If we accept that C02 causes some warming and also that the climate sensitivity is low (< 1), then this implies that increasing C02 levels will result in negative feedbacks, even if the temperature rise is very small. He wanted Richard Lindzen to tell him how this would impact the climate.
I’m a bit out of my comfort zone here, but I think it means that the hydrological cycle speeds up somewhat, resulting in a little more cloud and rain. Is this correct? If so, do we know how much?
Everybody agrees that the additional forcing for a doubling of CO2 would be about 3.8 additional watts. That would result in roughly an increase of 1.2 C. Let the feedback factor be f.
Then the ACTUAL forcing would be
1.2C/(1-f). If f is close to 1, you get infinity. Actual systems close to this model are hydrogen bombs, dynamite explosions, etc. Catastrophic AGWers believe the temperature increase would be about 3 C. That would make f= 0.6, caused by positive feedbacks from melting glaciers, increased water vapor in the atmosphere, etc. 1.2C/(1-0.6) = 3C If f is close to 0, you get 1.2 C for a doubling of CO2.
If there is negative feedback, say f = - 0.5, the temperature increase would be 1.2C/ (1 - (-0.5)) = 1.2C/(1 + 0.5) = 0.8 C
Over short temperature ranges, as at the end of the last glaciation 10,000 years ago, there can be strong positive feedbacks: melting glaciers from warmer temperatures decrease earth's albedo, leading to further warming, etc. Likewise falling snow due to cold weather increases earth's albedo, leading to further cooling and more snow, etc.
Over the course of the 4 billion years life has existed on earth, there must be strong negative feedbacks over greater temperature ranges, else the earth's oceans would have frozen solid or boiled away long ago.
In this paper, Hsien-Wang Ou argues that the effect of clouds must have acted as a strong stabilizer on earth's climate.
Thanks for your reply and for the references. Scrolling through your link to the Reference Frame, I noticed that Lubos' also has a comment about the L&D debate, which I entirely agree with - I thought the debate was a real breath of fresh air. Regards the Hsien-Wang Ou paper, I wasn't aware of this one, thank you for the tip. From the abstract, I think it is arguing that the oceans and clouds act as a thermostat to maintain temperature within the observed range. If so, this partially answers my question, but not entirely.
Let me reword. First up, I do accept the arguments for a likely low sensitivity, and understand that negative feedbacks will act to stabilise temperature increases. I think Hadi's point was that physical effects in the atmosphere cause the negative feedbacks. However neither he nor Richard Lindzen had time in the debate to explain what those physical effects were. My understanding was that he meant changes to cloud cover and rain fall. I wanted to know whether my understanding was correct, and if so what is the likely magnitude of such changes. Any more thoughts?
The forcing for water vapor is supposed to be about 15 watts for a doubling.
The increase in temperature from from a doubling of CO2, without feedback, is acknowledged by everyone to be about 3.8 watts/m^2, which would result in an increase of around 1C. I've seen actual estimates ranging from 0.7 C to 1.2 C. With a 1C increase, the saturation level of water vapor would increase 7%. That 7% increase implies a [(ln 1.07)/(ln 2)] * 15 watts = 0.0677/0.6931 = 1.47 watts/m^2.
So an initial 3.8 watt CO2 increase results in a Water Vapor multiplier of (3.8 + 1.47)/ 3.8 = 1.39. The final effective warming due to the multiplier effect would be 1/(1-.39) = 1/.61 = 1.64 1.64 * 3.8 = 6.23 watts. (390/383.77)^0.25 = 1.00403*287 = 288.157 (360/353.77)^0.25 = 1.00437*287 = 288.254
That's if there was NO increase in precipitation, NO change in convection, No change in clouds.
Trenbeth's figures give about 390 watts in heating the surface directly, 22 watts convection, and 78 watts in latent heat, somewhat higher than my computed estimate of 71.6 watts/m^2. Climate models predict an increase in precipitation less than the increase in humidity, around 3% rather than the full 7%. Multiplying my 71.6 watts by that 1.03 increse in precipitation gives 73.75, for an increase in watts of 2.1 in latent heat of vaporization. Half of that would be radiated to space, half back to earth, so the net increase would be 1.05 watt increase. Using Trenbeth's 78 watts gives a 2.34/2 = 1.17 watt increase. The net increase in SURFACE flux with a doubling of CO2 and water vapor feedback would be 6.23 -1.17= 5.06 watts. If there was a 3% increase in clouds in addition to 3% increased precipitiation, (And I think there's GOT to be an increase in clouds....... I.... wanna know have you ever seen the rain comin' down on a sunny day?) that same 3% increase would increse earth's albedo from about 0.3 to 1.03*0.3 = .309. The net wattage hitting earth's surface from the sun would drop from about 235 watts to 0.691/0.7 or 0.987*235. Multiply the wattage increase from the CO2 and water
vapor positive feedback by the cloud negative feedback and you get (240.06/235)*.987 = 1.008252 or a net increase of 1.94 watts, and a temperature increase to (391.94/390)^ 0.25 = 1.00124 * 288 = 288.357
Note that John Christy reported on an acutal experiment in increasing water vapor, due to irrigation of the San Joaquin Valley.
Alan: According to Trenberth it is a tragedy that reality does not match theory so we need to find out what is wrong with reality. One group wants to adjust their theory to fit reality and the other seems to want to adjust reality to fit their theory. This is why I read what Christy says and tend to ignore the advocates. I also follow Spencer's work.