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12.842 / 12.301 Past and Present ClimateFall 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

0.000004%O3*Ozone

0.00003%N2O*Nitrous Oxide

0.00005%H2Hydrogen

0.00017%CH4*Methane

0.0005%HeHelium

0.0018%NeNeon

0.0360%CO2*Carbon Dioxide

0.93%ArArgon

0 to 4%H2O*Water

20.95%O2Oxygen

78.08%N2Nitrogen

Percent VolumeChemical FormulaGas Name

Atmospheric Composition

* variable gases

Image courtesy of NOAA.

Image courtesy of NOAA.

0-100

Troposphere

Tropopause

Stratosphere

StratopauseMesosphere

Mesopause

Thermosphere

-80 -60 -40 -20 0 20

10000

20000

30000

40000

50000

60000

Alti

tude

(met

ers)

Temperature oC

70000

80000

90000

100000

Figure by MIT OpenCourseWare.

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

L0 −2S W≡ =1370 m0 24πd

Stefan-Boltzmann Equation: F T=σ 4−8 − −2 4σ = ×5.67 10 Wm K

4 27 −Sun: σT W= ×6.4 10 m→ T K6,000

Disposition of Solar Radiation:

( )

2Total absorbed solar radiation = −S a 1 π r0 p pa ≡ planetary albedo 30%p

2Total surface area = 4π rpS0 Absorption per unit area = − 1 a4 p

Absorption by clouds, atmosphere, and surface

Terrestrial Radiation:Effective emission temperature:

S4 0 σT a≡ − 1e p4

Earth: T K= =255 −18°Ce

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:

S4σ 0 T a= − 1 = TA 4 p e

Surface:SσT T4 4 0 ( )1 2 4

s = +σ A p− a = T4

→ =T TA e

4σ

σ e

1→ =T Ts e2 34 = 03 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.

-90 -6

-4

-2

0

2

4

6

-60 -30 Latitude

0 30 60 90

Atmosphere

Heat Transport by Oceans and Atmosphere

peta

Wat

ts

Ocean Transport

Total Transport Transport

Figure by MIT OpenCourseWare.