All matter, above absolute zero temperature, emits photons of energy as electromagnetic radiation, EMR. Colder the matter - the longer the wavelength and less energetic its EMR
The greenhouse gas effect, GHGE relies on idea that only greenhouse gases such as: CO2, and H2O absorb and emit EMR. It's true that CO2 and H2O absorb more (than non-GHG such as N2) at specific frequencies. It's untrue that O2 and N2 do not emit.
We're often shown a diagram which reveals the EMR absorption spectra of the greenhouse gases seen form space. They say these gases 'trap heat'.
What actually happens is: after photons are absorbed by CO2, an electron is excited to a higher energy orbital. Now comes thermalization: long before the electron will naturally decay, the molecule will collide with another air molecule such as O2, or N2. The excited CO2 orbital decays and the energy transferred as kinetic energy (to O2, or N2) which means N2 or O2 speed up slightly. They gain kinetic energy, which can be measured as a temperature increase
. So the energy is shared among all air molecules.
In the 'classical' GHGE hypothesis O2 and N2 do not emit. In reality, all matter emits EMR : so the characteristic spectrum is seen from space (see diagrams above). Some energy bands are in deficit (due to GHG), but other bands compensate and are in excess (of EMR emission) compared to what they'd otherwise give (due to extra emission by air). The more CO2 - the more thermalization - the more O2 and N2 will emit. It's important to remember that emission happens according to the 4th power of temperature. Even a small rise in temperature causes a much larger rise in photon energy emission. In short. The claim for a GHGE 'trapping heat' is based on an assumption that this GHGE makes the atmosphere more opaque to infra-red. That does not happen. It has never been observed. There is no evidence for many fundamental assumptions of the GHGE.
The only way a GHGE can warm the surface (in practice) is by ocean heating (which is 90%+ of climate warming). Because the heat capacity of water is 4 times greater than air and there is 270 times more ocean by mass than air. Ocean heating has happened at about 0.001 °C per year. Only 0.1 °C per century at most. But is not continuous. Oceans warm and cool. I'm nor ruling out faster warming than this. But faster warming is more likely in a colder earth. Earth is now in a mid-glacial called the holocene. When it's already warm, the maximum rate of warming will be slower than when earth is cooler. This is why the D-O events are ice age phenomena.
Note:
- The main CO2 band is 14992 nm = 667 cm–1
N2 emission band is 557.7 nm = 1793 cm–1
So the N2 emission band radiates more energetic radiation than CO2. It is on the borderline of the red light spectrum sitting in the same infrared area as radiation emitted from the sun. That may explain why this N2 emission band is often missing from diagrams of the IR. It is too far to the right in the diagram above and was truncated.
andthentheresphysics,
Thank you for your interest, and taking the time to read some of our summaries. 🙂
Your derivation of the dry adiabatic lapse rate is indeed a standard result, and as you correctly point out is independent of greenhouse gas concentrations.
However, in our “The physics of the Earth’s atmosphere” Paper 1 and Paper 2 (which you are referring to here), we were looking at a different phenomenon.
We were looking at the absolute temperature profile. That is, we weren’t just trying to understand how temperature changes with height (“lapse rate”), but trying to understand the absolute temperatures at each height (in Kelvin).
Surprisingly, when we analysed a large sample of weather balloons (several thousand for Paper 1, and more than 13 million for Paper 2), we found that the temperature profile for each balloon could be described entirely in terms of the thermodynamic properties of the bulk gases (i.e., nitrogen, oxygen & argon) and water vapour concentration.
This was an unexpected result (at least to us!). According to the greenhouse effect theory, the presence of greenhouse gases should be substantially altering the temperatures at different altitudes. Roughly speaking, they should be:
(1) Increasing the mean temperature (and height) of the troposphere, relative to a pure nitrogen/oxygen atmosphere.
(2) Decreasing the mean temperature of the stratosphere, relative to a pure nitrogen/oxygen atmosphere.
What we found is that the mean temperature profile throughout the troposphere and stratosphere is essentially the same as that of a nitrogen/oxygen atmosphere (with some variation near ground level due to water vapour).