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CHAPTER 7: THE GREENHOUSE EFFECT. MILLENIAL NH TEMPERATURE TREND [IPCC, 2001]. GLOBAL CLIMATE CHANGE SINCE 1850 [IPCC, 2007]. NOAA GREENHOUSE GAS RECORDS. RADIATION & FUNDAMENTAL RELATIONSHIPS. Electromagnetic energy at wavelength ( ) has associated frequency (f) and photon energy (E): - PowerPoint PPT Presentation
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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
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
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
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