ERTH 535: Planetary Climate Change (Spring 2018) Reading Assignment #6 Dr. Dave Dempsey Dept. of Earth & Climate Sci., SFSU

(Issued Friday., April 13 for classes beginning Monday, April 16)

## From The Earth System, Chapter 2: "Daisyworld: An Introduction to Systems" (pp. 21-34)

• Key Questions: all (p. 21)

• The Systems Approach (pp. 21-26)

The Essentials of Systems (p. 21)

1. What is a system? What are the parts of a system called, and what are some examples? What is meant by the state of a system?
2. Couplings (p. 22)

3. What is a coupling in a system? What is meant by a positive coupling and a negative coupling?

4. In a systems diagram, what symbols are used to represent positive and negative couplings, respectively? What symbol is used for system components?
5. Feedback Loops (p. 22)

6. What is feedback? What effect does a negative feedback loop have on a disturbance in a system? What does a positive feedback loop do to such disturbances?

7. Why does an odd number of negative couplings in a feedback loop make the loop a negative feedback loop, whereas an even number makes it a positive loop?
8. Equilibrium States (p. 23)

9. What is an equilibrium state of a system? How is a stable equilibrium state defined? An unstable equilibrium state?

10. What type of feedback is necessary to make an equilibrium state stable? What type makes an equilibrium state unstable?

11. What ball-in-valley-and-hill-terrain analogy illustrates that the stability of some stable equilibrium states can be limited, depending on the size of the disturbance?

12. Why would a system in unstable equilibrium be unlikely to remain there for very long? To what state would a system tend to change if it were disturbed from an unstable equilibrium state?

13. When is the stability of a system impossible to determine from system diagrams and must instead be evaluated mathematically?
14. Perturbations and Forcings (p. 24)

15. Volcanic eruptions typically inject sulfur dioxide gas (SO2) into the atmosphere. In what way does this perturb the earth's climate system (through its energy budget)? Why is it necessary to average together the climate response to several volcanic eruptions to increase our confidence that such eruptions actually impact the climate system as we think they do? How long to the effects of individual volcanic eruptions typically affect the climate system?

16. What distinction do the authors of our text make between a perturbation of a system and forcing of a system? What example to the authors mention of a forcing of the climate system?

17. What criticism of the Gaia hypothesis did Lovelock and Watson create Daisyworld to address?

• The Daisyworld Climate System (pp. 26-29) [You won't be held accountable for this section, though I think it's really interesting!]

• External Forcing: The Response of Daisyworld to Increasing Solar Luminosity (pp. 30-33)

1. Why can the response of a system to forcing be quite different from the system's response to a perturbation?

2. How would the response to forcing of a system with a negative feedback loop differ if the system lacked the feedback?
3. Response of Daisyworld Couplings to Forcing

Response of Equilibrium States to Forcing

Climate History of Daisyworld

The Lessons of Daisyworld

4. What two lessons about the earth's climate system do the authors draw from the behavior of Daisyworld?

5. Does the Gaia hypothesis claim that living organisms on earth modify the earth's climate system to optimize conditions for their own existence? Does Daisyworld behave that way?

6. Does the self-regulation of Daisyworld compensate entirely for the effect of forcing due to a steady increase in solar luminosity? Does the earth's climate system? If not, how do systems such as these typically respond instead? How does this response differ from that of a thermostat?

• Chapter Summary: all (p. 33)
• Review Questions: all (p. 34)
• Critical Thinking Problems: #s 1-3 (p. 34)

(In this section we are most interested in the basic idea of stability vs. instability in both simple, everyday systems and in the earth system. The text develops a box-and-arrow notation to portray positive and negative feedback relationships in a system—pay some attention to this pictoral notation and be aware of how different this is from a box-and-arrow budget diagram. The distinction between perturbations and forcings is important. For our purposes, the details of Daisyworld and the graphs used to illustrate what is going on on Daisyworld are much less important than a basic conceptual understanding of how positive and negative feedbacks work in Daisyworld.)

## From The Earth System, Chapter 3: "Global Energy Balance: The Greenhouse Effect" (pp. 53-55)

• Key Questions: #3, #4 (p. 36)

• Climate Feedbacks (pp. 53-55)

1. Why are climate feedbacks important in the context of changes in greenhouse gas concentrations such as those being caused by human activities?
2. The Water Vapor Feedback

3. What is meant by the condensation point of water vapor? Is the amount of water vapor present in the atmosphere (at least, near the earth's surface) close to the condensation point? Is carbon dioxide (CO2) close to its condensation point? What would happen to the amount of water vapor present in the atmosphere if the earth's surface temperature were to cool for some reason? What would happen to the amount of water vapor in the lower atmosphere if the earth's surface temperature were to warm? Why?

4. What would happen to the strength of the greenhouse effect, and hence surface temperature, in response to decreases and increases, respectively, in water vapor concentrations in the lower troposphere? What kind of feedback—positive or negative—do these couplings between temperature and water vapor concentration imply?
5. Snow and Ice Albedo Feedback

6. What couplings are there among surface temperature, snow and ice coverage, and albedo? What type of feedback—positive or negative—do these couplings imply? Why can't a simple one-dimensional climate model, which represents the temperature of the entire earth, capture this feedback very well quantitatively?
7. The IR Flux/Temperature Feedback

8. What couplings are there between the earth's surface temperature and the emission flux of longwave IR radiation? What feedback—positive or negative—do these couplings imply?
9. The Uncertain Feedback Caused by Clouds

10. Clouds both reflect solar radiation back to space, which tends to cool the planet, and absorb outgoing longwave IR radiation emitted by the surface and reemit its own, partly back to the surface, enhancing the greenhouse effect and warming the surface. High clouds (such as cirrus clouds, which tend to be thin and made of ice crystals) enhance the greenhouse effect more than they increase the planetary albedo, whereas low clouds (such as stratus clouds, which are relatively dense with water droplets made of liquid water) tend to increase planetary albedo more than they enhance the greenhouse effect. This presents problems trying to figure out whether cloud feedback in the climate system is net positive or negative. Why else is cloud feedback hard to determine?

11. Why isn't it enough to consider global average temperature alone when considering climate change?

• Chapter Summary: #'s 3, 4 (p. 55)
• Review Questions: #'s 11, 12 (p. 56)
• Critical Thinking Problems: #5 (p. 56)

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