(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)
- 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?
Couplings (p. 22)
- What is a coupling in a system? What is meant by a positive
coupling and a negative coupling?
- In a systems diagram, what symbols are used to represent positive and
negative couplings, respectively? What symbol is used for system components?
Feedback Loops (p. 22)
- 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
- 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
Equilibrium States (p. 23)
- What is an equilibrium state of a system? How is a stable equilibrium
state defined? An unstable equilibrium state?
- What type of feedback is necessary to make an equilibrium state
stable? What type makes an equilibrium state unstable?
- 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?
- 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?
- When is the stability of a system impossible to determine from system
diagrams and must instead be evaluated mathematically?
Perturbations and Forcings (p. 24)
- 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?
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?
- 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)
- Why can the response of a system to forcing be quite different
from the system's response to a perturbation?
- How would the response to
forcing of a system with a negative feedback loop differ
if the system lacked
Response of Daisyworld Couplings to Forcing
Response of Equilibrium States to Forcing
Climate History of Daisyworld
The Lessons of Daisyworld
- What two lessons about the earth's climate system do the authors
draw from the behavior of Daisyworld?
- 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?
- 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
- 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
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)
- Why are climate feedbacks important in the context of changes in greenhouse gas concentrations
such as those being caused by human activities?
The Water Vapor Feedback
- 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?
- 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?
Snow and Ice Albedo Feedback
- 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?
The IR Flux/Temperature Feedback
- 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?
The Uncertain Feedback Caused by Clouds
- 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?
- 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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