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, and absorptivity

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

• What are the major sources of GHG to the atmosphere?

1-Layer Model of the Greenhouse Effect

Surface Tsf

So/4(So/4)A

FsfIN Fsf

OUT

Atmosphere Tatm

(1-) FsfOUT

FatmOUT

FatmOUT

The Greenhouse Effect

289 K – 256 K = 33 K

Ttrue – T”bare rock”

Definition: Absorption of terrestrial long-wave radiation by the atmosphere, causing the surface T to be larger than the planet’s emission T (as determined from absorbed solar radiation flux).

The Greenhouse Effect is a

Hum

an-in

duced e

n...

A n

atura

l phen

ome.

..

87%

13%

1. Human-induced environmental problem

2. A natural phenomenon present on many planets

The physics of the Greenhouse Effect can best be described by analogy to

A g

reen

house

A s

olar p

owered

wat

..

Eggsh

ells

and o

rang...

53%

24%24%

1. A greenhouse2. A solar powered

water heater3. Eggshells and orange

peels in Earth’s energy drain

Earth’s Atmosphere

Measures of Composition

Physical Characteristics

•Pressure

•Temperature

•Regions

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

•Extends from Earth’s surface to about 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-trace components

Measuring Atmospheric Pressure

vacuum

A B

h

Patm at pt A and B is the same.

Height of fluid related to balance between 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 tendency to be pulled into space is balanced 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

altitude

An exponential decay is an exponential growth in reverse

Gases (air) are compressible fluids unlike liquids.

“Compressible” bricks of air stacked on each other

heig

ht

Vertical Profiles of Pressure

Pressure decreases exponentially with increasing altitude.

-”air gets thinner as you go up”

1 hPa = 1 mbar ~ 0.001 atm

ln(P) is a straight line when plotted vs. altitude.

Mean values for 30oN, March

What fraction of the atmosphere’s mass is below 15 km?

30%

60%

90%

10%

73%

17%

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

15 km

Vertical Profiles of Temperature

Temperature structure of atmosphere is complex.

Regions of lower atmosphere separated by behavior of T with altitude

Mean values for 30oN, March

Alt

itu

de (

km

)

Atmospheric Structure and Composition

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

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

• Air pressure and decrease exponentially with altitude (“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 “greenhouse gases” (G.H.G)?

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

• What makes one GHG “better” than another?

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 overall Greenhouse Effect.

Why are only some gases GHG?

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

IR radiation carries enough energy to make molecules vibrate 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 must be able to emit that radiation.

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

C OO- -+

C OO- -+

OHH

+ +

-

O-

HH+ +

OHH

+ +

- OH

H++

- OH H

++

-

Earth Atmosphere’s Absorptivity

Absorption Spectrum

Indicates the absorptivity we assumed in our 1-layer model

Emission Spectrum Taken From Space

Spectrum taken over Niger valley,N Africa

Emission from cold atmosphere and warm surface

“Atmospheric Window”

Addition of a GHG Absorbing at 11 m

1. Initial state

Addition of a GHG Absorbing at 11 m

2. Emission at 11 m decreases (cold atmosphere)

Addition of a GHG Absorbing at 11 m

3. New equilibrium: total emission must be same emission at other ’s must increase Earth surface must heat!

Because H2O vapor absorbs the larger fraction of OLR, reducing CO2

concentrations will not reduce the Greenhouse Effect

Tru

e

Fal

se

95%

5%

1. True2. False

GHG Ranking Factors

1. Amount: more there is, more radiation can be potentially absorbed

2. Ability: depends on the wavelength

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

Band Saturation

abili

ty t

o a

bso

rb

Fr

act

ion

ab

sorb

ed

1

maximum possible

Simulated effect of increasing [GHG] on

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

“Emission Height”

Temperature

Alt

itud

e (

z)

Tb4

Ts

a

b

Ta4

Emission to space from z = a carries much more energy than from z = b