1. Chapter 12—Part 1: The faint young Sun problem

2. Phanerozoic Time Ice age (Pleistocene) Dinosaurs go extinct Warm
First dinosaurs
Ice age
Age of fishes
First vascular plants on land
Ice age
Warm
First shelly fossils

3. Geologic time First shelly fossils (Cambrian explosion)
Snowball Earth ice ages
Warm (?)
Ice age
Rise of atmospheric O2 (Ice age)
Ice ages
Origin of life
Warm (?)

4. From a theoretical standpoint, it is curious that the early Earth was warm, because the Sun is thought to have been less bright 

5. The faint young Sun problem
Te = effective radiating temperature = [S(1-A)/4]1/4
TS = average surface temperature
Kasting et al., Scientific American (1988)

6. Positive feedback loops (destabilizing)
Water vapor feedback
Surface
temperature
Atmospheric
H2O
(+)
Greenhouse
effect
This feedback makes the faint young Sun problem worse

7. The faint young Sun problem
More H2O
Less H2O
Kasting et al., Scientific American (1988)

8. Positive feedback loops (destabilizing)
Snow/ice albedo feedback
Surface
temperature
Snow and ice
cover
(+)
Planetary
albedo
This feedback, which is not included in 1-D climate models,
would have further destabilized the early climate

9. Possible solutions to the FYS problem
Think back to the planetary energy balance equation
Te4 = S (1 – A) 4
Albedo changes?
A possible feedback involving biogenic sulfur gases and cloud condensation nuclei has been suggested (Rosing et al., Nature, 2010), but it is probably not strong enough to solve the problem
Geothermal heat?
Too small (0.09 W/m2 vs. ~240 W/m2 from absorbed sunlight

10. Possible solutions to the FYS problem
Increasing the greenhouse effect also works
Ts = Te + Tg
Possible greenhouse gases
NH3: Doesn’t work very well (photolyzes rapidly)
CO2: Works! (supplied by volcanoes)
CH4: Also works (probably requires life)
Sagan and Mullen (1972) liked methane (CH4) and ammonia (NH3) because they were aware that O2 levels were low on the early Earth…

11. Mass-independently fractionated S isotopes strongly
“Conventional” geologic indicators show that atmospheric O2 was low prior to ~2.2 Ga
Detrital
H.D. Holland (1994)
Mass-independently fractionated S isotopes strongly
support this conclusion

12. Witwatersrand gold ore
with detrital pyrite
(~3.0 Ga)
• Pyrite = FeS2
• Detrital pyrite is pyrite that
was not oxidized during
weathering
 Atmospheric O2 was low
when this deposit formed
P. Cloud, Oasis in Space (1988)

13. Caprock Canyon (Permian and Triassic) redbeds
Redbeds contain oxidized, or ferric iron (Fe+3)
Fe2O3 (Hematite)
Their formation requires the presence of atmospheric O2
Reduced, or ferrous iron, (Fe+2) is found in sandstones older than ~2.4 b.y. of age

14. Banded iron- formation or BIF (>1.8 b.y ago)
• Alternating layers of Fe-
rich and Fe-poor silica
Fe+2 is soluble, while
Fe+3 is not
• BIFs require long-
range transport of iron
 The deep ocean was
anoxic when BIFs
formed
(H.D. Holland, 1973)

15. What caused the rise of O2?
We are almost certain that the rise of O2 was caused by cyanobacteria, formerly known as blue-green algae
They’re not algae, though, because algae are eukaryotes (which have cell nuclei), whereas cyanobacteria are prokaryotes (lacking cell nuclei)

16. Different forms of cyanobacteria
a) Chroococcus b) Oscillatoria c) Nostoc
(coccoid) (filamentous) (heterocystic)
Nitrogen-fixing

17. Trichodesmium bloom
Cyanobacteria are important in the ocean today because they can fix nitrogen
Trichodesmium fixes N in the morning and makes O2 in the afternoon
Eukaryotic algae and higher plants all learned to photosynthesize from cyanobacteria 
Berman-Frank et al., Science (2001)

18. Ribosomal RNA
Ribosomes are organelles (inclusions) within cells in which proteins are made
Surprisingly, ribosomes contain their own RNA (ribonucleic acid)
The RNA is also the catalyst for protein synthesis, indicating that life may have passed through an “RNA World” stage
The RNA in ribosomes evolves very slowly, so looking at differences in the RNA of different organisms allows biologists to look far back into evolution
/mountains-beyond-mountains

19. (rRNA) tree of life cyanobacteria “Universal” Chloroplasts in algae
and higher plants
contain their own
DNA
Cyanobacteria form
part of the same
branch on the rRNA
tree (ribosomal RNA)
Interpretation (due
to Lynn Margulis):
Chloroplasts resulted
from endosymbiosis

