Eunice Foote - 19th century Climatologist

From Wikipedia

Foote conducted a series of experiments that demonstrated the interactions of the sun’s rays on different gases. She used an air pump, four mercury thermometers, and two glass cylinders. First she placed two thermometers in each cylinder, then by using the air pump, she evacuated the air from one cylinder and compressed it in the other. Allowing both cylinders to reach the same temperature, she placed the cylinders in the sunlight to measure temperature variance once heated and under different moisture conditions. She performed this experiment on CO2, common air, and hydrogen.

Of the gases she tested, Foote concluded that carbonic acid (CO2) trapped the most heat, reaching a temperature of 125 °F (52 °C). From this experiment, she stated “The receiver containing this gas became itself much heated—very sensibly more so than the other—and on being removed [from the Sun], it was many times as long in cooling.”

Her own two-page write is up here.

1. She pumped air out of one container into the other and left them in the sun for a while. Result:

Decompressed air – 88 F
Compressed air – 110 F.

“This circumstance must affect the power of the sun’s rays in different places, and contribute to produce their feeble action on the summits of lofty mountains.”

Yes, that is a large part of the actual explanation for sea-level temperatures, which is the gravity-induced lapse rate. Also, common sense tells us, it is pretty hard to heat up a vacuum (there is nothing to heat up), so decompressed air must heat up less.

2. She filled one container with moist air and one with dried air and left them in the sun for a while. Result:

Dry air – 108 F
Moist air – 120 F

[Explanation to follow once I've thought this through].

3. She filled the containers with different gases and left them in the sun for a while. Result:

Hydrogen – 104 F
Common air – 106 F
Oxygen – 108 F
CO2 – 125 F

“An atmosphere of [CO2] would give to our earth a high temperature; and if as some suppose, at one period of its history the air had mixed with it a larger proportion than at present, an increased temperature from its own action as well as from increased weight must necessarily have resulted”

Well, yes, of course. What are the specific heat capacities of those gases (in the 275 – 300 K range)?

Hydrogen – 14,025 J/kg/K
Common Air – 1,006 J/kg/K (or possibly 1,014 J/kg/K)
Oxygen – 916 J/kg/K
CO2 – 832 J/kg/K

Rather unsurprisingly, her experiment shows that things which require less energy to warm up, warm up the most. As ever, ‘back radiation’ has nothing to do with it. We’d have to adjust this for the mass of the gas compared to the mass of the glass containers and the specific heat capacity of glass (assuming they warmed to the same temperature), but the overall picture is clear enough.

On the impact on temperature, she is quite correct in a roundabout way.

A lower specific heat capacity means a higher lapse rate. If our atmosphere were 100% CO2, the lapse rate would be approx. 2K/km higher (assuming relative humidity stays the same). The average temperature of the atmosphere can’t increase as it is dictated by solar radiation. The average temperature is found half way up (approx. 5 km), so sea level temperatures would increase by approx. 10 K, and the temperature at the top of “lofty mountains” above 5 km altitude would fall.

But we would all have suffocated long before then.

In real life, we know that CO2 concentrations are likely to rise from pre-industrial 280 ppm to 500 or 600 ppm over the century or two.

If anybody can be bothered to work out the new average specific heat capacity of air will be when CO2 is 0.05% or 0.06% instead 0.028%, and then work out the new lapse rate (making some heroic assumptions as to whether and how much relative humidity would increase and moderate this) and the resulting impact on sea-level temperatures, then knock yourself out. Most calculators won’t have enough decimal places to give a meaningful answer, and even if it does, the additional sea-level temperature will be within the margin of error of even the most accurate thermometers.