By Javier Vinós
This post has been translated into German by Christian Freuer here.
Most people don’t have a clear understanding of the greenhouse effect (GHE). It is not complicated to understand, but it is usually not well explained. It is often described as “heat-trapping,” but that is incorrect. Greenhouse gases (GHG) do not trap heat, even if more heat resides within the climate system due to their presence in the atmosphere. The truth is that after adjusting to a change in GHG levels, the planet still returns all the energy it receives from the Sun. Otherwise, it would continue warming indefinitely. So, there is no change in the energy returned. How do GHGs produce GHE?
GHGs cause the atmosphere to be more opaque to infrared radiation. As solar radiation heats mainly the ocean and land surface of the planet, GHGs absorb thermal emission from the surface at the lower troposphere and immediately pass that energy along to other molecules (typically N2 and O2) through collisions that occur much faster than the time it would take to re-emit the radiation. This warms the lower troposphere. The density and temperature decrease rapidly through the troposphere, so molecules are colder and more separated at the upper troposphere. Now GHGs have a chance to emit IR radiation so when they finally collide with another molecule, they are colder so GHGs have a cooling effect in the upper troposphere and stratosphere.
Because GHGs make the atmosphere more opaque to IR radiation, when they are present the emission to space from the planet normally does not take place from the surface (as happens in the Moon). Part of it still takes place from the surface through the atmospheric window, but most of it takes place from higher in the atmosphere. We can define a theoretical effective emission height as the average height at which the Earth’s outgoing longwave radiation (OLR) is being emitted. The temperature at which the Earth emits is the temperature at the effective emission height in the atmosphere. That temperature, when measured from space is 250 K (-23°C), not 255 which is the calculated temperature for a theoretical blackbody Earth. That temperature corresponds to a height of about 5 km, which we call the effective emission height.
The last piece we need to understand the GHE is the lapse rate, which in the troposphere is positive, meaning that temperature decreases with height. Without a positive lapse rate, the GHE does not work. Since GHGs cause the planet to emit from a higher altitude, due to making the atmosphere more opaque to IR radiation, that altitude is colder due to the lapse rate. The Earth still needs to return all the energy received from the Sun, but colder molecules emit less. So, the planet will go through a period when it will emit less than it should, warming the surface and the lower troposphere until the new height of emission achieves the temperature necessary to return all the energy, at which point the planet stops warming.
The GHE simply states that the temperature at the surface (Ts) is just the temperature of emission (Te) plus the lapse rate (Γ) times the height of emission (Ze).
Ts = Te + ΓZe
Held & Soden (2000) illustrated it in figure 1:
This is how the GHE actually works. An increase in CO2 means an increase in the height of emission. Since the temperature of emission must remain the same, the temperature from the surface to the new height of emission must increase. The increase is small but significant. As Held and Soden say:
Held and Soden
“The increase in opacity due to a doubling of CO2 causes Ze to rise by ≈150 meters. This results in a reduction in the effective temperature of the emission across the tropopause by ≈(6.5K/km) (150 m) ≈1 K.”
So, the temperature at the surface must increase by 1K. That’s the direct warming caused by the doubling of CO2, before the feedbacks (mainly water vapor) kick in, further raising the height of emission.
This also has an interesting prediction. If the warming is due to an increase in CO2 when the increase takes place and the altitude of emission increases, the planet should emit less OLR as the new altitude is colder and a reduced OLR is the warming mechanism. Once the warming takes place, the OLR will become the same as before the GHG increase. It says so in Held and Soden’s figure 1 caption: “Note that the effective emission temperature (Te) remains unchanged.” Same Te, same OLR. So, if CO2 is responsible for the surface temperature increase, we should first expect less OLR and then the same OLR. If at any time we detect more OLR that would indicate another cause for the warming. Anything that makes the surface warmer, except GHGs, will increase the temperature of emission, increasing OLR.
