Sunday, August 21, 2011

Earth's Energy Budget Controversy: Circular Argument at its Finest

The argument over the impact of changing CO2 on the Radiation Energy Budget of the Earth frustrates the hell out of me. For CO2, the impact is on the 15% of the outgoing radiation absorbed by the atmosphere. Absorbed is the first problem. CO2 has an emission and absorption spectrum where it interacts with electromagnetic radiation. Absorbed does not mean "trapped", delayed is better but still subject to confusion, interacts is what happens and there are consequences following that interaction. The net effect is that it slows the flow of radiant heat from the Earth slightly. The 15% on the NASA chart is based on the total energy provided by the sun to keep things simple. The NASA cartoon or diagram is a very useful basic tool.

Before when I posted on the radiation budget as a puzzle, my answer was that the numbers on the NASA cartoon would not change or have very little change with twice as much CO2. I still believe my answer is 100% correct.

With the Radiation Budget provide by Kieth Trenbert, there are similarities and differences to the NASA cartoon. The biggest difference is that Trenberth uses radiant energy flows or fluxes to include what he perceives to be the balance with a minor adjustment for CO2 warming that is determined by a model because it cannot be accurately measured. There is nothing wrong with than as long as it is understood what the purpose of the work is. This work is responsible for the "travesty" quote by Trenberth where he thought it was a travesty that there was missing heat. Nothing particularly unusual about that, it is not an easy task to determine because there are inaccuracies and uncertainties. Because of the intent of his work, the values are recorded differently with more focus on the difference between the outward radiation and the inward radiation.

The simplified Trenberth radiation budget cartoon is useful for his intended purpose, but since it is not a direct comparison to the NASA budget cartoon, it creates a lot of valid questions for those that look at radiation and heat transfer in the old school way. The old school understands that the direction of heat flow is from warm to cool and while there is radiation from the sky to the surface, the net is from the surface to the sky, i.e. flow is always out at night and only in during the day. The modeled value that Trenberth uses is the energy imbalance caused by increased CO2 so his cartoon indicates that the sky is "physically" warming the surface. Whether that is possible according to the laws of physics is debatable. Technically, that is possible, as a general rule it is highly unlikely for any significant amount or time period. His cartoon indicates something that may be happening in the magnitude indicated by the model, that is not physically measurable. Could it happen? Yes. Would it last a significant time period? Not likely. There are constant brief radiation imbalances and adjustments, that is why the old school prefers to use the net flow, to simplify the situation.

Since Trenberth drew his cartoon the way he did, it is difficult see what change CO2 could have and why. Major problem for those trying to understand a rather complex issue. One person that is skeptical, Tall Bloke is his internet pen name, wanted to know what part of the down radiation is "new". That is a valid question that is not obvious in most diagrams of the "greenhouse" effect.

Neither Trenberth nor NASA makes it all that easy to answer the "new" energy question. "New" Energy may be defined as energy directly supplied to the Earth system. The sun warms the air above us and the clouds directly with approximately 19% of its total energy available at the Top of the Atmosphere (TOA). Moisture rising from the surface to the sky may be considered new, because it has latent, meaning hidden, heat. The latent heat is given up to the sky primarily by condensation when the water vapor becomes precipitation. As most everyone knows, the temperature of water remains constant while it is changing its state from solid to liquid or liquid to gas. So the evaporation of water does not have a major impact on surface temperature other than to stabilize the temperature somewhat during the evaporation and the main impact is felt with condensation when the heat is released. Since evaporation is at or near the surface and condensation starts high in the atmosphere on average, latent cooling is a major player accounting for 36% by NASA and 40% per Trenberth of the atmospheric warming while cooling the surface. If you combine the solar and latent factors, 61% (NASA) to 79% (Trenberth)of the warmth of the atmosphere is due to "new" energy.

This is very important for people trying to teach atmosphere radiation physics, the source of the heat and its location in the atmosphere. The reason being that radiative heat transfer of gases changes with the optical depth of the paths of radiative flow. This is known as the radiation window, the optical depth for different radiative frequencies. The higher the molecule is in the atmosphere, the cleaner the window is to space and the dirtier the window is to the surface.

While all that is fairly simple, here is were things start getting interesting. Greenhouse gases make the window dirty, it is much dirtier at the surface than it is at the top of the troposphere, that layer of the atmosphere we live in. Anyone that has climbed a mountain or flown in an airplane knows it get cooler the higher you go. That is because with the cleaner window up and the dirty window down, it is easier for the heat to leave going up so it leaves faster. In the middle layer of the troposphere it is a little easier up than down and at the surface it is barely easier to leave up and damn near impossible to go down. So knowing how much "new" energy there is and where it is important.

