Chapter 2 Systems approach
Objectives:
•Couplings & Feedback loops
•Equilibrium states
•Perturbations & Forcings
•CO2-temp.-photosyn. feedback system
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Body T
Blanket T
(a) Positive coupling
Blanket T
Body T
(b) Negative coupling
Body T incr. => blanket T decr.
Body T
(–)
Blanket T
(c) Negative feedback loop
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Correct setup
A’s body T
(–)
A’s blanket T
B’s body T
(–)
B’s blanket T
Incorrect setup
A’s body T
A’s blanket T
(+)
B’s blanket T
B’s body T
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Is this a positive or negative feedback loop?
A
B
C
D
H
G
F
E
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t
t
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Exceptions about positive feedbacks
• Positive
feedback
can =>
stable equil.
if state of
one
component
depends
only on the
present
state of the
other
component
Child’s noise level
parent’s anger level
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• If both
parent’s &
child’s
response
are strong
=>
unstable
equil.
Child’s noise level
parent’s anger level
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Equilibrium states
• Correct elec.
blanket setup with
negative feedback
loop =>
equilibrium state.
Stable eq.
state
Stable eq.
Stable eq.
state
state
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Unstable equilibrium states
• System rarely
stays at an
unstable equil.
state for long.
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Perturbations & forcings
• Perturbation = temporary disturbance of
system.
E.g. volcanic eruption => aerosol, but
effects disappear in 3 yrs.
• Forcing = more persistent disturbance of
system.
E.g. Gradual incr. in sun’s output.
Gradual incr. in atm. CO2.
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The average climate response to the five
largest volcanoes of the last 120 years.
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• Climate sensitivity:
The sensitivity of the climate systems to a forcing is commonly
expressed in terms of the global mean temperature changes that
would be expected after a time sufficiently long for both the
atmosphere and ocean to come to equilibrium with the change in
climate forcing.
An important factor of climate sensitivity is feedback of
components of the climate system. An example: if CO2
concentration doubles (forcing 4w/m^2), the temperature will increase
1.25C if no feedback. However, the situation is very complicated due to
feedbacks. For example, CO2 up => T up => ice melting => surface
color changes => more sun lights absorbed => T up….
feedbacks cause uncertainty in climate changes..
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Feedbacks
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Feedbacks
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CO2-temp.-photosyn. feedback system
Atm. CO2
(–)
T
(–)
Photosynthesis
6CO2+6H2O -> C6H12O6 + 6O2
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Water Vapor
Feedback
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warming
Decreased
snow and ice;
less reflectivity
More solar
radiation absorbed
at surface
Ice-Albedo Feedback
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Initial Change
Climate
warming
Reduced
Warming
Uncertain
Increased
clouds
Greater
reflected
radiation
Cloud Radiative Feedbacks
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FEEDBACKS INVOLVED IN GLOBAL WARMING
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CLIMATE SENSITIVITY
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CLIMATE SENSITIVITY
The ‘commitment’ to
future warming
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Uncertainty in Climate Change Study
Scenario uncertainty --- due to uncertainty of future
emissions of GHGs and other forcing agents;
Model uncertainty --- associated with climate models;
Natural variability --- Stochastic and Nonlinear problem;
Initial condition uncertainty and forcing and boundary –
the limitation of observation data and assimilation
technique
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•
•
Representative Concentration Pathways (RCPs)
The four RCPs --- RCP2.6, RCP4.5, RCP6, and RCP8.5
a possible range of radiative forcing values in the year 2100
relative to pre-industrial values (+2.6, +4.5, +6.0, and +8.5 W/m2,
respectively).
RCP 2.6 assumes that global annual GHG emissions peak
between 2010-2020, with emissions declining substantially
thereafter.
Emissions in RCP 4.5 peak around 2040, then decline.
In RCP 6, emissions peak around 2080, then decline.
In RCP 8.5, emissions continue to rise throughout the 21st
century.
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As observational records lengthen and models
improve, researchers should be able, within the
limitations of the range of natural variability, to
narrow that range in probable temperature in the
next few decades . It is also possible to use
information about the current state of the oceans
and cryosphere to produce better projections up
to a few years ahead.
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