1. MET 12 Global Climate Change - Lecture 5
Climate Feedbacks
Shaun Tanner
San Jose State University
Outline
Stability/instability
Feedbacks
Examples
Activity

2. That there is a link between CO2 and global temperatures is irrefutable as this diagram shows, but exact details of the link and all the possible feedback mechanisms are not clear.
The interaction of positive and negative feedback effects associated with the greenhouse effect is currently a critical issue for the scientific community.

3. Earth’s Climate
The Earth’s climate is fairly stable in terms of temperature
This can be visualized using in the following system diagram.
The idea is that even though the system may change away from it’s initial point, it will have the tendency to go back to ‘normal’ eventually. 2 3 1
Stable
Stable

4. Stability versus instability
Stable:
Instability:
Stable equilibrium
Unstable equilibrium

5. Stability versus instability
Stable:
Given a perturbation, the system tends to return to original state
Instability:
Given a perturbation, the system moves to another state.
Stable equilibrium
Unstable equilibrium

6. States of equilibrium
The system may have multiple states of equilibrium 2 3 1
Stable to small perturbations, until a big force perturbs the system into a new equilibrium

7. Climate Stability
The Earth’s climate changes as a result of internal/external forcing:
Changes in solar radiation
Changes in the earth’s orbit
Plate tectonics
Volcanoes
Human pollution etc.
These forcings can be thought of as a perturbation (or push) to climate stability.
These changes can be enhanced or diminished by positive or negative feedbacks

8. Feedbacks
Positive feedback:
Negative feedback:

9. Climate Feedbacks Positive feedback:
initial change reinforced by another process.
Negative feedback:
initial change counteracted by another process.

10. Positive Feedbacks Processes that accelerate a change
Note: Feedbacks cannot initiate change; they can only alter the pace of change
Important climate examples:
Ice-albedo feedback
Water-vapor feedback
Cloud feedback

11. Ice-Albedo Feedback (Cooling)
Initiating Mechanism
Earth Cools
Ice Coverage Increases
Albedo Increases
Absorption of Sunlight Decreases

12. Ice-Albedo Feedback (Cooling)
Initiating Mechanism
Earth Cools
Somehow this happens
Ice Coverage Increases
Positive Feedback
Albedo Increases
Absorption of Sunlight Decreases

13. Fill in the blanks Initiating Mechanism Earth Warms
31 of 31
Initiating Mechanism
increases, decreases, decreases
Decreases, decreases, increases
Increases, increases, increases
Decreases, decreases, decreases
Earth Warms
Ice Coverage ___________
Albedo _____________
Absorption of Sunlight _______

14. Ice-Albedo Feedback (Warming)
Initiating Mechanism
Earth Warms
Ice Coverage
Albedo
Absorption of Sunlight

15. Ice-Albedo Feedback (Warming)
Initiating Mechanism
Earth Warms
Ice Coverage Decreases
Positive Feedback
Albedo Decreases
Absorption of Sunlight Increases

16. Water Vapor Feedback (Warming)
Initiating Mechanism
Earth Warms
Evaporation
Atmospheric Water Vapor Content
Greenhouse Effect

17. Fill in the blanks Increases, increases, increases
33 of 36
Fill in the blanks
Increases, increases, increases
Increases, decreases, decreases
Decreases, increases, increases
Decreases, decreases, decreases

18. Water Vapor Feedback (Warming)
Initiating Mechanism
Earth Warms
Evaporation Increases
Positive Feedback
Atmospheric Water Vapor Content Increases
Greenhouse Effect Strengthens

19. Water Vapor Feedback (Cooling)
Initiating Mechanism
Earth Cools
Evaporation
Atmospheric Water Vapor Content
Greenhouse Effect

20. Water Vapor Feedback (Cooling)
Initiating Mechanism
Earth Cools
Evaporation Decreases
Positive Feedback
Atmospheric Water Vapor Content Decreases
Greenhouse Effect Weakens

21. Negative Feedbacks Processes that reduces an imposed change
Important examples:
Cloud feedback
Chemical weathering
Note: Positive/negative feedbacks have no relation to ‘good versus bad’, but are about how a system responds to a change.

22. Possible Role of Cloud in Warming or Cooling the Atmosphere
Finally in relation to CO2 or greenhouse warming there is the vexatious question of the role of cloud.
GO THROUGH
To some extent the role of cloud depends on its height in the atmosphere. Low level cloud tends toward warming through long wave trapping, while high level cloud leads to cooling through solar radiation reflection
In our final slide today, lets briefly look at surface characteristics.

23. Which feedback is positive?
Left
Right
32 of 36

24. Cloud Feedbacks (positive and negative)
Positive Feedback
Negative Feedback
Finally in relation to CO2 or greenhouse warming there is the vexatious question of the role of cloud.
GO THROUGH
To some extent the role of cloud depends on its height in the atmosphere. Low level cloud tends toward warming through long wave trapping, while high level cloud leads to cooling through solar radiation reflection
In our final slide today, lets briefly look at surface characteristics.

25. This is a positive feedback. This is a negative feedback.
As a result of the earth’s warming over the last 100 years, plants have been growing at a faster rate. Because plants absorb CO2, the increase in plant growth means that more CO2 is being absorbed from the atmosphere by plants.
This is a positive feedback.
This is a negative feedback.
This has nothing to do with feedbacks.

26. Activity
Imagine the Earth was to warm for some reason (initiating mechanism or perturbation)
Identify two positive feedbacks that would influence the earth’s climate and explain how each one works.
Identify two negative feedbacks that would influence the earth’s climate and explain how each one works.
Which feedback do you think is more uncertain. Explain your reasoning.