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Originally Posted by Windguy
No I like it. Thought provoking once again.
I would say Mars albedo effect of 30% since it's all desert rock, The fact that the atmosphere is nearly all CO2 and virtually nothing else. Close to a vacuum if you see and believe Total Recall. And CO2 as pointed out to me doesn't reabsorb it's own infrared radiation. So There is no H2O to help out of which is 95% of the earths GHG's. Therefore there is less absorption and re-radiation. And there the fact the planet is a lot cooler, so has anyone measured the spectrum of infrared light that leaves Mars? And since Co2 is logarithmic, it would be near the saturation limit since there is far less infrared light leaving Mars altogether.
One last point, Mars has a smaller circumference so there is more chance of the infrared leaving the planet after CO2 infrared emission. Since there is little chance of re absorption in the atmosphere. There would be a GHG effect on Mars, just not as anywhere near as noticable.
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Albedo for Mars is 15% per the wiki article on the subject. This is for visible light. The moon is about 12% albedo for comparison.
It doesn't matter that the atm is most all co2 as most all else, like nitrogen and o2 don't absorb in the IR.
Concerning broadening, it's all self broadening for the pressure as other molecules are quite low density.
Total atm pressure is less for two reasons. First off, gravity is only 38% of earth so a 1 pound mass of atmosphere (per square inch) that would generate 1 pound of force on earth is going to generate only 0.38 pounds of force on mars. The actual amount of atmosphere, which determines how much absorption can occur in the optical path is around 1.5% of earth. However, the pressure is more like 1/3 of that (switching units to mks - 600 Pa versus 101,300 Pa). That is quite the vacuum but your eyeballs aren't going to pop out and explode. Actually, they'll freeze rather quickly and you'll get the bends but you won't explode while you suffocate. So much for Total Recall.
The comment on absorption or reabsorption is TOTAL BS. Co2 will absorb and emit based upon its spectrum. Emission is a function of the gas temperature as well as upon the spectrum.
CO2 spectrum has nothing to do with H2O spectrums other than there is overlap. Even on earth the saturation is such that on strong peaks, the absorption pathlength is measured in cm not in meters at the surface. Numbers turn out something like an additional 18-20w/m^2 additional absorption for each doubling from all the calculations I've ever done but that includes all molecules including h2o. CO2 doubling numbers come out around 3.6w/m^2 for the doubling from 1750 and about 2.6W/m^2 for a doubling starting at 1976. Note this last one is a fairly refined one dimensional model rather than a moderately simple slab of atmosphere.
The difference between earth and mars is an additional 5 doublings or so over earth's 8 or 12 doublings for the actual additional pathlength. These of course are averages as peaks vary in absorption strength and strong peaks are extremely strong.
As mentioned, there is 40 times the amount of co2 in the mars pathlength over that of earth's.
H2O on earth is the vast majority of absorption accounting for the majority. One must be careful with the numbers like 95% as some numbers are done for political purposes rather than to clarify the situation. These use the irrelevent circumstance of what if it was the only gas in the atmosphere - what does it contribute rather than what does it contribute while other contributors remain unchanged. One way provides information, the other provides confusion and overestimations. Also, the 95% might include cloud effects rather than gas effects for h20 - that is entirely a different matter and is not calculable to significant accuracy as compared to straight radiative calculations.
Suffice to say though that if h2o vapor is 95% and one assumes co2 is essentially 5%, that is 1/20 of the effect. In reality, it's going to be more like 85 15 but that is somewhat more than 1/20 and when one has 40x the amount, there is some significant contributions, even log scale - probably at least 1/3 more absorption than on earth (not including pressure broadening).
It's safe to assume that mar's spectrum has been observed for ages or at least since the space program started in significant fashion. That is how atmospheric details get measured as well as temperature measurements get made.
there is less IR leaving but there is less visible light energy arriving. It's got to balance. And, evidently it balances rather well without virtually any atmospheric warming.
Note that the 95% contribution (to use your number) of h2o means that not only is there more possible atmospheric energy absorption and required atmospheric warming, it also means that the impact of the co2 is far less than a situation such as mars has without it. That means if you could attribute say 1.6 deg K to the mars atmosphere, for 16 doublings (earth has 11, mars an additional 5) in the log range with each doubling giving roughly a contribution of 0.1 deg K, that means that a doubling of co2 increases T by 0.1 when h2o isn't present but when h2o is present, it will be far less than that due to a fairly significant overap.
The circumference has no effect. Calculations are based upon per unit surface area. Absorption of incoming light is effectively a lambertian disk while emission is over the surface of a sphere.
chris,
I think that it was mentioned that this back of the envelope estimate didn't include differences in pressure broadening. However, I think you'll find there is far less difference than you might first believe as most of the absorption in the lower atmosphere where pressures are higher and lines broader are also reradiated numerous times at progressively higher altitudes which have significantly less pressures on earth hence emitting and absorbing narrower lines. In the mars case, all the lines are emitted and absorbed with somewhat narrower bandwidths even from the ground.
In either case, you'll note that the absorption lines of the spectrum only exist because the atmosphere above the emission area getting through is at a lower temperature than the emission area. Were the temperature the same, there would be no absorption lines and if the temperature in the atmosphere were higher, you would start to see emission lines.
Finally, it cannot be repeated enough to remind people that radiative transfer is only one means of thermal energy transport in an atmosphere. Convection is a very powerful transport mechanism in the troposphere. Above there in regions where it ceases being important, you're already at very low densities and there is little above to block the radiation.
Linear Mode