So, this is the test:
– Surface warming but less or same OLR: CO2 is guilty as charged
– Surface warming and more OLR: CO2 is innocent
And the test results can be evaluated for example with Derwitte and Clerbaux 2018:
Derwitte and Clerbaux 2018
“decadal changes of the Outgoing Longwave Radiation (OLR) as measured by the Clouds and Earth’s Radiant Energy System from 2000 to 2018, the Earth Radiation Budget Experiment from 1985 to 1998, and the High-resolution Infrared Radiation Sounder from 1985 to 2018 are analyzed. The OLR has been rising since 1985, and correlates well with the rising global temperature.“
CO2 is innocent. Its fingerprint is not found at the crime scene. Something else is warming the planet and causing the increase in OLR.
Bibliography:
Dewitte, S. and Clerbaux, N., 2018. Decadal changes of earth’s outgoing longwave radiation. Remote Sensing, 10(10), p.1539.
https://www.mdpi.com/2072-4292/10/10/1539/pdf
Held, I.M. and Soden, B.J., 2000. Water vapor feedback and global warming. Annual review of energy and the environment, 25(1), pp.441-475.
https://www.annualreviews.org/doi/pdf/10.1146/annurev.energy.25.1.441
Stephens, G.L., O’Brien, D., Webster, P.J., Pilewski, P., Kato, S. and Li, J.L., 2015. The albedo of Earth. Reviews of geophysics, 53(1), pp.141-163.
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This view is wrong for CO2. Looking at emission height is just plain bad science. Don’t get sucked in. What you really want to know is what changes in the total energy emitted when you change the concentration of a gas. Averaging out the emission heights for all GHGs is like averaging out the number of dollars and pennies you have and saying increases in the number of any denomination you have has the same effect.
For CO2 the average emission height is in the Stratosphere. If all else were held equal and you raised that height, you should emit more energy and thus cool the planet. However, even that is the wrong answer for other reasons.
GHGs both absorb and emit energy. As a result opacity looks entirely different than we are used to with stuff than only absorbs energy (like mist). In addition, the atmosphere exists in a gravitational field which changes the concentration of these absorbers/emitters as you go higher. You can’t apply what might seem like common sense.
For well mixed GHGs the ability to absorb/emit stays balanced at all altitudes as you increase their concentrations. This means the flow of energy stays constant. The effective opacity stays the same.
It’s the combination of absorption and emittance that determines the final result. Absorption does increase slowing down the flow of energy. However, emittance also increases which means more energy is flowing. The two values cancel out and the energy flow per unit time remains constant.
For CO2 the only increase in energy from increasing the concentration comes from the fact that more surface energy is absorbed low in the Troposphere at the edges of the 15 mm frequency band.
If you calculate the Earth’s total thermal energy released to space for the case of no atmosphere and an albedo of 30% you will get 240 w/m2. If you now add an atmosphere of any concentration of CO2, water vapor, and other gasses the total thermal energy released to space will remain 240 w/m2.
While I agree with most of what you say I think it is even more complicated than this. You would be correct if there weren’t other molecules floating around. But what gets absorbed doesn’t always get emitted as radiation. Sometimes it get passed on as kinetic energy to other molecules (e.g. oxygen, nitrogen, water vapor, other CO2, etc). How that gets handled via conduction and convection also plays a part in the heat profile as well as the absorption and emitting of IR by oxygen and nitrogen. O2 and N2 don’t need to have the same IR efficiency as CO2 in order to have an impact since there is so much more O2 and N2 in the atmosphere.
Whether all that can be measured adequately, especially since it is a time function, leaves me with a lot of doubt.
/sarc/ perhaps we should focus on getting some O2 and N2 out of the atmosphere instead of CO2?
I just Googled for NASA statements of CERES trends, and saw that incoming absorbed radiation and outgoing longwave IR are both increasing, but incoming has been increasing more than outgoing, and the imbalance is in the direction of warming the surface and increasing. Income of absorbed radiation is expected to be increasing because of decreasing surface albedo.
So, all this endless discussion about Convection is meaningless. The Earth orbits in a vacuum, heated by a star. The amount of energy in the atmosphere is determined by two things, energy in and energy out. Energy in seems relatively constant, although the amount of UV seems to vary a bit. Energy out is determined by how much radiation goes to Space, from the Land, mostly dirt and rock, from the Ocean liquid, from reflectance from Albedo, and from the Atmosphere. Albedo is not constant, mostly determined by clouds, can be measured by reflectance from the New Moon, this data is hard to find. It is called Earth-Shine, try to find the records.
