ingliz wrote:@Truth To Power
What keeps the Earth warm enough to be habitable, if not CO2?
The sun, obviously.
But there are actually three main atmospheric factors that keep the earth warmer than the moon, which is -- again, obviously -- at the same distance from the sun as the earth:
1. Most importantly, the density of the earth's atmosphere makes the surface much warmer than the temperature of the upper troposphere/lower stratosphere where outward infrared radiation balances the incident solar energy. Atmospheric density is why the surface of Venus is hotter than Mercury despite being a lot farther from the sun and having a much higher albedo, and the surface of Mars is much colder than the surface of the earth, despite the fact that Mars has more CO2 in its atmosphere and a lower albedo.
2. Next most important, the water vapor in the earth's atmosphere absorbs infrared radiation from the surface, acting as an insulating blanket ("greenhouse" gas is a misnomer, as greenhouses work by blocking loss of solar heat by convection, not radiation). The magnitude of this effect is determined by the infrared absorption properties of the water molecules and the number of molecules present in the atmosphere, not the mass
of gas, which is how concentration is measured. Water vapor is a much more effective infrared absorber than CO2, and the total number of water molecules in the atmosphere is about eight times the number of CO2 molecules, so water vapor is a far more important "greenhouse" gas than CO2. Moreover, water vapor is much more plentiful near the earth's surface and at the lower latitudes that get more sun, because it is warmer; so in terms of actually warming the earth's surface, water vapor is probably about 100 times as important as CO2.
3. Third in importance is CO2, but its effect is theoretically greatest at very low concentrations -- much lower than have ever actually occurred on earth. The lowest concentration of CO2 is believed to have been ~180ppm during the depths of the Ice Ages, which occurred because sea water dissolves more CO2 at lower temperatures -- and the oceans hold about 50x as much CO2 as the atmosphere. We are now at ~420ppm, and in the distant past the concentration was far higher: ~4,000ppm in the Cambrian Era, and as much as 300,000ppm before the emergence of photosynthetic plants. However, if there were no
CO2 in the atmosphere, the earth would enter a positive ice-albedo feedback loop, and probably freeze completely. Some people claim this theoretical
possibility shows CO2 is the most important greenhouse gas; but even a very small amount of CO2 -- maybe 1ppm -- would be enough to reverse the feedback loop, releasing enough water vapor to melt some ice, release more water vapor, etc., and once the initial positive water vapor feedback is established, additional CO2 is almost irrelevant.
You may now consider yourself at least minimally informed on the subject.