I’ve seen these issues come around before, but I don’t remember seeing anyone “connect the dots”.
First, a reminder of my earlier Tropopause Rules where we saw that below the tropopause CO2 does nearly nothing, but above the tropopause, in the stratosphere, it cools via radiation.
In many ways the paper about a 0.1 bar tropopause is just saying the same thing, but for a generalized ‘all atmospheres with SW absorbing gasses’. This Google Books entry lists those shortwave absorbing gases for Earth as being H2O (water vapor), O2 and O3 (Oxygen and Ozone), NO2, and NO3, and leaves out SO2 as it “almost corresponds with the strongest Ozone band”. So basically, water vapor and life derived oxygen and nitrite / nitrate.
Note that CO2 is not part of this block of gases.
In this graph, you can see that the CO2 band only radiates in the stratosphere. Below the dashed line of the tropopause it does nothing radiatively.
Also notice that Ozone is a ‘hot spot’ in the stratosphere as it is absorbing a lot of UV light, and down in the troposphere it is water vapor that’s picking up the heat (and moving it skyward as water vapor is lighter than air and rises to make clouds and rain).
So what’s new here, now? First off, this paper talks about that 0.1 bar (more or less) constant tropopause height, but is paywalled so the abstract is all we get…
RE GEOSCIENCE | LETTER
Common 0.1 bar tropopause in thick atmospheres set by pressure-dependent infrared transparency
T. D. Robinson & D. C. Catling
Nature Geoscience 7, 12–15 (2014) doi:10.1038/ngeo2020
Received 28 March 2013 Accepted 29 October 2013 Published online 08 December 2013
A minimum atmospheric temperature, or tropopause, occurs at a pressure of around 0.1 bar in the atmospheres of Earth1, Titan2, Jupiter3, Saturn4, Uranus and Neptune4, despite great differences in atmospheric composition, gravity, internal heat and sunlight. In all of these bodies, the tropopause separates a stratosphere with a temperature profile that is controlled by the absorption of short-wave solar radiation, from a region below characterized by convection, weather and clouds5, 6. However, it is not obvious why the tropopause occurs at the specific pressure near 0.1 bar. Here we use a simple, physically based model7 to demonstrate that, at atmospheric pressures lower than 0.1 bar, transparency to thermal radiation allows short-wave heating to dominate, creating a stratosphere. At higher pressures, atmospheres become opaque to thermal radiation, causing temperatures to increase with depth and convection to ensue. A common dependence of infrared opacity on pressure, arising from the shared physics of molecular absorption, sets the 0.1 bar tropopause. We reason that a tropopause at a pressure of approximately 0.1 bar is characteristic of many thick atmospheres, including exoplanets and exomoons in our galaxy and beyond. Judicious use of this rule could help constrain the atmospheric structure, and thus the surface environments and habitability, of exoplanets.
Note that Mars is out as the atmosphere is too thin and it lacks SW absorbing gases. Venus is out due to other reasons. But for many other planets, the tropopause will be near 0.1 bar, or at about 1/10th of an Earth atmosphere density. That also means more than 90% of all the Earth atmosphere is BELOW the tropopause in the zone where infrared radiation is essentially meaningless as it does not travel far. I’ve bolded the bit in the abstract that points out that “at higher pressures, atmospheres become opaque to thermal radiation”. Memorize that. Show it to all your friends (and the occasional enemy ;-)
So below 1/10 bar, IR doesn’t get the job done and it is all about convection. I’d also suggest that some details will depend on evaporation, conduction, precipitation… but they are secondary. With less evaporation, you would just get more convection until the heat is moved back up. Which is why we ignore the day at our peril.
So right off the bat we can say that for 90% of the atmosphere, CO2 “back radiation” is entirely a non-starter. And in the 10% where it does radiate, it radiates to space, not the surface, as the top of the tropopause stops the shortwave radiation “right quick”. It’s a one way diode for heat to go up the convection escalator and then radiate only to space.
Does a very good job of summarizing the paper.
In terms of comparative planetology, we have a Figure 2 very different situations: while some atmospheres are dominated by CO2 or N2, other are mainly composed of H2 and He, the lightest gases available with a substantially different chemistry. Distances from the Sun are also extremely variable, causing a broad range of temperatures from the hot Venus to the cold Uranus and Neptune. However, it is striking to note that all of them (well, all except Venus for reason yet to discuss) display a tropopause at very much the same level. But, why this happens?