The only thing CO2 changes is the opacity of the Atmosphere to outgoing radiation. CO2 absorbs in the 15-micron band. A bit more CO2 at the surface does not change much, as it already absorbs all it can in the first 10 meters from the surface. It rarely re-radiates down here, as collisions with Air absorb this energy as it has for the entire history of the Earth.
The absorbance band of CO2 is around 15 microns wavelength. This corresponds to a temperature of emittance and of absorption of -80 C. So, more CO2, high in the sky where it is much colder that at the Surface, does exactly what Javier says, raises the altitude at which the Atmosphere is freely able to radiate to Space, thus lowering the temperature at which the Atmosphere is freely able to radiate to Space, thus retaining some more energy equals Heat in the Atmosphere.
But, as Javier also says, no one knows how much, this cannot be calculated from First Principles.
I will not reply to Dr. Eschenback again, he does not understand the first word of this, never passed any classes in Thermo nor Heat Transfer, all he knows is what NASA and NOAA say…
Moon
Speaking of energy in and energy out. The surface of the Earth radiates about 390 w/m2. Of that 240 w/m2 is radiated to space. At all times 390 w/m2 must be returned to the surface to maintain equilibrium. Estimates from NASA and others suggest the thermal radiation from the Sun is about 160 w/m2 so that leaves 230 w/m2 that must be returned to the surface from some other source.
Without CO2 and water vapor it would be difficult to find that level of energy returning to the surface because without GHGs most of the radiated energy would pass directly to space. So if GHGs can return that much energy why not a few more w/m2 further heating the surface?
In a 1D conceptual model:
The Total OLR is the combination of Cloudy Sky OLR and Clear Sky OLR.
The Total OLR is the sum of transmitted flux from the active planetary surface and that emitted from atmosphere.
The active planetary surface observed from space includes the land “dirt and rock” and that from the condensed matter surfaces (ocean and cloud).
At any given time the active radiating planetary surface observed from space is the 2/3ds cloud fraction. The remaining 1/3 is the Earth surface land and ocean.
The gap between the Earth surface and the active radiating cloud surface, of which occupies 2/3s of total radiating surface at any given time, is completely dominated by non radiative flux K.
The dominance of non-radiative process delivering power to the active radiating surface in atmospheric budgets does not displace the Ein and Eout paradigm.
The error is reducing the internal power dynamics to radiative process. These dynamics are known to meteorologists as weather.
Atmospheric Ein = +Solar +Oceanic Flux
Atmospheric Eout = -OLR – Oceanic Flux
Earth System Ein = Solar
Earth System Eout = -OLR
The weighted effective planetary radiating surface temperature can be thought of as:
1/3 289K + 2/3 273K (roughly 2km cloud radiating height).
Planetary effective radiative surface temperature = 279K
279K delivering 240 Wm-2 power density suggests a total system emissivity about 0.7.
The weighted effective planetary emissivity:
1/3 x Blackbody Surface + 2/3 Cloud Emissivity (~0.6).
Planetary effective emissivity ~0.7
Further still – these ratios are absolutely everywhere in the earth system.
Take Schmidt’s diagram posted upthread by Javier
The surface budget components reflect the ratios:
170 = Net SW
16 + 100 = K
Net LW = 53
Surface Net SW *1/3 = Surface Net LW
Surface Net SW * 2/3 = K
This causes dissonance for people who wish to reduce the Earth system climate dynamics to radiative flux mechanisms within atmosphere, but in fact there is no net flux of LW within atmosphere whatsoever. The only LW flux is that which is emitted to space (more or less). This last statement often gets me into trouble here.
If one wishes to get even more daring, the albedo can be derived as a simple reciprocal of emissivity for the opaque effective radiating surface (i.e. earth surface + cloud):
where the albedo = (1-0.7)
“The absorbance band of CO2 is around 15 microns wavelength. This corresponds to a temperature of emittance and of absorption of -80 C.”
It has absolutely nothing to do with -80ºC