This Week: The Greenhouse Effect

• Reading: Continue Chapter 3• Problem Set 2 Due in Discussion Fri

This Week: The Greenhouse Effect (GHE)

• Atmospheric structure, composition, andabsorptivity

• Which gases contribute to the GHE andwhy are some better than others?

• What are the major sources of GHG tothe atmosphere?

1-Layer Model of the Greenhouse Effect

Surface Tsf

So/4 (So/4)A

FsfIN Fsf

OUT

Atmosphere Tatm

(1-ε) FsfOUT

εFatmOUT

εFatmOUT

The Greenhouse Effect

289 K – 256 K = 33 K

Ttrue – T”bare rock”

Definition: Absorption of terrestrial long-wave radiation by the atmosphere, causingthe surface T to be larger than the planet’semission T (as determined from absorbedsolar radiation flux).

The Greenhouse Effect is a

Hum

an-in

duced e

n...

A n

atura

l phen

ome.

..

87%

13%

1. Human-inducedenvironmental problem

2. A natural phenomenonpresent on many planets

The physics of the Greenhouse Effect canbest be described by analogy to

A g

reen

house

A s

olar p

ower

ed w

at..

Eggsh

ells

and o

rang...

53%

24%24%

1. A greenhouse2. A solar powered water

heater3. Eggshells and orange

peels in Earth’s energydrain

Earth’s Atmosphere

Measures of Composition

Physical Characteristics

•Pressure

•Temperature

•Regions

Earth’s Atmosphere•Thin collection of mainly gasesand some condensed phases

•Extends from Earth’s surface toabout 100 Km.

•Primary components (% by volume)•N2 (78%)•O2 (21%)•Argon (0.9%)•H2O vapor (0.00001 – 4%)•CO2 (0.038%)

•Many trace and ultra-tracecomponents

Measuring Atmospheric Pressure

vacuum

A Bh

Patm at pt A and B is the same.

Height of fluid related to balancebetween gravity and Patm

P(z2)

P(z1)

Gravity

Pressure Gradient Force

Are these two forces always in balance?

Barometric Law—”Hydrostatic Equation”

The atmosphere’s tendencyto be pulled into space isbalanced by gravity.

Announcements

• Office Hours Today– 4-5pm in 506 ATG– 5-6pm in 406 ATG

• Go to Focus the Nation on Thursday

• JISAO lectures (see course website)

Today

• Review Pressure vs Altitude

• Temperature vs Altitude

• Atmospheric Absorptivity, key players

Pressure Decreases Exponentially w/Altitude

P

altitudeAn exponential decay is anexponential growth in reverse

Gases (air) are compressiblefluids unlike liquids.

“Compressible” bricks ofair stacked on each other

heig

ht

Vertical Profiles of Pressure

Pressure decreases exponentiallywith increasing altitude.

-”air gets thinner as you go up”

1 hPa = 1 mbar ~ 0.001 atm

ln(P) is a straight line whenplotted vs. altitude.

Mean values for 30oN, March

What fraction of the atmosphere’s mass isbelow 15 km?

30%

60%

90%

10%

73%

17%

1. 30%2. 60%3. 90%

15 km

Vertical Profiles of Temperature

Temperature structure ofatmosphere is complex.

Regions of loweratmosphere separated bybehavior of T with altitude

Mean values for 30oN, March

Alt

itud

e (k

m)

Atmospheric Structure and Composition

• The atmosphere is a collection of ideal gases P = ρRT

• Pressure is force/area; difference in airpressure will cause motion

• Air pressure and ρ decrease exponentially withaltitude (“air gets thinner”)

• T decreases from 0 – 15 km (troposphere),increases from 15 – 50 km (stratosphere),decreases again from 50 – 80 km (mesosphere)

Key Points

The Greenhouse Effect (GHE)

• What gases contribute to the G.H.E.?

• What’s special about these “greenhousegases” (G.H.G)?

• How does adding a GHG to theatmosphere warms the surface?

• What makes one GHG “better” thananother?

Solar and Terrestrial Emission Spectra

Assuming black bodies

What Gases are Greenhouse Gases (GHG’s)?

Greenhouse gases absorb terrestrial outgoing long-wave radiation

I.e. they absorb infrared (IR) radiation

Several different gases give rise to the overallGreenhouse Effect.

Why are only some gases GHG?

The answer lies in our analogy to charges on springsinteracting with EM radiation.

IR radiation carries enough energy to make moleculesvibrate and rotate.

Announcements

• Office Hours Today– 4-5pm in 506 ATG

• Go to Focus the Nation on Thursday

• JISAO lectures (see course website)

Greenhouse Gases Absorb IR Radiation

Kirchoff’s law: to absorb radiation, the molecules mustbe able to emit that radiation.

For gas to absorb IR radiation: must generateoscillations in E&M fields when vibrate and rotate

C OOδ- δ-δ+

C OOδ- δ-δ+

OHH

δ+ δ+

δ-

Oδ-

HHδ+ δ+

OHH

δ+ δ+

δ- OH

Hδ+δ+

δ- OH H

δ+δ+

δ-

Earth Atmosphere’s Absorptivity

Absorption Spectrum

Indicates the absorptivity weassumed in our 1-layer model

Emission Spectrum Taken From Space

Spectrum takenover Niger valley,N Africa

Emission from cold atmosphere and warm surface

“Atmospheric Window”

Addition of a GHG Absorbing at 11 µm

1. Initial state

Addition of a GHG Absorbing at 11 µm

2. Emission at 11 µmdecreases (cold atmosphere)

Addition of a GHG Absorbing at 11 µm3. New equilibrium:total emission must be sameemission at other λ’s mustincreaseEarth surface must heat!

Because H2O vapor absorbs the larger fraction ofOLR, reducing CO2 concentrations will not reduce

the Greenhouse Effect

Tru

e

Fals

e

95%

5%

1. True2. False

GHG Ranking Factors

1. Amount: more there is, more radiation canbe potentially absorbed

2. Ability: depends on the wavelength

3. Location: both where in the atmosphere andwhere (λ) in the outgoing radiation spectrum

Band Saturation

λ

abili

ty t

o ab

sorb

ε Fr

acti

on a

bsor

bed

1

φ

λ

maximum possible

Simulated effect ofincreasing [GHG] on ε

Intrinsic to GHG,doesn’t depend on [GHG]

“Emission Height”

Temperature

Alt

itud

e (z

)

σTb4

Ts

a

b

σTa4

Emission to space fromz = a carries much moreenergy than from z = b