Water Vapor Feedback.
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[W]ater vapor, confessedly the greatest thermal absorbent in the atmosphere, is dependent on temperature for its amount, and if another agent, as CO2, not so dependent, raises the temperature of the surface, it calls into function a certain amount of water vapor which further absorbs heat, raises the temperature and calls forth more vapor ...
TC Chamberlin (1905)
as quoted in Held and Soden (2000)
Held and Soden (2000), Ann. Rev. Energy and Environment.
IPCC (2001) Third Assessment Report, Chapter 7.
Wentz and Schnabel (2001), Nature
Soden et al. (2002), Science.
Santer et al. (2005), Science.
Soden et al. (2005), Science.
Soden and Held (2006), J. Climate.
Bony et al. (2006), J. Climate.
Primer in realclimate.org writen by Brian Soden in June 2006.
dX/dt = Q + F(X)
X = state of climate system (temperature, etc)
Q = Forcing, independent of X.
examples: insolation, CO2, etc
F = Feedbacks, dependent on X.
examples: water vapor, clouds, ocean circulation, etc.
Line between feedback and forcing can be unclear: e.g. methane in atmosphere…
S = Te4
S is incoming solar radiation
Te = effective emission temperature.
Surface T ~ 290K, Te ~ 255K.
Te = T-Ze
Implies emission coming from ~5km up.
∂T/ ∂(logCO2)= o ~ 1°C
∂T/∂(logCO2) = o·(1-H20)-1
H20 provides measure of water vapor feedback. Models indicate ~0.4 for fixed relative humidity.
So: ∂T/∂(logCO2) ~ 1.7°C
If H20 > 1 runaway warming (cooling).
Largest greenhouse effect
<- Most water vapor
<- Warmest SSTs
Wentz and Schabel (2001)
Mt. Pinatubo errupted in 1991, cooling Earth from aerosols.
Can a model represent the satellite-observed moisture changes?
From Soden et al (2002, Science)
All linked to some degree with clouds processes.
From IPCC Reports:
Current observational estimates
Current model estimates
(open histogram - 82 models)
(shade - Normal fit)
From Bony et al (2006), from Forster and Collins (2004)