Titan is my new favorite moon/planet (it orbits Saturn and is a bit bigger than our Moon). It has an atmosphere that is strikingly similar to Earth's - it's mainly N2, with 5.65% CH4 to spice things up and the surface pressure is 1.48 times as much as Earth's surface pressure.
Harry Dale Huffman pointed out that comparing the surface temperatures of Venus ~737K (where atmospheric pressure is 92.1 times as much as on Earth. And it's nearly all CO2!) with the surface temperature of Earth 288K is a diagonal comparison. A direct comparison is the temperature of Venus' atmosphere at the altitude where atmospheric pressure happens to be equal to Earth's surface pressure versus Earth's surface temperature, so we are comparing 338K with 288K.
Then you just adjust Venus' temperature down to compensate for the fact it is nearer the Sun and the solar radiation it gets is more intense. The adjustment factor is the fourth root of (2,601 W/m2 ÷ 1,361 W/m2) = 1.91 ^ 0.25 = 1.176*. Divide 338K by 1.176 = 287.4K, that's as close to 288K as makes no difference, job's a good 'un.
HDH does not claim to be able to explain why this is so (see discussion here), but that's just how science works. First step is observe stuff, recognize clear patterns, and then you try and work out why. His guess appears to be that you can ignore a planet's albedo when looking at temperatures, because higher temperature causes clouds; clouds increase the albedo; thus reducing incoming solar radiation; which would reduce the temperature. So we would end up in a circular calculation. Or something like that.
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OK, let's strap on our space suits, fire up the rockets and head off to Titan. Temperature at the surface is measured/estimated to be 93.7K. It receives 14.7 W/m2 solar radiation (same as Saturn, but knock off 0.8% because it is in Saturn's shadow 0.8% of the time and add on 0.08 W/m2 which Titan receives from Saturn itself).
Let's adjust Earth's surface temp (288K) down using the same method as above: 288K x ((1,361 W/m2 ÷ 14.7 W/m2)^0.25) = 92.8K. That's pretty close to 93.7K!
Where the wheels come off a bit is that HDH's direct comparison would be the temperature on Titan at the altitude where pressure = Earth's surface pressure versus Earth's surface temperature. With a lapse rate of ~1 K/km, that would be at altitude 7.8 km where the temperature is 85.9K (against predicted 92.8K). Which is still close enough, I think.
Which all demonstrates that a planet's albedo and the precise composition of its atmosphere are probably irrelevant, and whether or not the constituent gases can 'trap' radiation is almost certainly less than irrelevant. Unless we blame the ~7 degree extra temperature on the naughty 5.65% CH4? For comparison, that's about 4.3 kg of CH4/m2, or a bit less than the 6kg of CO2/m2 on Earth. But if CH4 does have such a strong effect, then the effect of CO2 must be +/- zero, or else the 'greenhouse effects' on Earth and Titan would cancel out, and the Earth-Venus comparison wouldn't work at all.
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* The short cut is divide distances from the Sun and take the square root = (149.6m km ÷ 108.2m km) ^ 0.25 = 1.176.
Debate Magazine
