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12.842 / 12.301 Past and Present Climate
Fall 2008
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Climate Physics and Chemistry
Role of the Atmosphere in Climate
(Read Hartmann, Chapters 1 and 2)
Ways by which the atmosphere
influences climate:
• Strong effects on radiative transfer,
including filtering of ultraviolet radiation
• Large advective and convective heat
transfer
• Main driver of ocean circulation
• Important role in biogeochemical cycles
Atmospheric Composition
Gas Name
Nitrogen
Chemical Formula
N2
Percent Volume
78.08%
Oxygen
O2
20.95%
*Water
H2O
0 to 4%
Argon
Ar
0.93%
CO2
0.0360%
Neon
Ne
0.0018%
Helium
He
0.0005%
*Methane
CH4
0.00017%
Hydrogen
H2
0.00005%
N2O
0.00003%
O3
0.000004%
*Carbon Dioxide
*Nitrous Oxide
*Ozone
* variable gases
Image courtesy of NOAA.
Image courtesy of NOAA.
100000
Thermosphere
90000
Mesopause
80000
Altitude (meters)
70000
60000
Mesosphere
Stratopause
50000
40000
30000
Stratosphere
20000
Tropopause
10000
Troposphere
0
-100
-80
-60
-40
-20
Temperature oC
Figure by MIT OpenCourseWare.
0
20
Figures removed due to copyright restrictions.
See Figure 1.6 and Figure 1.7 in Hartmann, Dennis L. Global Physical Climatology. Reading, MA: Academic Press, p.411. ISBN: 0123285305.
Elements of Thermal Balance:
Solar Radiation
• Luminosity: 3.9 x 1026 J s-1 = 6.4 x 107 Wm-2
at top of photosphere
• Mean distance from earth: 1.5 x 1011 m
• Flux density at mean radius of earth
L
−2
0
S ≡
= 1370 Wm
0
2
4π d
Stefan-Boltzmann Equation: F =σ T 4
−8
−
2
−
4
σ =5.67×10 Wm K
Sun:
→
7
4
−
2
σ T =6.4×10 Wm
T 6,000 K
Disposition of Solar Radiation:


2
Total absorbed solar radiation = S 1 − a  π r
0
p p
a ≡ planetary albedo ( 30% )
p
Total surface area = 4π r 2
p
S
Absorption per unit area = 0 1 − a 
p
4 
Absorption by clouds, atmosphere, and surface
Terrestrial Radiation:
Effective emission temperature:
S
σ T 4 ≡ 0 1 − a 
e
4 
p
Earth: T = 255 K = −18°C
e
Observed average surface temperature = 288K = 15°C
Highly Reduced Model
• Transparent to solar radiation
• Opaque to infrared radiation
• Blackbody emission from surface and each layer
Radiative Equilibrium:
Top of Atmosphere:
S


4
4
0
1− a  = σT
σT =

A
p
e
4 
→
TA = Te
Surface:
S0
4
σ Ts = σ TA + (1 − a p ) = 2σ Te
4
4
4
1
→ Ts = 2 4 Te = 303 K
Surface temperature too large
because:
• Real atmosphere is not opaque
• Heat transported by convection as well as
by radiation
Figures removed due to copyright restrictions.
See Figures in Hartmann, Dennis L. Global Physical Climatology. Reading, MA: Academic Press, p.411. ISBN: 0123285305.
6
4
Ocean Transport
Atmosphere
Transport
Total Transport
petaWatts
2
0
-2
-4
-6
-90
-60
-30
0
30
60
Latitude
Heat Transport by Oceans and Atmosphere
Figure by MIT OpenCourseWare.
90