1. How the Greenhouse Effect Works/Feedback factors
Chapter 3—Parts 3&4
How the Greenhouse
Effect Works/Feedback factors

2. Climate feedbacks
The greenhouse effect itself can be calculated quite accurately
Example: Doubled CO2
The direct temperature effect of doubled CO2 (with no feedbacks) is to increase surface temperature by ~1.2oC
In the language of Daisyworld (and Earth 2)
T0 = 1.2oC

3. Climate feedbacks For doubled CO2: T0 = 1.2oC
But the predicted equilibrium response from climate models is
2oC < Teq < 5oC
Hence, in the models at least, there are positive feedbacks that tend to amplify the forcing by CO2. What are these?

4. Climate feedbacks Water vapor feedback Ice/snow albedo feedback
Cloud feedback

5. Water vapor feedback (+)  Positive feedback loop Surface temperature
Atmospheric
H2O
(+)
Greenhouse
effect
 Positive feedback loop

6. Snow/ice albedo feedback
Surface
temperature
Snow and ice
cover
(+)
Planetary
albedo
 Another positive feedback loop

7. What about clouds? Some reflection Cirrus clouds (Thin)
10 km
Cirrus clouds
(Thin)
More reflection
Altitude
Cumulus/stratus clouds
(Thicker)

8. What about clouds? Cirrus clouds High and cold Altitude
10 km
Cirrus clouds
High and cold
Tc4
Altitude
Cumulus/stratus clouds
Tw4
Low and warm
Tw4
Ts4 Tc
Temperature Tw Ts

9. What about clouds? Cumulus and stratus clouds Cirrus clouds
Low and warm
Small greenhouse effect
Big effect on albedo
These clouds cool the climate
Cirrus clouds
High and cold
Large greenhouse effect
Smaller effect on albedo
 These clouds warm the climate

10. Cloud feedback
Most models predict that cloudiness should increase as the climate warms
If low clouds increase the most, then the feedback will be negative
If high clouds increase the most, then the feedback will be positive
The balance of evidence suggests that cloud feedback is negative. However, this is highly uncertain, as clouds are sub-grid-scale in size and are therefore difficult to model.

11. Short-wavelength effect of clouds

12. Long-wavelength effect of clouds

13. Net Cloud Radiative Forcing
Net Cloud Radiative Forcing

14. Now, let’s go back and look at some fundamental properties of greenhouse gases 

15. Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas concentration (%) ppm
N2 78
O2 21
Ar 0.9
H2O variable CO CH
N2O
O to 0.01
(stratosphere-surface)

16. Composition of the Atmosphere
Air is composed of a mixture of gases:
Gas concentration (%)
N2 78
O2 21
Ar 0.9
H2O variable
CO ppm CH
N2O
O to 0.01
(stratosphere-surface)
greenhouse
gases

17. Greenhouse Gases

18. Water
Methane

19. N2 O2 N  N O = O Non-greenhouse Gases
What distinguishes these gases from
greenhouse gases?

20. N  N O = O Non-greenhouse Gases Answer: Symmetry!
(Technically speaking, greenhouse gases
have a dipole moment whereas N2 and O2
don’t)

21. O H H Oxygen has an unfilled outer shell
(−) O H H
(+)
Oxygen has an unfilled outer shell
of electrons (6 out of 8), so it wants
to attract additional electrons. It gets
them from the hydrogen atoms.

22. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.

23. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.
Water H O H
oxygen is more electronegative than hydrogen

24. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.
Water
Electron-poor region H O H
oxygen is more electronegative than hydrogen
Electron-rich region

25. Molecules with an uneven distribution of electrons are especially good absorbers and emitters.
These molecules are called dipoles.
Water
Electron-poor region:
Partial positive charge H O H
oxygen is more electronegative than hydrogen
Electron-rich region:
Partial negative charge

26. Vibration Molecules absorb energy from radiation.
The energy increases the movement of the molecules.
The molecules rotate and vibrate.
stretching
bending
Vibration

27. Approximate absorption regions
H2O O3
CO2
H2O
Radiant energy
Sun
Earth
0.1
1.0 10 15
100
 (m)

28. Thermal IR Spectrum for Earth
H2O pure rotation
H2O vibration/rotation
CO2 (15 m)
(6.3 m)
O3 (9.6 m)
Note that wavelength increases towards the left in this
diagram..
Ref.: K.-N. Liou, Radiation and Cloud Physics Processes in the Atmosphere (1992)

29. Distribution of Gases in the Atmosphere
Most gases are well mixed and distributed evenly
throughout the lowermost 100 km of the atmosphere.
Examples: O2, N2, Ar, CO2, freons
Gases with short lifetimes are not well-mixed.
Example: O3

30. Structure of the Atmosphere
Pressure = force per unit area
(exerted by a gas or liquid
on a surface)
At sea level, P = 1 atmosphere
= bar (or 1013 mbar)
Pressure decreases away from the Earth’s surface. The air becomes “thin” at high elevations.

31. The Barometric Law Pressure declines exponentially with altitude
Thus, it forms a (nearly) straight line when plotted on a log scale 

32. 100 80
Altitude
(km) 60 40 20
Pressure (mbar)

33. 100 80
Pressure decreases away from the Earth’s surface.
Altitude
(km) 60 40 20
Pressure (mbar)

34. 100 80
Altitude
(km) 60 40 20
( oC)
Temperature (K)

35. 100 80
Altitude
(km) 60 40 20
Temperature decreases
Temperature (K)

36. 100 80
Altitude
(km) 60 40 20
Troposphere
0-10 km
Temperature decreases
Temperature (K)

37. 100 80
Altitude
(km) 60
Temperature increases 40 20
Troposphere
0-10 km
Temperature (K)

38. 100 80
Altitude
(km) 60
Temperature increases
Stratosphere
10-50 km 40 20
Troposphere
0-10 km
Temperature (K)

39. 100 80
Mesosphere
50-90 km
Temperature decreases 60
Stratosphere
10-50 km 40 20
Troposphere
0-10 km
Temperature (K)

40. Temperature increasesoC
Thermosphere
90 + km
100
Temperature increases 80
Mesosphere
50-90 km 60
Stratosphere
10-50 km 40 20
Troposphere
0-10 km
Temperature (K)

41. Troposphere
heated by convection
turbulent, mixed
contains all weather (wind, rain, clouds, etc.)
water is important in this region
Stratosphere
not well mixed, or “stratified”
cold at base, warmer in upper region
ozone present
ozone heats upper region by absorbing uv
radiation

42. Stratosphere 10-50 km Troposphere 0-10 kmoC
100 80 60
ozone
Stratosphere
10-50 km 40 20
water
Troposphere
0-10 km
Temperature (K)