Nitrogen isn't a greenhouse gas (and CO2 isn't diatomic). It reflects very little of the sun's thermal radiation. If you keep an atmosphere of equal density but with materials that do not facilitate thermal retention, there is a massive drop in average temperature. Earth's would, with only the removal of the 0.04% of it that is CO2, drop by several degrees C. Same is true for Venus, but on a far more drastic scale.
I derped on the diatomic, my mistake, I was typing and checking tabs and so forth, I meant CO2 works out to have the same molar heat but even there I might have run afoul of trying to track down the info on different tabs and missed that the conditions under which N2 has ~29 J/K aren't exactly the same as where CO2 has ~29 J/K (so I think they'd be 20/29 N2/CO2 together), but it doesn't actually change the main point: unless you're trying to stack the deck and set up conditions which support one argument but not another. All atmospheres in the solar system exhibit an increasing temperature gradient from the 0.1 Bar altitude down to the 1 Bar+ altitudes, or from the top to the bottom of the region which mixes convectively. The temperature at the altitude at which a planet hits 0.1 Bar lets you work out the temperature at any point below it.
Whether we're looking at mostly hydrogen or nitrogen, carbon dioxide or a mishmash of moist oxygen and nitrogen,
they all exhibit that profile in the lower regions. You might notice Titan on there is steeper than Venus despite being something like 95% N2 down low with the rest being largely farts, it would need to be taller from the surface to the tropopause if Venus was covered with an N2 atmosphere, but arguing it would just fall over and snow out when it hasn't managed to do that anywhere else in the solar system seems a bit much.
Also, uh, not gonna jump on it since as I showed we all make mistakes, but reflecting solar thermal has nothing to do with anything, and one of the key concepts in the radiative greenhouse effect model is that CO2 in the upper atmosphere is better at cooling
because it is more radiatively active, and this is then said to raise the tropopause and drag the surface temperature upwards.
An incredibly massive atmosphere of N2 being swapped in on Venus for CO2, if going by the simplified "it just doesn't emit radiation" model for convenience here, would have no way to lose heat outside of conduction against the surface, and thus convection would lift parcels and establish the familiar tropospheric temperature/pressure profile and in the lack of outside energy input it would gradually eject faster molecules and relax, but until the Sun shuts off that isn't a reasonable assumption to work under.
My words were specifically about a scenario in which we successfully break climate stability for good, ensuring that Earth's organic and water mass are increasingly converted into atmosphere. This is particularly severe as the majority of the greenhouse effect is caused by not CO2 but water vapor, the latter is simply not increasing. Were Earth's rather substantial supply of water enter the atmosphere alongside its carbon, extreme alterations would occur. Whether or not that would actually happen is a question of the exact circumstances of humanity's suicide and the long-term corrective effect of the carbon and water cycles.
Bleh, I don't feel like going down the "constant relative humidity assumption" road.
See the point above. CO2's exact temperature effect isn't going to be contained in this margin because it's not a matter of the CO2 getting hot, it's a matter of it getting in the doorway and keeping IR from escaping in diverse atmospheric conditions. Prepare to get very rich if you can actually determine CO2's climate sensitivity in WolframAlpha, because there's been a lot of people working on it.
I'd say I was giving a constraint on the far end: there's no way anybody can claim with a straight face that doubling CO2 concentration would give 22 K of warming without missing that such a claim implies we should be something like 10 K warmer than pre-industrial periods currently. Obviously we aren't, so it seems clear to me that CO2 doubling doesn't produce
the amount of warming needed to call Venus proof of the CO2 greenhouse effect.Arguably it's worse because it shouldn't be increasing on a linear scale like I used above, probably need to go logarithmic or something at the higher concentrations.
Tossing in additional handwaved "well of course it isn't just because the CO2 got hot, but in the end CO2 did it" just sounds like a way to get around the issue that you're willing to accept a scenario where it's already hot and credit CO2 for it, but above when I suggested an N2 scenario you dismissed it because you want to assume that it's starting out cold and apparently without the cloud decks? As it stands there is no way to credit any form of the radiative greenhouse effect/related properties of CO2 with more than 20 K or so of the difference between here and there, short of declaring that
something arbitrarily changes the situation there and then only allowing that
something to include some way of boosting the greenhouse related properties of CO2.
On the other hand, as I explained, having that much atmosphere is going to mean the 0.1 Bar altitude is much higher, and whatever the temperature there is (which I guess I should have specified to be the same as the CO2 atmosphere one is in my thought experiment to cut the "ah, but if I just say N2 starts cold it doesn't work" stuff short) the temperature profile below it is a matter of the masses involved.
Unless you want to propose that Hydrogen has some sort of greenhouse effect and that is why Jupiter exhibits the same 0.1 > 1+ Bar profile, but that would basically be trying to credit the greenhouse effect with creating the adiabatic lapse rate, wouldn't it?
Also, you didn't answer my actual question. You don't need to know the exact sensitivity to give a fairly confident answer; no more than Galileo needed to explain how the Earth could possibly orbit the Sun in the face of God's primacy over the universe.
I'm pretty sure I showed that the radiative properties of CO2 can not account for the temperature of Venus without inserting a hidden fudge-factor or assumption somewhere which actually explains it before giving credit to CO2. If CO2 does actually explain it then it doesn't work very well for explaining things here, water vapor or not. You may note I left Mars out of the discussion, despite the mostly CO2 atmosphere it doesn't even have a 0.1 Bar altitude (outside of a warm day at the bottom of Hellas Planitia?) so it has to sit in the corner and think about what it's done.
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Topic: SCIENCE, Gravitational waves, and the whole LIGO OST! (Read 516840 times)