As a note, some try to explain the decrease in temperature with altitude with only the gas laws and gravity. That is not a complete explanation because of the three types of heat transfer, conductive, convective and radiant, conductive and convective decrease with altitude while radiant increases with altitude, all have to be considered for a valid explanation.

The new energy from latent heat rising and solar warming of the clouds is easiest to locate, it is about midway to the top of the troposphere where it is easier out than in. So more CO2 will make the up dirtier and the down dirtier which will result in very little change to the impact of these "new" energies. The CO2 may slightly increase the cooling relative to these "new" energies, but not much, at least now right away. The solar absorbed by the atmosphere is a little different. If it is spread uniformly from the top down, then CO2 will increase the top absorption and decrease the bottom absorption which will result in more cooling. If most of the solar absorbed by the atmosphere is near the surface, likely, then the impact is much more dependent on the radiation spectrum of the absorbing molecules and the spectrum of the molecules being energized by collision with the absorbing molecules. That is pretty complicated.

For old energy you have conduction or thermal updrafts and radiation from the Earth's surface. These will be most impacted by the increase in CO2 because they start at the dirtiest window. CO2 will only make it dirtier. So while more CO2 will not "trap" the heat, it will make its road to the TOA longer or slower. Longer is a good term because the path length of the radiation decreases with a dirtier window. The distant to the top is the same, but the heat has to take more, shorter paths and detours.

So most of the confusion is that the pros don't explain that CO2 only has a major impact on about a third of the heat energy traveling from the surface to space. Looking at Trenberth's cartoon it is easy to imagine that two thirds of the energy at the surface is due to down welling radiation, but two thirds of that two thirds is due to "new" energy that will have little change due to changing CO2. So in my opinion, Trenberth's cartoon would be much improved if it separated the distribution of the down welling radiation by source, if his intent is to explain the greenhouse effect.

The huge amounts of energy fluxes Trenberth show up and down are just as misleading as trying to explain the decrease in temperature with altitude with just the gas laws and gravity. The impact of radiation decreases with altitude while conduction and convection increases. Molecules with heat energy are constantly moving and collide with other molecules more often when there is a higher concentration of molecules per volume. The collisions transfer energy via conduction basically. When they are in contact they can transfer energy without having to radiate that energy. There is more to it than that, but the process is more conductive that radiative. When molecules transfer heat radiatively, they emit or absorb photons of energy within their radiative spectrum. With collision, CO2, water vapor or any molecule can swap energy with any other molecule most likely nitrogen which makes up most of our atmosphere or oxygen which comes second. Nitrogen and oxygen swap energy with other molecules nearly exclusively by collision. They do emit radiant energy, only the amount emitted via radiation is miniscule compared to collision and much lower than CO2 or water vapor. What nitrogen and oxygen do though is transfer energy from one type of greenhouse molecule to another. Radiative transfer is restricted to the spectrum of the molecule emitting, collisional transfer is not. So when a CO2 molecule bumps into a nitrogen molecule, then the nitrogen bumps into a water vapor molecule, that photon of energy from the CO2 can be emitted by the water vapor at a different wavelength. Neat huh? So now the CO2 energy can increase conduction also called thermals or rising air and latent heat. So the radiative heat transfer picture gets muddy near the surface. The "greenhouse" gases can absorb photons of radiation but it is almost impossible for them to radiatively emit because of collisions. So the down welling radiation which does exists causes an increase in the overall rate of energy transfer between molecules but reduces the rate of flow of heat to the TOA. This is a bit of a paradox, increased down welling radiation higher in the atmosphere does cause the surface to be at a higher temperature, but conduction and convection are the means of heat transfer that increase at the surface, since emission of radiation by "greenhouse" gases is pretty well maxed out at the surface.

There are plenty points to debate on the climate change front, but the fact that there is warming of the sky that warms the Earth is not one. That warming can be expressed as net radiation flux increasing, causing slower cooling or individual fluxes up and down or sideways doing that same thing, but either way the sky has a temperature and anything with a temperature radiates energy which impacts the radiate energy flow rate of any other object, dependent on the temperature difference between the objects, the radiation window between the objects, the absorption/emission spectrum of the objects, the effective thermal conductivity and their separation. Simple right?

Update: Since I am talking about the sky, I should have had the effective thermal conductivity which I added in bold. In a vacuum things are purely radiative, in the atmosphere, even pretty high, there are molecules that can collide. So the temperature difference and the effective thermal conductivity play a role that decreases with increased altitude, minor I know, but I was being a smart ass.

As usual this was just a quick post while waiting on something to dry, so there may be a few typos or minor errors, but it should be pretty accurate.

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