1. Environmental Physics Office Hours ( Arnaud Czaja) Thursdays, 13.00-14.00 (Huxley Room 726)

2. Lecture 8 Water vapour feedback

3. 8.1 Motivation: Water vapour and the greenhouse effect

4. “Top of Atmosphere” (TOA) Surface A simple model of the greenhouse effect (lecture 7) Radiation of surface Atmospheric layer (absorbs all radiation from surface and re-emits at its own temperature) Solar input

5. “Top of Atmosphere” (TOA) Surface A simple model of the greenhouse effect (lecture 7) Radiation of surface Atmospheric layer (absorbs all radiation from surface and re-emits at its own temperature) Solar input A measure of the greenhouse effect: G = upward radiation of surface minus upward Radiation at TOA

6. =upward (longwave) radiation at TOA

7. Observed Greenhouse effect Water vapour content Sea surface temperature 180 50 10 303 273 NB: All are annual mean data (from Held & Soden, 2000)

8. Pierrehumbert et al., 2006, AGU Monograph. Trapping of radiation by CO2 (330ppmv) only Trapping of radiation by CO2 + observed water vapour No atmospheric absorbers Upward radiation at TOA from full radiative calculation (January) Trapping of radiation by CO2 + water vapour at saturation

9. Pierrehumbert et al., 2006, AGU Monograph. Trapping of radiation by CO2 (330ppmv) only Trapping of radiation by CO2 + observed water vapour No atmospheric absorbers Upward radiation at TOA from full radiative calculation (January) Trapping of radiation by CO2 + water vapour at saturation Water vapour vs. CO2: -more abundant (amount of CO2 in atmosphere is only a few percent of that of water vapour) -larger impact on greenhouse effect (as a result of being more abundant) -a strong function of climate (and thus can not be considered externally prescribed as is CO2 to a certain extent) --------> FEEDBACK!!

10. What is meant by a function of climate: Upper tropospheric RH on April 27, 1999Low level enthalpy 0100 2.5 3.5

11. What is meant by a function of climate: Upper tropospheric RH on April 27, 1999Low level enthalpy 0100 2.5 3.5 Some control though…. Clausius-Clapeyron relation (2 nd law of thermodynamics)

12. Dependence of globally averaged water vapour upon surface temperature (IPCC-AR4 models) Held and Soden (2007) ΔeΔe ΔeΔe ΔTs Clausius-Clapeyron scaling (7% / K) satisfied! SRES A1B 20 th century (control) 0 1K 0 4K