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

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Environmental PhysicsOffice Hours (Arnaud
Czaja)Thursdays, 13.00-14.00(Huxley Room 726)
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Lecture 8Water vapour feedback
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8.1 MotivationWater vapour and the greenhouse
effect
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A simple model of the greenhouse effect (lecture
7)
Top of
Atmosphere (TOA)
Atmospheric layer (absorbs all radiation from
surface and re-emits at its own temperature)
Surface
Radiation of surface
Solar input
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A simple model of the greenhouse effect (lecture
7)
Top of
Atmosphere (TOA)
A measure of the greenhouse effect G upward
radiation of surface minus upward Radiation at
TOA
Atmospheric layer (absorbs all radiation from
surface and re-emits at its own temperature)
Surface
Radiation of surface
Solar input
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upward (longwave) radiation at TOA
7
180
Observed Greenhouse effect
50
50
Water vapour content
10
303
Sea surface temperature
273
NB All are annual mean data (from Held Soden,
2000)
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Upward radiation at TOA from full radiative
calculation (January)
No atmospheric absorbers
Trapping of radiation by CO2 (330ppmv) only
Trapping of radiation by CO2 observed water
vapour
Trapping of radiation by CO2 water vapour at
saturation
Pierrehumbert et al., 2006, AGU Monograph.
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Upward radiation at TOA from full radiative
calculation (January)
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) --------gt FEEDBACK!!
No atmospheric absorbers
Trapping of radiation by CO2 (330ppmv) only
Trapping of radiation by CO2 observed water
vapour
Trapping of radiation by CO2 water vapour at
saturation
Pierrehumbert et al., 2006, AGU Monograph.
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What is meant by a function of climate
Upper tropospheric RH on April 27, 1999
Low level enthalpy
3.5
0
100
2.5
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What is meant by a function of climate
Upper tropospheric RH on April 27, 1999
Low level enthalpy
Some control though. Clausius-Clapeyron
relation (2nd law of thermodynamics)
3.5
0
100
2.5
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Dependence of globally averaged water vapour upon
surface temperature (IPCC-AR4 models)
SRES A1B
?e
Clausius-Clapeyron scaling (7 / K) satisfied!
?Ts
0
4K
20th century (control)
?e
?Ts
1K
0
Held and Soden (2007)