Transcript Document

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):
h=6.62x10-34 Js
E  hf 
hc

,
Also often use wavenumbers notation:
c
f

c=3.0x108 m/s
1

EMISSION 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:
Here DF is the radiation flux emitted in [, +D]
is the flux distribution function characteristic of the object

Total radiation flux emitted by object:
F    d 
0
BLACKBODY RADIATION
Objects that absorb 100% of incoming radiation are called blackbodies
For blackbodies,  is given by the Planck function:
2
2
p
hc
b 
 hc 




5
 kT  
 e
 1




F  T 4
Function of T
only! Often
denoted B(,T)
  2p 5k 4/15c2h3 is the
Stefan-Boltzmann constant
max = hc/5kT
Wien’s law
max
KIRCHHOFF’S LAW:
Emissivity e(,T) = Absorptivity
For any object:
Illustrative example:
Kirchhoff’s law allows
determination of the
emission spectrum of
any object solely from
knowledge of its
absorption spectrum
and temperature
…very useful!
SOLAR RADIATION SPECTRUM: blackbody at 5800 K
GREENHOUSE EFFECT:
absorption of terrestrial radiation by the atmosphere
ABSORPTION OF RADIATION BY GAS MOLECULES
•
…requires quantum transition in internal energy of molecule.
•
THREE TYPES OF TRANSITION
– Electronic transition: UV radiation (<0.4 mm)
• Jump of electron from valence shell to higher-energy shell,
sometimes results in dissociation (example: O3+hn gO2+O)
– Vibrational transition: near-IR (0.7-10 mm)
• Increase in vibrational frequency of a given bond
requires change in dipole moment of molecule
– Rotational transition: far-IR (10-100 mm)
• 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 CO2
Δp  0
forbidden
Δp  0
allowed
Δp  0
allowed
Greenhouse gases = gases with vib-rot absorption features at 5-50 mm
• Major greenhouse gases: H2O, CO2, CH4, O3, N2O, CFCs,…
• Not greenhouse gases: N2, O2, Ar, …
EFFICIENCY OF GREENHOUSE GASES FOR GLOBAL WARMING
The efficient GGs are the ones that absorb in the “atmospheric window” (8-13
mm). Gases that absorb in the already-saturated regions of the spectrum are
not efficient GGs.
RADIATIVE EQUILIBRIUM FOR THE EARTH
Solar radiation flux intercepted by Earth = solar constant FS = 1370 W m-2
Radiative balance c
effective temperature of the Earth:
= 255 K
where A is the albedo (reflectivity) of the Earth
SIMPLE MODEL OF GREENHOUSE EFFECT
IR
VISIBLE
Incoming
solar
FS / 4
FS / 4
Reflected
solar
FS A / 4
Energy balance equations:
• Earth system
FS (1  A) / 4  (1  f ) To4 + f  T14
Transmitted
surface
• Atmospheric layer
(1  f ) To4
f  To4  2 f  T14
Solution: 
f  T14
Atmospheric
emission
f  T14
Atmospheric
emission
1
4
 To=288 K
 F (1  A)  e f=0.77
To   S

f
4(1

)


 T1 = 241 K
2 

Atmospheric layer (T1)
abs. eff. 0 for solar (VIS)
f for terr. (near-IR)
Surface emission
FS A / 4
 To4
Earth surface (To)
Absorption efficiency 1-A in VISIBLE
1 in IR
EQUILIBRIUM RADIATIVE BUDGET FOR THE EARTH
Kevin Trenberth, BAMS, 2009
The ultimate models
for climate research
TERRESTRIAL RADIATION SPECTRUM FROM SPACE:
composite of blackbody radiation spectra emitted from different altitudes
at different temperatures
HOW DOES ADDITION OF A GREENHOUSE GAS WARM THE EARTH?
Example of a GG absorbing at 11 mm
1.
1. Initial state
2.
2. Add to atmosphere a GG
absorbing at 11 mm;
emission at 11 mm
decreases (we don’t see
the surface anymore at
that , but the atmosphere)
3.
3. At new steady state, total
emission integrated over all ’s
must be conserved
e Emission at other ’s must
increase
e The Earth must heat!
RADIATIVE FORCING OF CLIMATE DF
Reflected solar
FSA/4
atmospheric
emission
f T14
Flux
out
Flux in
solar radiation
FS/4
surface
emission
(1-f) To4
greenhouse layer
(H2O, clouds, CO2, CH4, …)
Efficiency f
• Radiative equilibrium: DF = (Flux in) – (Flux out) = 0
• Increase greenhouse efficiency f e Flux out decreases e DF > 0; WARMING
• Increase solar reflection e Flux in decreases e DF < 0; COOLING
• Radiative forcing DF predicts equilibrium surface temperature response DTo :
DTo =  DF. In our 1-layer model,   [4(1f/2)T3o]-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, RESPONSE
Positive feedback from water vapor causes rough doubling of 
IPCC [2007]
GLOBAL WARMING POTENTIAL (GWP):
foundation for climate policy
• The GWP measures the integrated radiative forcing over a time
horizon Dt from the injection of 1 kg of a species X at time to,
to +Dt
relative to CO2:
GWP 

to
to +Dt

DF1 kg X dt
DF1 kg CO2 dt
to
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