1. Atul Kapur akapur@rsmas.miami.edu from a paper by S. Manabe and Richard T. Wetherald 1966

2. No atmosphere Fixed emissivity and absorptivity at a given layer But in reality Absolute Humidity is a strong function of temperature? Fixed distribution of absorbers including water vapor (or Absolute humidity) (Manabe and Srickler, 1964)

3. Why fixed Relative Humidity? DJF JJA Zonal-mean Absolute humidity (g kg -1 ) DJF JJA Zonal-mean Relative humidity (%) (Peixoto and Oort, 1992) Strong seasonal variation in Absolute Humidity Weak seasonal variation in Relative Humidity

4. No atmosphere Fixed emissivity and absorptivity at each layer Fixed distribution of absorbers including water vapor (or Absolute humidity) But Absolute Humidity is a strong function of temperature? (Manabe and Srickler, 1964) Fixed distribution of Relative humidity (Telegadas and London, 1954) (Manabe and Wetherald, 1967)

5.  Mixing ratio is now allowed to change with change in temperature  Another degree of freedom PARTIALLY released  Constrained by the condition of fixed Relative Humidity

6.  Mixing ratio of CO 2 is assumed to be constant (300 ppm by volume) (Herring and Borden, 1965) Ozone (London, 1956) Cloud characteristics

7. Radiative (Fixed abs. humidity) Radiative (Fixed relative humidity) Radiative- Convective (Fixed relative humidity) Cooler Atmospheric Temperature + Fixed relative humidity Less moisture Less greenhouse effect Further temperature drop at the surface (Hergesell, 1919) Self Amplification effect

8. Increase in moisture in a given volume of air Warmer Atmospheric Temperature + Fixed relative humidity OLR Increase in height of effective source of OLR OLR lesser (than in case of fixed absolute humidity) Slower approach towards equilibrium Lesser Radiative cooling Increase in latent energy of air Increase in effective heat capacity Slower Approach towards equilibrium Approach of mean temperature towards equilibrium Fixed abs. humidity (I) Fixed RH with C p dry (II) Fixed RH with effective C p (III)

9. OLR Higher value of Solar constant + Fixed relative humidity Increase in temp. + But OLR less than expected Further inc. in temp. to increase OLR at top of atmosphere Higher sensitivity of temperature upon Solar constant

10. Fixed abs. humidity Fixed RH Solar constant Surf Temp.  For fixed RH, equilibrium temp. is almost twice as sensitive as for fixed absolute humidity  Difference in sensitivity decreases with temp. Mixing ratio of water vapor is lower at lower temperatures Self-amplification effect

11.  More CO 2 results in warmer troposphere and warmer surface  More CO 2 results in colder stratosphere  Stratospheric temp. much more sensitive to CO 2 than troposphere. Inc. in CO 2 + Fixed relative humidity Increase in temp. + OLR less than expected Increased sensitivity to CO 2 Sensitivity of temperature upon CO 2 almost double as compared to fixed absolute humidity

12.  Larger albedo colder the temperature  Influence of surface albedo vanishes with height Sensitivity upon surface albedo almost double as compared to fixed absolute humidity

13.  Time required for radiation-condensation relaxation is almost double than that required for radiation relaxation.  For fixed RH, sensitivity of surface temp. upon solar constant, cloudiness, surface albedo, and CO2 content is almost twice as compared to that for fixed absolute humidity  Doubling of CO2 with fixed relative humidity increases surface temp. by about 2.3°C.