Lecture 6: The faint young Sun problem_Part 1_CO2 feedbacks

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Lecture 6 The faint young Sun problem_Part 1_CO2
feedbacks
Abiol 574
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Ice age (Pleistocene)
Dinosaurs go extinct
Phanerozoic Time
First dinosaurs
Ice age
First vascular plants on land
Ice age
Age of fish
First shelly fossils
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Geologic time
First shelly fossils (Cambrian explosion)
Snowball Earth ice ages
Warm
Rise of atmospheric O2 (Ice age)
Ice age
Warm (?)
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The faint young Sun problem
Kasting et al., Scientific American (1988)
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Was the young Sun really faint?

• Solar luminosity is a strong function of solar
  mass L? M?4
• Planetary orbital distance varies inversely with
  solar mass a M?1
• Solar flux varies inversely with orbital
  distance S a2
• Flux to the planets therefore goes as
• S M?6

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Ly ? spectrum of ? Eridani(from HST)
Estimated stellar emission line
Astrospheric absorption
ISM absorption
B. Wood et al., Ap. J. 574, 412 (2002)
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Estimated mass loss rate vs. stellar age
Sun
Wood et al. (2002)
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Integrated mass loss vs. time
200 Myr
Wood et al. (2002)

• The Sun was probably back on the standard solar
  evolution
• curve by 4.4 Ga (i.e., 4.4 Gyr ago)

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Terrestrial solutions to the FYS problem

• Albedo changes? Unlikely..
• Hard to have decreased cloudiness on a warm early
  Earth
• Increased greenhouse gas concentrations
• NH3 Doesnt work (photolyzes rapidly)
• CO2 Works! (supplied by volcanoes)
• CH4 Also works (probably requires life)
• Important to understand climate feedbacks

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Systems Notation
system component
positive coupling
negative coupling
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Positive Feedback Loops(Destabilizing)
Water vapor feedback
Surface temperature
Atmospheric H2O
()
Greenhouse effect
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The faint young Sun problem
More H2O
Less H2O
Kasting et al., Scientific American (1988)
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Positive feedback loops(destabilizing)
Snow/ice albedo feedback
Surface temperature
Snow and ice cover
()
Planetary albedo
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The Carbonate-Silicate Cycle
(metamorphism)

• Silicate weathering slows down as the Earth
  cools
• ? atmospheric CO2 should build up

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Negative feedback loops(stabilizing)
The carbonate-silicate cycle feedback
Rainfall
Surface temperature
Silicate weathering rate
(-)
Atmospheric CO2
Greenhouse effect
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CO2 vs. time if no other greenhouse gases
(besides H2O)
Snowball Earth events
J. F. Kasting, Science (1993)
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pCO2 from Paleosols (2.8 Ga)
Absence of siderite (FeCO3) places upper bound
on pCO2

• May need
• other green-
• house gases
• (CH4?)

Todays CO2 level (3?10-4 atm)
Rye et al., Nature (1995)
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Conclusions

• Higher atmospheric CO2 levels are a good way of
  compensating for the faint young Sun
• But, other greenhouse gases (e.g., CH4) could
  also have been important in the Archean when
  atmospheric O2 was low