20. They can’t do the whole thing, though
Back to climate…
So, reduced gases like CH4 and NH3 probably played some role in solving the faint young Sun problem
They can’t do the whole thing, though
NH3 is photolyzed (broken apart) by solar UV radiation and turns into N2 and H2
CH4 is also photolyzed and forms organic haze particles, which can create an anti-greenhouse effect
This brings us back to CO2…

21. The carbonate-silicate cycle
Atmospheric CO2 builds up as the climate cools, so higher
atmospheric CO2 is a natural solution to the FYS problem

22. Negative feedback loops (stabilizing)
The CO2-climate feedback
Rainfall
Surface
temperature
Silicate
weathering
rate
(−)
Greenhouse
effect
Atmospheric
CO2

23. CO2 vs. time if no other greenhouse gases (besides H2O)
One can easily solve the FYS problem with CO2 (and H2O), if one
is allowed enough CO2
J. F. Kasting, Science (1993)

24. CO2 from paleosols
Some geochemists have attempted to place limits on CO2 based on
geochemical indicators such as paleosols (ancient soils)
The latest estimates (Kanzaki & Murakami, 2015) are consistent with
the climate calculations
That said, there are still reasons to think that other greenhouse gases
were present
Catling & Kasting, in prep.

25. Why CH4 was important on the early Earth
Atmospheric O2 levels were low
Substrates for methanogenesis should have been widely available, e.g.:
CO2 + 4 H2  CH4 + 2 H2O
Methanogens are thought to be evolutionarily ancient 

26. (rRNA) tree of life Methanogenic bacteria Universal
Root?
Methanogens are all
found within one branch
of the Archaea, not far
from the presumed root
of the tree of life
Courtesy of
Norm Pace

27. CH4 is a good greenhouse gas because it absorbs in the 8-12 m
Window region
CH4 is a good greenhouse gas because it absorbs in the 8-12 m
“window” region where H2O and CO2 absorption is weak
Figure courtesy of Abe Lerman, Northwestern Univ.

28. Feedbacks in the methane cycle
Furthermore, there are strong feedbacks in the methane cycle that would have helped methane become abundant
Doubling times for thermophilic methan-ogens are shorter than for mesophiles
Thermophiles will therefore tend to outcompete mesophiles, producing more CH4 and further warming the climate

29. CH4-climate positive feedback loop
Surface
temperature
CH4
production
rate
(+)
Greenhouse
effect

30. But
If CH4 becomes more abundant than CO2, organic haze begins to form, as it does on Saturn’s moon, Titan 

31. Titan’s organic haze layer
The haze is formed from UV photolysis of CH4
It creates an anti-greenhouse effect by absorbing sunlight up in the stratosphere and re-radiating the energy back to space
This cools Titan’s surface
Image from Voyager 2

32. Possible Archean climate control loop
production
(–)
Surface
temperature
Haze
production
Atmospheric
CH4/CO2
ratio
(–)
CO2 loss
(weathering)

33. “Daisyworld” diagram for Archean climate
Greenhouse
effect
Haze formation
Effect of CH4
on surface
temperature
Surface temperature
Atmospheric CH4 concentration

34. “Daisyworld” diagram for Archean climate
Greenhouse
effect
Haze formation
Effect of CH4
on surface
temperature
Effect of
surface
on CH4 • P2
Surface temperature • P1
Atmospheric CH4 concentration

35. “Daisyworld” diagram for Archean climate
Greenhouse
effect
Haze formation • P2
Point P2 is stable
the Archean Earth
should have been
covered in organic
haze
Surface temperature • P1
Atmospheric CH4 concentration

36. CH4/CO2/C2H6 greenhouse with haze
Late Archean Earth?
Paleosols
Water freezes
When one puts all of this together, one comes up with this complicated
figure (Fig in ed. 3 of The Earth System)
The methane greenhouse ended, though, when atmospheric O2 went
up at ~2.4 Ga, possibly triggering the Paleoproterozoic glaciations
J. Haqq-Misra et al., Astrobiology (2008)

37. Geologic time First shelly fossils (Cambrian explosion)
Snowball Earth ice ages
Warm (?)
Ice age
Rise of atmospheric O2 (Ice age)
Ice ages
Origin of life
Warm (?)

38. Huronian Supergroup (2.2-2.45 Ga)
High O2
Redbeds
Glaciations
Detrital
uraninite
and pyrite
Low O2
S. Roscoe, 1969

39. Conclusions
So, reduced gases like CH4 probably played some role in solving the faint young Sun problem during the Archean
The CH4 went away when O2 levels rose, possibly triggering the Paleoproterozoic glaciations
High atmospheric CO2 was also involved, though
The high CO2 was a natural consequence of the negative feedback provided by the carbonate-silicate cycle