This is the intriguing question studied by Robinson and Catling in their work. For doing so, they developed a relatively simple 1-D model to account for the temperature structure of the atmospheres under very different conditions. This model accounts for the solar and infrared radiation absorption but also for diffusion and convection. However, it is known that the infrared opacity is a power law of pressure. The exact coefficient depends on which kind of absorption is dominant at each atmospheric level. In the higher atmosphere, absorption is dominated by Doppler broadening of the lines produced by the atmospheric constituents (yielding a power of 1), while below the middle stratosphere the absorption is mainly pressure-broadened and collision-induced resulting in an infrared opacity proportional to the square of pressure, regardless of which particular species is absorbing radiation. This model is able to give an analytical expression of temperature and, setting the derivative of this function to zero, it is fairly simple to find not only the minimum of temperature (namely the tropopause) but also the conditions under which such a minimum will develop.
I note in passing that a power law with a squared function in it that starts of 1/10th the total is going to be very effective rather quickly at stopping IR from penetrating back down against convective rise.
He does (kind of) explain the Venus differences:
Venus has been already stated as lacking a proper tropopause. This is true for the global mean, but not for some latitudes of the planet with a stratospheric temperature inversion, where a tropopause can indeed be found. So Venus is only an exception in the global mean.
That “over averaging” thing again. Averaging out the parts where it still holds.
And I found this bit particularly interesting:
In the near future, we expect to retrieve spectra of exoplanet atmospheres; assuming a 0,1 bar pressure tropopause can help to determine the surface temperature in a much more accurate way and therefore to determine the feasibility of finding liquid water on them.
So with just the spectra of the atmosphere, some idea of the level of astral radiation, and this formula, we can figure out the surface temperature of the planet and the water phase as liquid, solid, or gas. But per the Global Warmers, just not on Earth? Eh?…
Now, just a quick note before the next paper / link. It references SED, but does not translate that to words. I believe it means Spectral Energy Distribution. As our sun has just gone a bit quiet and TSI didn’t drop much, but the SED shifted dramatically away from UV, I think this matters.
Now, on to a bit of NASA speak…
This includes the graphs from the 0.1 bar paper, and also comments on the usabability of this method to finding potentially habitable worlds. This is an excerpt of their comments on that paper:
This model was recently used to explain why Earth, Jupiter, Saturn, Titan, Uranus, and Neptune all share a common tropopause temperature minimum in their atmosphere at 0.1 bar pressure (Robinson and Catling 2013). The explanation lies in the physics of infrared radiative transport, and should apply to countless worlds outside the Solar System. Furthermore, the assumption of a 0.1 bar tropopause can be used to help constrain surface pressure or surface temperature on an exoplanet, the combination of which determine habitability.
So we have a rule that applies to at least 6 major bodies of our solar system, that can be applied to planets anywhere in the galaxy, and can constrain surface pressure or surface temperature. And we know that the surface pressure on Earth is constrained to 1 Bar, so it is not the variable. I think that means this formula is going to be constraining surface temperature when applied to Earth… Just saying…
They go into albedo aspects a bit more than I would expect, and cover some other turf too, but one particular paragraph a ways into it caught me eye.
Shields, Meadows, Bitz, Pierrehumbert and collaborators used a hierarchy of climate models to explore the effect of the interaction between the parent star’s radiation and the planet’s wavelength dependent reflectivity (from surface ice and snow, and atmospheric absorption) on planetary climate. Their results indicate that planets orbiting cooler, redder (M-dwarf) stars are less sensitive to decreases in stellar insolation (as shown in Figure 2 below), and episodes of low-latitude glaciation may be less likely to occur on M-dwarf planets in the habitable zone than on planets orbiting stars with high visible and near-UV output. This is due to absorption of near-infrared radiation by lower-albedo surface ice and atmospheric absorption by CO2 and water-vapor. However, at the outer edge of the habitable zone, high levels of CO2 mask the ice-albedo effect, leaving the traditional limit of the outer edge of the HZ unaffected by the spectral dependence of ice and snow albedo (Shields et al., 2013). Ongoing simulations also indicate that the amount of increased stellar flux required to melt a planet out of a snowball state is highly sensitive to host star SED. We find that a distant frozen M-dwarf planet orbiting beyond the outer edge of its star’s habitable zone without a continuously active carbon cycle is likely to melt more easily out of global ice cover as its host star ages and its luminosity increases (Shields et al., in prep).
I’ve bolded a bit.
To me, this seems to indicate that SED matters rather a fair amount. Shifts of SED can shift habitable zone of the orbit space, and it can change how hard or easy it is to exit an snowball state. We, not being around a redder dwarf, will be a bit more sensitive to SED as it interacts with surface albedo such as ice and snow.
As I read this it is focused on red dwarfs, so I’m less clear on how this applies to our yellow medium sized star with a load of UV. But it is pretty clear that shifts of UV vs IR matter, potentially a lot, and interacts with ice cover and water vapor and oxygen content. It also seems to say that high levels of CO2 (though it doesn’t say what those are) can mask the ice-albedo effect. So, since IR is absorbed by the ice, having more CO2 means the ice is less likely to melt? Do I have that right?
IMHO, this is going to take some more thinking. It is also very obvious that this is NOT in the climate models. “Settled science” my SED…