Environmental Physics
Office Hours (Arnaud Czaja)
Thursdays, 13.00-14.00(Huxley Room 726)
Lecture 8Water vapour feedback
8.1 Motivation:Water vapour and the
greenhouse effect
“Top ofAtmosphere” (TOA)
Surface
A simple model of the greenhouse effect(lecture 7)
Radiation of surface
Atmospheric layer(absorbs all radiationfrom surface and re-emitsat its own temperature)
Solar input
“Top ofAtmosphere” (TOA)
Surface
A simple model of the greenhouse effect(lecture 7)
Radiation of surface
Atmospheric layer(absorbs all radiationfrom surface and re-emitsat its own temperature)
Solar input
A measure of the greenhouse effect:
G = upward radiation
of surface minus
upward Radiation at TOA
I
=upward (longwave)radiation at TOA
Observed Greenhouse effect24
WminITG clears
Water vapour content2
2
kgminM OH
Sea surface temperature
KinSST
180
50
50
10
303
273
NB: All are annual mean data (from Held & Soden, 2000)
Pierrehumbert et al., 2006, AGU Monograph.
Trapping of radiation by CO2 (330ppmv) only
Trapping of radiation by CO2 + observedwater vapour
No atmospheric absorbers
Upward radiation at TOA from full radiative calculation (January)
Trapping of radiation by CO2 + water vapourat saturation
Pierrehumbert et al., 2006, AGU Monograph.
Trapping of radiation by CO2 (330ppmv) only
Trapping of radiation by CO2 + observedwater vapour
No atmospheric absorbers
Upward radiation at TOA from full radiative calculation (January)
Trapping of radiation by CO2 + water vapourat saturation
Water vapour vs. CO2:
-more abundant (amount of CO2 in atmosphereis 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 notbe considered externally prescribed as is CO2to a certain extent)
--------> FEEDBACK!!
What is meant by a function of climate:
sate
eRH
Upper tropospheric RH on April 27, 1999Low level enthalpy
qLTCh vp
0 1002.5
3.5
What is meant by a function of climate:
sate
eRH
Upper tropospheric RH on April 27, 1999Low level enthalpy
qLTCh vp
0 1002.5
3.5Some control though….Clausius-Clapeyron relation(2nd law of thermodynamics)
Dependence of globally averaged water vapour upon surface temperature (IPCC-AR4 models)
Held and Soden (2007)
Δe
Δe
ΔTs
ΔTsClausius-Clapeyron
scaling (7% / K)satisfied!
SRES A1B
20th century(control)
0 1K
0 4K