1. Climate feedbacks: Water vapor, snow/ice albedo, and clouds
Chapter 3—Part 4
Climate feedbacks: Water vapor, snow/ice albedo, and clouds

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
Feedback factor: f  Teq/ T0
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. Systems Notation = system component = positive coupling
= negative coupling

6. Positive Coupling Atmospheric CO2 Greenhouse effect
An increase in atmospheric CO2 causes
a corresponding increase in the greenhouse
effect, and thus in Earth’s surface temperature
Conversely, a decrease in atmospheric CO2
causes a decrease in the greenhouse effect

7. Negative Coupling An increase in Earth’s albedo causes a
(reflectivity)
Earth’s
surface
temperature
An increase in Earth’s albedo causes a
corresponding decrease in the Earth’s surface
temperature by reflecting more sunlight back to
space
Or, a decrease in albedo causes an increase in
surface temperature

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

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

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

11. 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

12. 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

13. 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.