Water Vapor Feedback is defined as the increase in atmospheric greenhouse effect (also called Radiative Forcing) due to the increase in atmospheric water vapor caused by warming of the earth’s surface, primarily that of the oceans. The effect on average global temperature after doubling present (400 ppm) CO2 of increased water vapor is 0.22 C, making the total effect of doubling CO2 = 1.05 C.
- Compute ratio of δP/P/δT for water in the vicinity of 15C
- List water vapor pressure at temperatures around 15C (current earth surface temp.)
- Use average pressure (P) over temperature intervals.
- Reference: Keenan & Keyes, The Thermodynamic Properties of Steam, John Wiley & Sons, 1938, p. 28
- Data in above reference is in English Units (psia & deg F)
- Data used:
- 58F, 0.2386 psia
- 59F, 0.2473 psia
- 60F, 0.2563 psia
- 61F, 0.2655 psia
- for 59F, δP = 0.2563 – 0.2386 = 0.0177 psia, δT = 2F, (1/P)*δp/δT = 0.0395 F-1 = 0.0711 C-1
- for 60F, δP = 0.2655 – 0.2473 = 0.0182 psia, δT = 2F, (1/P)*δp/δT = 0.0358 F-1 = 0.0644 C-1
- Compute δT of earth’s surface by increasing CO2 using Method B, as follows:
- Using ModTran, compute earth’s heat radiating to space H1 for baseline conditions = 242.188 w/m2
- Similarly, compute H2 for doubling CO2 to 800 ppm = 239.739 w/m2
- Adjust, by trial & error, Ground Temperature Offset at 800 ppm to match H1: Offset T= 0.83 C
- Compute δP/P by the formula δP/P/δT * δT = 0.0711 C-1* 0.83 C = 0.059
- The increased water vapor ratio = 1 + 0.059 = 1.059. Enter this into “Water Vapor Scale” (was initially 1.0) and run Modtran at this H2O level and 800 ppm (H3): heat radiating to space = 239.08 w/m2
- Adjust, by trial & error, Ground Temperature Offset at this condition to match H1 (242.188 w/m2): Offset T = 1.05 C
The H2O Feedback for doubling CO2 to 800 ppm = 1.05 C – 0.83 C = 0.22 C.