Water vapor is the most powerful greenhouse gas
The skeptic argument...Water vapour is the most important greenhouse gas. If you get a fall evening and the sky is clear, heat will escape, the temperature will drop and you get frost. If there's cloud cover, the heat is trapped by water vapour as a greenhouse gas and the temperature stays warm. If you go to In Salah in southern Algeria, they recorded at noon 52°C. By midnight, it's -3.6°C. That’s a 56°C drop in temperature in 12 hours. It's caused because there is very little water vapour in the atmosphere and is a demonstration of water vapour as the most important greenhouse gas (source: Interview with Tim Ball).
What the science says...
Water vapour is indeed the most dominant greenhouse gas. The radiative forcing for water is around 75 W/m2 while carbon dioxide contributes 32 W/m2 (Kiehl 1997). Water vapour is also the dominant positive feedback in our climate system and a major reason why temperature is so sensitive to changes in CO2.
Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature. Water vapour is brought into the atmosphere via evaporation - the rate depends on the ocean and air temperature and is governed by the Clausius-Clapeyron relation.
If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two. Similarly, if somehow moisture was sucked out of the atmosphere, evaporation would restore water vapour levels to 'normal levels' in short time.
Water Vapour as a positive feedback
As water vapour is directly related to temperature, it's also a positive feedback - in fact, the largest positive feedback in the climate system (Soden 2005). As temperature rises, evaporation increases and more water vapour accumulates in the atmosphere. As a greenhouse gas, the water absorbs more heat, further warming the air and causing more evaporation.
How does water vapour fit in with CO2 emissions? When CO2 is added to the atmosphere, as a greenhouse gas it has a warming effect. This causes more water to evaporate and warm the air more to a higher (more or less) stabilized level. So CO2 warming has an amplified effect, beyond a purely CO2 effect.
How much does water vapour amplify CO2 warming? Without any feedbacks, a doubling of CO2 would warm the globe around 1°C. Taken on its own, water vapour feedback roughly doubles the amount of CO2 warming. When other feedbacks are included (eg - loss of albedo due to melting ice), the total warming from a doubling of CO2 is around 3°C (Held 2000).Water vapor is the most powerful greenhouse gas[/QUOTE
End the part from Rocks. Start the part from Code.
First off, Thank you for that.
According to this, a doubling of CO2 increases the Temp by 1 degree. So when we get to 560 PPM (280 X 2) we can expect to see a CO2 driven rise of 1 degree. This will triple, according to the above, to a 3 degree rise from other forcers. Following that, a second doubling will have the same effective rise and a successive doubling will do so again, but a different species will have risen to track this since we cannot breath that mix of air.
Dinosaurs make a come back.
The moral of the story is that at 560 ppm, we get a three degree rise. At that level, it is doubtful that any more albedo can be found so the max increase according to the formula is two degrees for the next doubling. This means a max increase due to GHG's of 5 degrees when we hit 1120 ppm.
MIT thinks that we will hit the 5 degree rise by 2100. Interesting. I wonder if candle lit dinners will still be possible. Forest fires may be at an end with all that CO2 in the air.
