More CO2: Promoted From Comments
Lumo writes (in the comments):
Dear CIP, I said CO2 was irrelevant for a particular temperature graph we were discussing - and the word "irrelevant" has an effective meaning: the CO2's effect is undetectable with any statistical significance. Believe me that I do know the greenhouse effect, I believe it, understand it, and have been lecturing on it.
OK, my bad - I misunderstood your remark, but whether or not CO2 is relevant to the increased temperatures in middle England is not a settled question. You continued:
Venus' CO2 partial pressure in the atmosphere is something like 300 000 times higher than it is on the Earth: the total pressure is 100 times higher than here, and 96% instead of 0.038% of the atmosphere is CO2. Be sure you can't get there to the Venus-like mass of CO2 in the atmosphere by burning fossil fuels - not even all fossil fuels that exist anywhere on Earth (insufficient by many orders of magnitude). The only way you get there is for you to be a young Earth creationist and pray to God to multiply your CO2 by one million - and you must know very well that at the level of science, all the miraculous effects attributed to the climatically irrelevant gas are just religious fantasies.
These is not enough fossil organic carbon for a Venusian atmosphere, of course, but even a 20K temperature increase would be pretty catastrophic. Once water can no longer condense (as on Venus), geological processes gradually dump the inorganic carbon into the atmosphere. There is a whole lot of that.
By the way, CO2 greenhouse effect adds something like 400 deg Celsius on Venus. If the impact were linear, it would be 400/300,000 = 1 millidegree on the Earth. The fact that it may be about 2 degree Celsius on the Earth (the CO2 greenhouse effect from all natural and man-added CO2) shows that the dependence is sublinear - closer to logarithmic, although the simple log dependence breaks down at very high concentrations like those on Venus.
But it's not a linear effect, as you note. It's approximately logarithmic, and Venus has about 2^18 times as much CO2 as Earth. If the effect were truly logarithmic, and Venus's greenhouse were due purely to CO2, that would imply a climate sensitivity of approximate 390/18 = 20 K per doubling - many time the actual sensitivity of 2-5 K.
If the Earth's CO2 were it's only greenhouse gas, the CO2 greenhouse effect would be about 10 K. Because there are other greenhouse gases, much of the CO2 absorption is redundant, so that the net additional warming due to CO2 is only about 1/3 as much - say 3 K. This fact is important, since that 10 K worth of CO2 stays in the atmosphere even if the amount of water vapor (the really big GHG) decreases. Otherwise, the Earth's climate would be less stable, since cooling decreases the H2O vapor (and the methane too) which produces further cooling, and so on.