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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
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
From before:
Natural climate variations
time scale
type according to book
~few years
“historical”
~10-1000 years
“historical / millenial”
~10,000 years
“orbital”
millions of years
“tectonic”
Long-term climate
changes
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
Map of Earth’s lithopheric plates
Seafloor
spreading
here
Subduction
& mountain
building
here
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).
Changing continent positions:
Assembly of supercontinent Pangaea
Rate of
volcanism
Changes in amount of uplift of continental rock
could regulate amount of weathering
“Uplift
weathering
hypothesis”
Get uplift mainly
when continents
collide
Why increased rock fragmentation leads
to more weathering:
Weathering depends on surface area
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
Climates on three planets today
Venus
Earth
Mars
avg. temp.
460 oC
15 oC
-55 oC
avg. distance
to sun
0.7 x Earth
1.5 x Earth
solar energy
2 x Earth
0.44 x Earth
input (flux)
Climates on three planets today
avg. temp.
Venus
Earth
Mars
460 oC
15 oC
-55 oC
Just right
greenhouse
warming
285 oC
31 oC
5 oC
avg. temp.
with no
175 oC
-16 oC
-60 oC
greenhouse
Too cold
Phase diagram for water
Venus, Earth, Mars with no greenhouse effect (& same pressure):
-16 C
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?
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
Earth should
have been
frozen before
1.8 b.y. ago
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
GCM results: the effect of different CO2 levels
CO2 as Earth’s thermostat
-- where is carbon (C) stored on Earth?
-- how is C exchanged between different
reservoirs?
Where is carbon stored on Earth?
Carbon reservoirs today
Limestone
(carbonate)
rock: CaCO3
How is C exchanged between different reservoirs?
Carbon cycle
C exchange between
rocks & ocean +
atmosphere
Note: low rates
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
Volcanic eruptions
(Regulated by plate tectonics)
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 !
Chemical weathering (hydrolysis):
-- chemical reaction of minerals with water to
form different minerals
CaSiO3 + H2O + CO2
CaCO3 + SiO2 + H2O
mineral
in rock
mineral
rain
atm
Makes carbonic acid H2CO3
mineral
Chemical weathering (hydrolysis):
CaSiO3 + H2O + CO2
silicate
rock
rain
atm
CaCO3 + SiO2 + H2O
limestone /
carbonate
-- removes CO2 from atmosphere, puts it in
limestone (or carbonate) rock
-- proceeds faster if more precipitation,
higher temperature, more vegetation
(Why?)
Chemical weathering (hydrolysis):
CaSiO3 + H2O + CO2
silicate
rock
rain
atm
CaCO3 + SiO2 + H2O
limestone /
carbonate
-- removes CO2 from atmosphere, puts it in
limestone (or carbonate) rock
-- proceeds faster if more precipitation,
higher temperature, more vegetation
(Why?-- carbonic acid)
Temperature - weathering feedback:
Temperature - weathering feedback:
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
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.
But there were times in Earth’s
history when the (presumed CO2)
thermostat was not so effective...
This led to
icehouse & greenhouse conditions
Past icehouse conditions
evidence for multiple glaciations
Major glaciation 550-850 m.y. ago
3 glaciations
last 500 m.y.
Glacial striations
in Alaska
Formed by
movement of ice
over rock
Positioning of large
landmasses over polar
regions help cause
glaciation
Note: Polar positioning is
not the only reason we had
past icehouse climates
Past greenhouse conditions
fossil evidence for warm conditions
100 Ma ago (Cretaceous period)
Dinosaurs
Warm-climate flora
O-isotope
data, deep
oceans:
~13 oC
cooling in
last 50 m.y.
100 m.y. ago (Cretaceous):
• Supercontinent Pangaea breaking apart
• High sea level
GCM models including changes in plate
position and CO2 fail to fully explain Cretaceous
climate
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
Today
Then
Model simulation of Cretaceous ocean salinity
Highly saline water is dense and
can sink, even if warm
If heat in
Cretaceous
oceans
transported more
efficiently,
would tend to
equalize
temperatures
more…
...discrepancies
between models
& geologic
evidence would
be explained