CHAPTER 7: THE GREENHOUSE EFFECTCHAPTER 7: THE GREENHOUSE EFFECT

MILLENIAL NH TEMPERATURE TREND [IPCC, 2001]MILLENIAL NH TEMPERATURE TREND [IPCC, 2001]

GLOBAL CLIMATE CHANGE SINCE 1850 [IPCC, 2007]GLOBAL CLIMATE CHANGE SINCE 1850 [IPCC, 2007]

NOAA GREENHOUSE GAS RECORDSNOAA GREENHOUSE GAS RECORDS

RADIATION & FUNDAMENTAL RELATIONSHIPSRADIATION & FUNDAMENTAL RELATIONSHIPS

Electromagnetic energy at wavelength () has associated frequency (f) and photon energy (E):

Also often use wavenumbers notation:

,hc c

E hff

h=6.62x10-34 Jsc=3.0x108 m/s

1

Here is the radiation flux emitted in [

is the flux distribution function characteristic of the object

Total radiation flux emitted by object:

EMISSION OF RADIATIONEMISSION OF RADIATION

Radiation is energy transmitted by electromagnetic waves; all objects emit radiation

One can measure the radiation flux spectrum emitted by a unit surface area of object:

0

d

BLACKBODY RADIATIONBLACKBODY RADIATION

Objects that absorb 100% of incoming radiation are called blackbodies

For blackbodies, is given by the Planck function:

k 4/15c2h3 is theStefan-Boltzmann constant

max = hc/5kT Wien’s law

Function of Tonly! Often denoted B(T)

max

2

5

4

2

e 1

b

hc

kT

hc

T

KIRCHHOFF’S LAW: KIRCHHOFF’S LAW: Emissivity Emissivity TT) = Absorptivity) = Absorptivity

For any object: …very useful!

Illustrative example:

Kirchhoff’s law allowsdetermination of the emission spectrum of any object solely from knowledge of its absorption spectrum and temperature

SOLAR RADIATION SPECTRUM: blackbody at 5800 KSOLAR RADIATION SPECTRUM: blackbody at 5800 K

GREENHOUSE EFFECT:GREENHOUSE EFFECT:absorption of terrestrial radiation by the atmosphereabsorption of terrestrial radiation by the atmosphere

ABSORPTION OF RADIATION BY GAS MOLECULESABSORPTION OF RADIATION BY GAS MOLECULES

• …requires quantum transition in internal energy of molecule.

• THREE TYPES OF TRANSITION

– Electronic transition: UV radiation (<0.4 m)

• Jump of electron from valence shell to higher-energy shell, sometimes results in dissociation (example: O3+hO2+O)

– Vibrational transition: near-IR (0.7-10 m)

• Increase in vibrational frequency of a given bond

requires change in dipole moment of molecule

– Rotational transition: far-IR (10-100 m)

• Increase in angular momentum around rotation axis

THE GREENHOUSE EFFECT INVOLVES ABSORPTION OF NEAR-IR TERRESTRIAL RADIATION BY MOLECULES UNDERGOING VIBRATIONAL AND VIBRATIONAL-ROTATIONAL TRANSITIONS

NORMAL VIBRATIONAL MODES OF CONORMAL VIBRATIONAL MODES OF CO22

forbidden

allowed

allowed

Δp 0

Δp 0

Δp 0

Greenhouse gases = gases with vib-rot absorption features at 5-50 m

• Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,…

• Not greenhouse gases: N2, O2, Ar, …

EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMINGEFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING

The efficient GGs are the ones that absorb in the “atmospheric window” (8-13 m). Gases that absorb in the already-saturated regions of the spectrum are not efficient GGs.

RADIATIVE EQUILIBRIUM FOR THE EARTHRADIATIVE EQUILIBRIUM FOR THE EARTH

Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2

Radiative balance effective temperature of the Earth:

= 255 K

where A is the albedo (reflectivity) of the Earth

SIMPLE MODEL OF GREENHOUSE EFFECTSIMPLE MODEL OF GREENHOUSE EFFECT

Earth surface (To) Absorption efficiency 1-A in VISIBLE 1 in IR

Atmospheric layer (T1)abs. eff. 0 for solar (VIS) f for terr. (near-IR)

/ 4SF

Incoming solar

/ 4SF

Reflectedsolar

/ 4SF A

/ 4SF A4oT

Surface emission

4(1 ) of T

Transmittedsurface

41f T4

1f T

Atmosphericemission

Atmosphericemission

Energy balance equations:• Earth system

4 41(1 ) / 4 (1 )S oF A f T f T

• Atmospheric layer4 4

12of T f T

Solution:1

4

(1 )

4(1 )2

So

F AT

f

To=288 K f=0.77T1 = 241 K

VISIBLE IR

EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTHEQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH

Kevin Trenberth, BAMS, 2009

The ultimate models for climate research

TERRESTRIAL RADIATION SPECTRUM FROM SPACE:TERRESTRIAL RADIATION SPECTRUM FROM SPACE:composite of blackbody radiation spectra emitted from different altitudes composite of blackbody radiation spectra emitted from different altitudes

at different temperaturesat different temperatures

HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?

1.1. Initial state

2. 2. Add to atmosphere a GG absorbing at 11 m; emission at 11 m decreases (we don’t see the surface anymore at that but the atmosphere)

3. At new steady state, total emission integrated over all ’s must be conserved Emission at other ’s must increase The Earth must heat!

3.

Example of a GG absorbing at 11 m

RADIATIVE FORCING OF CLIMATE RADIATIVE FORCING OF CLIMATE F

solar radiationFS/4

Reflected solarFSA/4

surfaceemission(1-f)To

4

atmosphericemissionfT1

4

greenhouse layer(H2O, clouds, CO2, CH4, …)Efficiency f

Flux inFlux out

• Radiative equilibrium: F = (Flux in) – (Flux out) = 0• Increase greenhouse efficiency f Flux out decreases F > 0; WARMING• Increase solar reflection Flux in decreases F < 0; COOLING• Radiative forcing F predicts equilibrium surface temperature response To : To = F. In our 1-layer model, f/2T3

o]-1 = 0.3 K m2 W-1;in research climate models, ranges from 0.3 to 1.4 K m2 W-1 depending on model

CLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSECLIMATE CHANGE FORCINGS, FEEDBACKS, RESPONSE

Positive feedback from water vapor causes rough doubling of

IPCC [2007]

GLOBAL WARMING POTENTIAL (GWP):GLOBAL WARMING POTENTIAL (GWP):foundation for climate policyfoundation for climate policy

• The GWP measures the integrated radiative forcing over a time horizon t from the injection of 1 kg of a species X at time to, relative to CO2:

2

1 kg X

1 kg CO

GWP

o

o

o

o

t t

t

t t

t

F dt

F dt

Gas Lifetime

(years)

GWP for time horizon

20 years 100 years 500 years

CO2 ~100 1 1 1

CH4 12 63 23 7

N2O 114 279 300 158

CFC-12 (CF2Cl2) 100 10340 10720 5230

HFC-134a (CH2FCF3) 14 3580 1400 4

SF6 3200 15290 22450 32780