1. Greenhouse Gases and Climate Modeling
GLOBAL ENERGY BUDGET - 4
Greenhouse Gases and Climate Modeling

2. WHY DO SOME GASES CONTRIBUTE TO THE GREENHOUSE EFFECT & OTHERS DO NOT?
Gas molecules absorb/emit radiation in two ways
Changing the rate at which the molecule rotates
Changing the amplitude with which a molecule vibrates

3. CHANGE IN ROTATION Molecules rotate at discreet frequencies
If the frequency of the incoming wave is just right, the molecule absorbs the photon
The molecule emits the photon when the rotation slows down
Depends on structure of molecule

5. H2O ROTATION BAND Strong absorption feature of Earth’s atmosphere
H2O molecule absorbs IR radiation of 12μm or longer
Virtually 100% of infrared radiation > 12μm absorbed

6. H2O ROTATION BAND

7. CHANGE IN AMPLITUDE OF VIBRATION
If the frequency at which the molecule vibrates matches frequency of incoming wave, molecule absorbs photon and vibrates more
Bending mode of CO2 allows molecule to absorb IR radiation about 15 μm λ

8. CHANGE IN AMPLITUDE OF VIBRATION

9. 15 μm CO2 BAND Strong absorption feature of Earth’s atmosphere
Important to climate because it occurs near peak of Earth’s outgoing radiation
very little of Earth’s outgoing radiation can escape because it is absorbed by CO2

10. OTHER GREENHOUSE GASES
CH4, N2O, O3 and freons
More effect on outgoing radiation than low concentrations would suggest
Absorb at different wavelengths than H2O & CO2

11. O2 & N2 Poor absorbers of IR radiation Perfectly symmetrical molecules
Electromagnetic fields unable to interact with symmetrical molecules

12. EFFFECT OF CLOUDS ON RADIATION BUDGET
Quantification of effect difficult
Many types of clouds
Cumulus – water
Cumulonimbus – water
Stratus – water
Cirrus – ice

13. CLOUD TYPES

14. CLOUD EFFECTS Day – cool Earth by reflecting sunlight back to space
Without clouds albedo would be ~0.1
At 0.1 Te would increase 17C
Night – warm Earth – re-emit outgoing IR radiation

15. CLOUD EFFECTS Stratus – low, thick Cirrus – high, thin Increase albedo
Reflect incoming solar radiation
Radiate at higher temperature, and according to Stefan-Boltzmann law radiate more energy to space
Cirrus – high, thin
Increase greenhouse effect
Ice crystals more transparent to incoming solar radiation
Radiate at lower temperatures and according to Stefan-Boltzmann law radiate less energy to space

16. CON TRAILS FROM JETS?

17. EARTH’S GLOBAL ENERGY BUDGET

18. PRINCIPLE OF PLANETARY ENERGY BALANCE
At the top of the atmosphere, the net downward solar radiation flux (incoming minus reflected), must equal the outgoing infrared flux

19. CLIMATE MODELING Climate system complex
Computer models based on data used to simulate climate systems
GCM – General Circulation Model (aka Global Climate Model) - includes
3-d representation of atmosphere (winds, moisture, energy)
Weather (clouds, precipitation)
Require huge amounts of computer power

20. One Dimensional Climate Model
Radiative-Convective Model (RCM)
Climate system approximated by averaging incoming solar and outgoing IR over Earth’s entire surface
Vertical dimension divided into layers
Temperature of each layer calculated
Energy received or emitted
Convection
Latent heat release

21. RCMs Allow estimation of greenhouse effect magnitude
uses concentrations of greenhouse gases in atmosphere
Models accurately predict ∆Tg (33C)
Allow prediction of temperature increase due to GHG
Doubling CO2 from 300ppm to 600ppm would produce a 1.2C increase
The temperature change ∆T0 in the absence of any climate system feed back loops

22. CLIMATE FEEDBACKS
Amplify or moderate radiative effect due to GHG concentrations
Water Vapor Feedback
Snow and Ice Albedo Feedback
The IR Flux/Temperature Feedback
The Cloud Feedback (Uncertain)

23. THE WATER VAPOR FEEDBACK
If Earth’s surface temperature , then water vapor  (precipitation)
If water vapor , then greenhouse effect , and surface temp 
If Earth’s surface temperature , then water vapor  (evaporation)
If water vapor , then greenhouse effect , and surface temp 

24. THE WATER VAPOR FEEDBACK

25. THE WATER VAPOR FEEDBACK
Incorporated into RCM by assuming fixed relative humidity in troposphere
RCM predicts doubling CO2 doubles the equilibrium change in surface temperature compared to the effect without water vapor

26. Mathematically Speaking . . .
Comparing equilibrium temperature with and without water vapor feedback (from Ch 2)
∆Teq = ∆T0 + ∆Tf
∆Teq = 1.2C+ 1.2C  2.4C

27. The Feedback Factor
The ratio of the equilibrium response to forcing (the response with feedback) to the response without feedback
 = temperature change with feedback = C  2
temperature change w/out feedback C
Negative feedback loop if 0 <  < 1
Positive feedback loop if 1 < 
STRONGLY POSITIVE

28. SNOW & ICE ALBEDO FEEDBACK
Snow & ice have higher albedo than land & water
Increases in snow and ice coverage should decrease surface temperature
Positive feedback loop
Snow & ice restricted to middle & high latitudes, 2- or 3-d models are required

29. SNOW & ICE ALBEDO FEEDBACK

30. THE IR FLUX/TEMPERATURE FEEDBACK
Strong negative feedback loop
Stabilizes Earth’s climate on short time scales
If Earth’ surface temperature , outgoing IR flux , if outgoing flux , surface temperature would 
More energy is lost from the system
System can fail if the atmosphere contains too much water vapor
Venus – runaway Greenhouse Effect

31. THE IR FLUX/TEMPERATURE FEEDBACK

32. THE CLOUD FEEDBACK (UNCERTAIN)
Adds significant uncertainty to climate models
Clouds can warm or cool, depending on height
Form at some locations and not others
Most current GCMs
Net positive feedback for doubled CO2
Increase in cirrus clouds (warming) outweighs any increase in stratus clouds (cooling)