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