Environmental Physics:Climate Dynamics aspects

Arnaud Czaja

Space & Atmospheric Physics

Overview (Lectures #7-14)

• Greenhouse effect (#7, i.e. today)• Water Vapour feedback (#8)• Cooling mechanisms for the planet (#9)• Climate models (#10) + MATLAB session• Climate timescales (#11)• Climate sensitivity (#12)• Modes of climate variability (#13)• Discussion of a hot Topic (#14)

Our own EP Climate Model!

Atmosphere

Tropics

ExtraTropicsOcean

OH 2AT

1ST 2ST

1OT 2OT

AH

OH

Our own EP Climate Model!

Atmosphere

Tropics

ExtraTropicsOcean

OH 2AT

1ST 2ST

1OT 2OT

AH

OH

(i) Use of model is not mandatory and thus themodel is not examinable

(ii) MATLAB session on Thursday Feb 7 (5-6pm)(come if you wish, this is not mandatory)

Lecture 7The Greenhouse effect

Lecture 7The Greenhouse effect

Student: Oh yeah! It’s the trapping of radiation by the

atmosphere

Lecture 7The Greenhouse effect

Lecturer: Let’s be a bit more precise!!

7.1 Motivation:Observations of Earth from

Space

Total radiation seen by a satellite

(GERB project in Space & Atmospheric Physics)

NB: Photon energy is the main form of energyexchangedbetween theearth and space

LW SW

Long wavelength photons (emitted

by the Earth)

Short wavelength photons (reflected

from the Sun)

+

Terrestrial radiation λ≈10-20μm

1 erg s-1 cm-2 = 10-3 W m-2

Sun radiation Solar radiation λ≈0.2-1μm

To zero order…

• Solar photons are absorbed by the atmosphere (clouds, water vapour, ozone)

• …but much less so than are the terrestrial photons.

• This is because of the atmospheric composition, with tri-atomic molecules efficient at interacting with λ≈10-20μm photons (“greenhouse absorbers”).

Atmospheric composition (fraction of total molecules)

Main Greenhouse gas (GHG) is Water vapour (variable ≈1%)

Man influenced GHG

Pre-industrial IPCC (2007)

CO2 280ppmv 380 ppmv

CH4 700ppbv 1800 ppbv

N2O 270ppbv 320 ppbv

NB: 1ppmv = 1 molecule out of a million 1ppbv = 1 molecule out of a billion

N2

O2

CO2

What is meant by variable H2O…

sate

eRH

Upper tropospheric RH on April 27, 1999Low level enthalpy

qLTCh vp

0 1002.5

3.5

7.3 Observations of the greenhouse effect (G)

I

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)

Imperial College London

28 May 200320

Solar spectr

um

Lean (1991)

[adapted by Lockwood]