the speed of the evaporative process in the Knudsen layer above. Up to now it has been treated as a static phenomenon and I think that is wrong.
I propose that the dynamic nature of that layer actually works to prevent any change in the temperature of the molecules in the Knudsen layer from having any negative effect on the rate of energy flow from ocean to air.
Instead the rate of energy flow from ocean to air would be primarily governed by the rate of evaporation and not by temperature differentials.
In other words, the same package of infrared energy in the air will provoke two simultaneous effects. One effect will be to create a change in temperature differential between the topmost molecules of the water surface and the layers of water below. The other effect will be an increase in the rate of evaporation.
Of the two effects by far the greatest in terms of a change in the speed of energy transmission is the evaporative process. Thus the effect of Fourier’s Law (see below) is more than cancelled by the effect of the change in the rate of evaporation so instead of the energy flow from ocean to air decreasing it actually increases from any increase in downwelling IR.
The Realclimate description relies on Fourier’s Law alone which states that one of the parameters that controls energy flow between objects at different temperatures is the size of the temperature differential.
That is perfectly correct but that relates to conduction and not evaporation. If there were no evaporation then Fourier's Law would set up a smooth temperature gradient to the surface and there would be no Knudsen Layer.
The thing is that the evaporative process is much more powerful in shifting energy upward than that process which Fourier describes. That is why one gets that cooler layer and a temperature discontinuity at about 1mm deep instead of at the surface. That cooler layer is entirely a consequence of evaporation and nothing to do with conduction or radiation.
The existence of that cooler layer is evidence that the rate of evaporation is the primary influence on variability in the rate of ocean energy loss (apart from internal ocean circulation variability which is not relevant here) and it follows that more evaporation for the same rate of conduction and radiation (from a stable temperature differential) will send that cooler layer deeper and/or intensify the temperature differential between it and the ocean bulk below.
Changes in the evaporation rate from, say, higher or lower humidity in the air above, changes in windiness or changes in ocean surface area caused by waves will have a similar effect in varying the characteristics of that 1mm cooler layer.
So if one increases the rate of downwelling IR (thereby increasing the evaporation rate) then the increase in upward energy flow caused by the fall in the temperature of that 1mm layer will be greater than the decrease in upward energy flow that will result from any reduced temperature differential between the topmost Knudsen layer and the ocean bulk arising from the application of Fourier's Law.
With evaporation being the more powerful effect the rate of energy flow to the air above is likely to increase rather than decrease and the 1mm deep layer descend and/or intensify despite a warming of the topmost few microns.
As I said, Realclimate and those who rely on that source may have got the sign wrong.
So, faster evaporation caused by any increase in downwelling IR probably leads to a faster energy flow out of the oceans despite the potential constraint of Fourier’s Law. More IR means more evaporation which means a faster energy flow from ocean to air.
I know that is heresy but is it right or wrong ?”
Could an increase in greenhouse gases actually have a cooling effect over water by speeding up the rate of evaporation from the oceans thereby extracting energy faster from the oceans, speeding up the hydrological cycle and pushing energy faster to space ?
The thing is that despite many attempts I have never found an independent scientific authority to support the Realclimate assertion that extra IR at the surface does actually reduce the rate of energy loss from oceans to air, yet it is parroted automatically as gospel truth by AGW believers.
Another question seems to be whether the extra energy required by the evaporative phase change from water to vapour comes from the downwelling IR itself, from the air or from the water. Latent heat cannot be taken directly from the IR flux because the energy in it does not become heat until the IR has already interacted with water or air. For the energy required to come from the air the IR has to warm the air first but in fact the IR warms the topmost few microns of water first. Thus the latent heat must be taken from the water unless the air is already warmer and that is a very rare, localised temporary situation even in the tropics.
If the energy required for the extra evaporation does all or mostly come from the water then as I have explained it has the potential to more than offset the expected reduction in energy flow that would otherwise be caused by a warmer topmost few microns according to Fourier’s Law.
If evaporation contributes only a portion of the cooling of that 1mm layer (some or even most being attributable to conduction and radiation) then more evaporation would still cause even more cooling of that layer and would still be a mechanism for maintaining or increasing the energy flow to the air rather than decreasing it.
What is needed is firm evidence either way and the Realclimate description does not deal with the issue.
In any event whether more CO2 warms or cools the world the effect is trivial as against natural variability as I have explained elsewhere and would just be dealt with by a miniscule adjustment in the air circulations as the overall system maintains stability.
Here I have set out reasons why the effect of greenhouse gases may be different depending on whether the surface beneath is water or land.
It appears that conventional climatology holds good over land but possibly not over water.
Extra greenhouse gases have a warming effect over land but a possible cooling effect over water.
The Earth’s surface is 70% water.
Other characteristics of the Earth will affect the net position such as the distribution of the land and sea surfaces but given the predominance of ocean surfaces and the fact that most energy comes in at the equator which is mostly oceanic then it seems most likely that the net global effect of more greenhouse gases is actually a miniscule cooling rather than a miniscule warming.
Still nothing to worry about though.