1. Climate over the long term (Ch. 3 - 6 highlights)
Long-term climate changes
Plate tectonics
What maintains Earth’s habitability?
Faint Young Sun paradox
CO2 : Earth’s thermostat *
Past icehouse conditions
Past greenhouse conditions
* Critical idea

2. Long-term climate changes
Long-term: consider how Earth’s climate has changed
over last few hundred m.y.
Why study long-term changes?
-- Helps us to fundamentally understand how
Earth’s climate system works
-- If we don’t know this, we can’t evaluate
how climate might change in future

3. From before:
Natural climate variations
time scale type according to book
~few years “historical”
~ years “historical / millenial”
~10,000 years “orbital”
millions of years “tectonic”
Long-term climate
changes

4. Plate Tectonics Theory that the upper portion of Earth is subdivided
into ~dozen large pieces (lithospheric plates)
that move relative to one another
Most volcanic activity occurs at plate boundaries,
either where plates are moving apart (divergent
margin), or where they are moving towards
each other (convergent margin)
“tectonic” means any large scale Earth movement

5. Map of Earth’s lithopheric plates

6. Subduction
& mountain
building
here
Seafloor
spreading
here

7. Plate tectonics can affect climate because:
(1) Continents can change position
This strongly affects ocean currents.
(2) It controls the rate of volcanism
(high when plates moving fast, low otherwise).
(3) It controls the rate of weathering
(high when more continents collide and
more mountains formed).

8. Changing continent positions:
Assembly of supercontinent Pangaea

9. Rate of
volcanism

10. Changes in amount of uplift of continental rock
could regulate amount of weathering
“Uplift
weathering
hypothesis”
Get uplift mainly
when continents
collide

11. Why increased rock fragmentation leads to more weathering:
Weathering depends on surface area

12. What maintains Earth’s habitability?
Earth’s climate “just right”
-- at present
-- mostly over geologic time
geologic evidence (e.g. sedimentary
rocks) & biologic evidence (fossils)
indicates liquid water stable at
surface for most of Earth history
-- not always true in past, however

13. Venus Earth Mars Climates on three planets today
avg. temp oC oC oC
avg. distance x Earth x Earth
to sun
solar energy 2 x Earth x Earth
input (flux)

14. Venus Earth Mars Climates on three planets today
avg. temp oC oC oC
greenhouse oC oC oC
warming
avg. temp oC oC oC
with no
greenhouse
Just right
Too cold

15. Phase diagram for water

16. Venus, Earth, Mars with no greenhouse effect (& same pressure):

17. Faint Young Sun paradox
(1) Astrophysical models indicate that sun’s
brightness should have increased
significantly over age of solar system
(2) So why wasn’t Earth frozen earlier?

18. Solar luminosity -- what we mean by sun’s “brightness”
not same as albedo!
luminosity = energy / (area * time) = Watts / m2
at surface of sun; we call this flux away from sun
-- flux decreases as distance from sun increases
because solar energy spread over a larger area
(spreads over surface area of sphere = 4 * pi * r2)
-- models suggest sun’s luminosity increased by ~30% over age of solar system

19. Earth should
have been
frozen before
1.8 b.y. ago

20. CO2 : Earth’s thermostat?
CO2 is a greenhouse gas, helping to make
Earth habitable today
The amount of CO2 in the atmosphere may
have varied in the past to keep Earth
comfortable

21. GCM results: the effect of different CO2 levels

22. CO2 as Earth’s thermostat
-- where is carbon (C) stored on Earth?
-- how is C exchanged between different reservoirs?

23. Carbon reservoirs today
Where is carbon stored on Earth?
Carbon reservoirs today
Limestone
(carbonate)
rock: CaCO3

25. How is C exchanged between different reservoirs?
Carbon cycle
C exchange between
rocks & ocean +
atmosphere
Note: low rates

26. Focus on C exchange between rocks and atmosphere:
volcanic eruptions add C to atmosphere
(as CO2), remove it from rocks
chemical weathering of rocks either adds or removes C from atmosphere, depending on type of rock weathered; we’ll consider
removal of C from atmosphere

27. Volcanic eruptions
(Regulated by plate tectonics)

28. More volcanism earlier in Earth history?
-- Yes, more plate tectonic activity
-- Could get more CO2 in atmosphere, stronger
greenhouse
-- But unlikely that this alone exactly balanced
variations in solar luminosity
No reason for volcanic activity on Earth
to be related to solar luminosity !

29. Chemical weathering (hydrolysis):
-- chemical reaction of minerals with water to form different minerals
CaSiO H2O + CO CaCO SiO H2O
mineral rain atm mineral mineral
in rock
Makes carbonic acid H2CO3

30. Chemical weathering (hydrolysis):
-- removes CO2 from atmosphere, puts it in limestone (or carbonate) rock
-- proceeds faster if more precipitation, higher temperature, more vegetation
CaSiO H2O + CO CaCO SiO H2O
silicate rain atm limestone /
rock carbonate
(Why?)

31. Chemical weathering (hydrolysis):
-- removes CO2 from atmosphere, puts it in limestone (or carbonate) rock
-- proceeds faster if more precipitation, higher temperature, more vegetation
CaSiO H2O + CO CaCO SiO H2O
silicate rain atm limestone /
rock carbonate
(Why?-- carbonic acid)

32. Temperature - weathering feedback:

33. Temperature - weathering feedback:

34. CO2 : Earth’s thermostat?
The amount of CO2 in the atmosphere may
have varied in the past to keep Earth
comfortable
Chemical weathering (hydrolysis) was probably
important in regulating this
The weathering process involved a negative
feedback

35. Can weathering explain the Faint Young Sun Paradox?
If colder (lower solar luminosity), weathering rates should have been less...
… more CO2 stored in atmosphere, less in rocks...
… more greenhouse effect, higher temperature.
So: Yes, in principle.

37. icehouse & greenhouse conditions
But there were times in Earth’s
history when the (presumed CO2) thermostat was not so effective...
This led to
icehouse & greenhouse conditions

38. Past icehouse conditions
evidence for multiple glaciations
3 glaciations
last 500 m.y.
Major glaciation m.y. ago

39. Glacial striations
in Alaska
Formed by
movement of ice
over rock

40. Positioning of large landmasses over polar regions help cause glaciation
Note: Polar positioning is not the only reason we had past icehouse climates

41. Past greenhouse conditions
fossil evidence for warm conditions
100 Ma ago (Cretaceous period)
Dinosaurs
Warm-climate flora

42. O-isotope
data, deep
oceans:
~13 oC
cooling in
last 50 m.y.

43. 100 m.y. ago (Cretaceous):
Supercontinent Pangaea breaking apart
High sea level

44. GCM models including changes in plate position and CO2 fail to fully explain Cretaceous climate

45. What led to greenhouse conditions in the Cretaceous?
Probably 2 factors important
(1) Higher CO2 in atmosphere
-- faster plate movement led to more
volcanic emission of CO2
-- there was less removal of CO2 from
atmosphere by weathering because
there were few high mountains
(no plate collisions)
(2) Heat was transported in oceans differently than
today

46. Today
Then

47. Highly saline water is dense and
Model simulation of Cretaceous ocean salinity
Highly saline water is dense and
can sink, even if warm

48. If heat in
Cretaceous
oceans
transported more
efficiently,
would tend to
equalize
temperatures
more…
...discrepancies
between models
& geologic
evidence would
be explained